Phosphors Based on Phosphates of Nazr2(PO4)3 and Langbeinite Structural Families A

ISSN 2079-9780, Review Journal of Chemistry, 2018, Vol. 8, No. 1, pp. 1–33. © Pleiades Publishing, Ltd., 2018

Phosphors Based on Phosphates of NaZr2(PO4)3 and Langbeinite Structural Families A. E. Kanunova,* and A. I. Orlovab a Russian Federal Nuclear Center, All-Russia Research Institute of Experimental Physics, Sarov, Nizhny Novgorod oblast, 607188 Russia b Lobachevsky State University, Nizhny Novgorod, 603950 Russia *e-mail: [email protected]

Abstract—The review covers aspects of modeling the composition and luminescent properties of phosphates of NaZr2(PO4)3 (NZP) and langbeinite structural families. Based on the analysis of phosphates structure, a plausible algorithm of the use of crystal chemistry data for modeling compositions, structure, and properties of new compounds is proposed. The following properties determined by requirements to materials for the chosen purposes are studied: behavior on heating and in water systems, luminescence, and biocompatibility. Prospects for the use of such data for solving various problems of materials science, LED technologies, bioimaging, and X-ray induced photodynamic therapy of oncological diseases are shown.

Keywords: phosphates, lanthanides, structural type, NaZr2(PO4)3, NZP, langbeinite, phosphors, LED technologies, bioimaging, X-ray induced photodynamic therapy DOI: 10.1134/S207997801801003X

Table of contents 1. Introduction 1.1. Phosphate Phosphors for LED Technologies 1.2. Inorganic Phosphors for Intracellular Bioimaging 1.3. Inorganic Phosphors for X-ray Induced Photodynamic Therapy of oncological Diseases

2. NaZr2(PO4)3 and Langbeinite Structural Families 2.1. NZP Family 2.2. Lb Family 2.3. Crystal Chemistry Approach in the Design of New Phosphate Phosphors. Choice of Formula Composi- tions 3. Synthesis 3.1. Brief Review of Methods for the Preparation of Orthophosphates of NZP and Lb Families 4. Features of Phase Formation in Systems of Lanthanide-Containing Phosphates with NZP and Langbeinite Structures 4.1. Phase Formation 4.2. Structural Data 5. Luminescent Properties 5.1. Phosphate Phosphors for LED Technologies 5.2. Phosphate Phosphors for Bioimaging 5.3. Phosphate Phosphors for X-PDT 6. Other Properties: Behavior on Heating, Chemical Stability, Biocompatibility 6.1. Behavior on Heating 6.2. Chemical Stability 6.3. Biocompatibility 7. Conclusions

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1. INTRODUCTION The need in new “on a plan” functional materials, including those favoring the better quality and longer duration of human life [1–5] constantly increases under the conditions of continuously developing science and high technology. Tasks of the development and improvement of methods of the preparation and study of such materials are included in the List of Russian Critical Technologies. Within these tasks, an innovative direction of present-day inorganic chemistry and materials science is the development of new ecologically safe and biocompatible phosphors with adjustable properties made as nanocrystalline powders and ceramics. The control of their composition and, correspondingly, properties opens wide possibilities for the use of such materials in promising industrial and biomedical technologies, also as energy-efficient sources of while light, biocompatible optically active substances for monitoring pathological processes in tissues of living systems (bioimaging), and X-ray induced photodynamic therapy of oncological diseases (X-PDT).

1.1. Phosphate Phosphors for LED Technologies Energy-saving technologies in lighting have a need in new compounds and materials on their basis for economic and ecologically safe light sources. Among them of interest are LED technologies and the development of white light-emitting diodes. Two methods are known to obtain white light: mixing colors according to the RGB (red, green, blue) technology and application of phosphors on industrially pro- duced light-emitting diodes (LEDs), emitting in the blue (Fig. 1a) or ultraviolet (Fig. 1b) spectral regions. White LEDs coated by phosphors are significantly cheaper than LED RGB matrix panels. Because of a combination of different phosphors, white light with coordinates close to {0.33; 0.33} in the color scale of the International Commission on Illumination for these devices can be obtained in a simpler way. The most widespread design includes a InxGa1 – xN light-emitting diode (λ = 460 nm) [6–10] and a phosphor based on YAG:Ce3+ [11–13], which converts part of radiation of the light-emitting diode to light in a wide spectral band with the maximum in the yellow region due to photoluminescence (Fig. 1a). Being mixed, radiation of the phosphor and the light-emitting diode give white light. However, the white light obtained in this case does not possess the maximum intensity. It is increased using phosphors with other color possibilities. Phosphor materials must satisfy certain requirements, such as (1) safe composition, (2) chemical and thermal stability, (3) possibility of regulation of optical properties by changing composition (is desirable), and (4) simplicity of synthesis and low cost. These requirements necessitate the improvement of the properties of already known materials and the search for new compositions, possessing better characteristics, and also the sophistication of methods of their preparation. A special place among the known phosphors is occupied by Eu2+-containing phosphors, adapted to blue or UV spectral region [14]. Emission and absorption spectra of Eu2+ contain wide bands correspond-

Phosphor (a) (b) I absorption 1.0 Combined UV Light-emitting spectrum Phosphor diode emission 0.8 Phosphor 0.6 emission

0.4 InGaN LED 0.2

UV Visible region IR 0 400 450 500 550 600 650 700 750 380 555 755 λ, nm λ, nm

Fig. 1. Method of creation of while light-emitting diodes: (a) blue light-emitting diode coated with a yellow phosphor; (b) UV light-emitting diode coated with blue, green, and red phosphors.

REVIEW JOURNAL OF CHEMISTRY Vol. 8 No. 1 2018 PHOSPHORS BASED ON PHOSPHATES 3 ing to transitions from the excited 4f 65d1 state to the ground 4f 7 state. As 5d orbitals are outer orbitals, the positions of energy levels and, correspondingly, excitation and emission wavelengths, strongly depend on the “host” crystal (matrix) [14]. Therefore, the choice of a matrix is a critical parameter in determining optical properties of the Eu2+ cation. Eu3 +, Sm3+, and Mn2+ cations are also used as luminescence activators in many phosphors [15]. The most intense luminescence bands characteristic for Eu3+ and Sm3+ cations correspond to 4f–4f transitions (red luminescence) on excitation in the UV or blue spectrum region [16]. The emission of the Mn2+ cation covers a wide frequency region and, as was found in [17], with an increase in the effect of crystal field, is shifted from the green to the red region. It is known from the published data that salt compounds with tetrahedrally coordinated oxoanions— phosphates, silicates, vanadates, molybdates, etc.—are studied as crystal matrices containing Eu2+, Eu3+, Sm3+, and Mn2+ activator cations [18–23]. It should be noted that phosphate-based phosphors offer evident advantages, as they differ by stable physical and chemical properties, safety, and also by the low cost of the starting components [22, 23]. Phosphate phosphors activated by Eu2+, Eu3+, Sm3+, and Mn2+ and known from the literature and the corresponding regions of spectral band maxima are summarized in Table 1. Note that the number of publications on the study of lanthanide-containing inorganic compounds of oxide and salt character for optical applications constantly grows every day, because of which corresponding information is sketchy. As methods for the preparation such materials, researchers use the solid-phase method and the sol– gel technology, including the Pechini citrate method. The final temperature of synthesis is in the range 700–1300°C. An important requirement for phosphors is a possibility of the regulation of optical properties by changing their composition. In this regard, attention should be paid to substances with the structures of NaZr2(PO4)3 (NZP, NASICON) and langbeinite (Lb, basic analogue of K2Mg2(SO4)3). Possibilities of these structural types, because of their wide isomorphism, open a promising direction of scientific research and assume the preparation of a wide range of substances (individual compounds and solid solutions) with adjustable specified properties, including optical ones [65–68]. In this case, the crystal chemistry approach is the basic one. However, the data on such substances for optical applications with the main activator cations are sketchy and quite limited. However, as described above, there are extremely wide possibilities for the preparation new compounds with useful optical properties, also regulated in the desirable direction, from these classes of substances using the crystal chemistry approach.

1.2. Inorganic Phosphors for Intracellular Bioimaging The use of inorganic compounds as phosphors for intracellular bioimaging or labeling individual sub- cellular structures is a fundamental issue of bioengineering (genetic, cellular, tissue, etc.) and an innovative method of the study of mechanisms of physiological and pathological processes in living systems. Flu- orescent labels are detected as individual samples by standard fluorescence microscopy, which allows the visualization of processes at a level of individual cell structures and molecules [69–75]. The main “target” molecules are antibodies, to which various labels can be attached by chemical methods [75]. The necessary requirements to such materials are their biological inertness, presence of photoluminescence in the visible spectral region with an acceptable intensity, and safety of excitation sources for biological samples. Several classes of compounds are studied for these purposes, i.e., (1) organic: fluorescent proteins; (2) quantum dots; and (3) inorganic composites of complex composition. Fluorescent proteins. Fluorescent proteins of the GFP (green fluorescent protein) family are the most native structures ensuring the complete elimination of the problem of biodegradation and affinity to any cell type [76, 77]. However, a small number of these compounds known by now hinder observations in any spectral region [78–80]. The fluorescence of GFP depends on the pH and temperature of the medium and can be completely quenched on protein denaturation [79]. In addition, fluorescent proteins, as all other proteins, possess antigen properties, so that their use for labeling antibodies is inexpedient. An important fact is the high cost of the preparation of fluorescent proteins and work with them. Quantum dots. Semiconductor quantum dot nanocrystals consist of 103–105 atoms of elements groups 2–6 or 3–5 of the Periodic System (CdHg, CdSe, CdTe, CdS, ZnS, PbS, InP, InAs, etc.), and their size is 2–10 nm [69]. They offer a number of advantages, such as photostability and bright emission in the wide optical region on excitation by a source of infrared radiation [69, 81–84]. However, the main prob-

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Table 1. Positions of maxima (or range) of luminescence bands in the visible region and spectra of luminescence excitation for the known phosphate phosphors Positions of band maxima, nm Phosphor Reference excitation emission Simple phosphates with a layered structure 2+ Li(Ca,Sr,Ba)PO4:Eu 350−400 450−480 [24–27] 2+ NaCaPO4:Eu 390 506 [28] 3+ NaCaPO4:Eu 393 594–621 [29] 2+ K(Ca,Sr,Ba)PO4:Eu 292–325 425–473 [25, 30–32] 2+ CsMgPO4:Eu 420 650 [33] 3+ LaPO4:Eu 254 575–625 [34] Phosphates with mineral whitlockite structure 2+ 2+ 2+ Ca9(La,Eu,Lu)(PO4)7:Eu , Eu –Mn 257–365 485–645 [35–37] 2+ 3+ 2+ 2+ Ca8Mg(Gd,Lu)(PO4)7: Eu , Eu , Eu −Mn 365–400 480–615 [14, 38, 39] 2+ 2+ 2+ (Ca,Mg,Sr)9Y(PO4)7:Eu , Eu −Mn 280–360 435–490, 632 [40] 2+ Ca2.89Mg0.11(PO4)2:Eu 350 413 [41] 3+ Ca9Bi(PO4)7:Eu 393 592, 615 [42] 2+ Ca3–x–ySry(PO4)2: Eu 365 400−700 [43] Phosphates with mineral apatite structure 2+ Ca5(PO4)3Cl:Eu 375 459 [44, 45] 2+ 2+ Ca5(PO4)3Cl:Eu −Mn 375 585 [44, 45] 2+ Sr5(PO4)3Cl:Eu 345 445 [46, 47] 3+ BiCa4(PO4)3O:Eu 269, 395 570−620 [48] 2+ Sr5Cl0.75F0.25(PO4)3:Eu 277, 341 445 [49] Pyrophosphates 2+ 2+ 2+ (Ca,Sr)2P2O7:Eu , Eu –Mn 300–330 415–600 [50, 51] 3+ Li2BaP2O7:Eu 252–467 581–622 [52] 3+ Li2BaP2O7:Sm 227–476 525–650 [52] Other phosphates 2+ 2+ Ca3Mg3(PO4)4:Eu , Mn 300 450, 625 [53] 2+ (Ba,Sr)10(PO4)4(SiO4)2:Eu 315, 350 507 [54] 2+ Ba7Zr(PO4)6:Eu 370 585 [55] 3+ K3Y(PO4)2:Sm 405 600−700 [56] 3+ Ba3Y(PO4)3:Sm 401 600 [57] 2+ Ca5(PO4)2(SiO4):Eu 365 495 [58] 2+ 3+ Na5Al(PO4)2F2:Eu −Eu 265, 325 420, 530, 615 [59] 2+ 2+ 2+ SrMg2(PO4)2:Eu ,Eu −Mn 375 420, 675 [60–62] 2+ 2+ 2+ SrZn2(PO4)2:Eu ,Eu −Mn 365 416, 613 [60–62] 3+ Na2Ba1–xSrxMg(PO4)2:Eu 220, 300 585–625 [63] 3+ K3Gd5(PO4)6:Eu 394 575–625 [64]

REVIEW JOURNAL OF CHEMISTRY Vol. 8 No. 1 2018 PHOSPHORS BASED ON PHOSPHATES 5 lem limiting the use of quantum dots is connected with their toxicity and ability to initiate morphological changes in the cells, internal parts of a body, and DNA [70, 84–89].

Fluoride-based inorganic biosensors. Nanodimensional particles on the basis of fluorides (LaF3, YF3, GdF3, NaYF4, GdOF) are of interest as energy converters from the near IR to the visible spectral region; correspondingly, by their optical characteristics they can be used as biotags for the visualization of cancer tumors [90]. A drawback of fluorides is in the strict requirements to synthesis conditions, i.e., specially pure oxygen- and water-free atmosphere. Inorganic composites of complex composition. The effect of toxicity can be reduced using substances formed by biogenous elements, for example, Ca, P, Si, etc. The published data also cover phosphors on the basis of silicates and phosphates containing alkaline-earth elements: MgSiO3, CaMgSi2O6, 2+ 3+ 2+ Sr2MgSi2O7, Ca0.2Zn0.9Mg0.5Si2O6 with Eu , Dy , and Mn impurities [91, 92]; Ca9Eu(PO4)7 [37] and Ca10.5–1.5(x + y)ErxYby(PO4)7 [93]. The last two compounds are chemical analogues of biogenous mineral whitlockite β-Ca3(PO4)2 [94]. Mixed oxide compounds Gd14 – x – yErxYbyB6Ge2O34 and Gd11 – n – mYbnErmSiP3O26 were described in [95]. Silicates containing Eu2+, Dy3+, and Mn2+ cations are characterized by an emission band in the red region of visible spectrum at 600–750 nm with an intensity maximum at 690 nm on excitation by an UV source with λexcit ~ 350 nm. For phosphates with the structure of whitlockite Ca10.5 – 1.5(x + y)ErxYby(PO4)7, emission was observed in the visible spectral region (λem = 525, 550, 650 nm) on excitation with an IR source (λexcit = 977 nm) by an upconversion mechanism. Because of its safety (green luminescence) and high intensity, it is acceptable for the visualization of living systems. Phosphates of this kind correspond to the criterion of safety by results of testing their toxicity [93]. The shortcomings of silicate and phosphate compounds are the high temperature of synthesis (~1150°C) and intermediate pressing stages [91, 93]. In addition, β-whitlockite as a biogenous mineral forms the main part of bone tissue, and lanthanides found in its composition can be accumulated in it. Compounds capable of emitting light in the visible spectral region on excitation by an IR source with λexcit ~ 980 nm (anti-Stokes shift, including the upconversion mechanism) are most acceptable for bioimaging tasks. Such emission can be attained on the simultaneous presence Er3+ and Yb3+ lanthanides in the matrix [96–100]. As follows from the summary of the published data, the most promising direction in the creation of fluorescent tags by the moment is the synthesis of such compounds combining useful properties of natural fluorophores (fluorescent proteins) and quantum dots. Russian analogues of such nanomaterials are not known. The use of principles of crystal chemistry modeling allows the prediction of new compositions of compounds of anticipated structures. In the present work, this approach is used as a basis for the development of biocompatible phosphates of structural families NaZr2(PO4)3 and langbeinite with high concentrations of calcium and also of magnesium, silicon, and lanthanide cations (Er3+, Yb3+) in their compositions.

1.3. Inorganic Phosphors for X-ray Induced Photodynamic Therapy of Oncological Diseases Inorganic compounds bearing lanthanides are studied as phosphors for specific applications in medi- cine, i.e., X-ray induced photodynamic therapy of oncological diseases (X-PDT). This method consists in the introduction of photosensitizers, enhancing the sensitivity of tumors to light, in an organism. Under the effect of light, the photosensitizer generates active forms of oxygen (singlet oxygen, peroxides, etc.), which destroy pathogenic cells [101–105]. A sophistication of the X-PDT method is the development of a pharmacological preparation, which, along with a photosensitizer, contains a colloidal solution of a nanophosphor, emitting light with the wavelength necessary for the excitation of a photochemical reaction under the influence of high-energy photons, which easily penetrate into organism tissues (Fig. 2) [105]. Such a phosphor must satisfy the following requirements: (1) be safe for an organism by its chemical composition; (2) possess particle size of 60–90 nm; (3) on excitation by X- or γ-rays, emit in a very narrow wavelengths region, necessary for the implementation of a photochemical transformation of the used photosensitizer [105]. Chlorin, purpurine, and benzoporphyrin derivatives are the most efficient and safe among the studied photosensitizers. Preparations on the basis of such compounds possess a photodynamic therapy spectrum with absorption maxima in the region 620–700 nm: “KillerRed®” λ = 610 ± 5 nm [106, 107], “Foskan®” λ = 652 ± 5 nm [108], “Fotoditazin®” λ = 662 ± 5 nm [109, 110], and “Fotosens®” λ = 670 ± 5 nm [108]. The attention of researchers is concentrated on the development of phosphor materials with emission in the above regions of optical spectrum for activating the known photosensitizers.

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X- or γ-rays Photosensitizer Singlet oxygen generation

3 → 1 D D* O2 O2 Nanophosphor–converter of radiation Visible light Killing pathogenic cell

Fig. 2. An X-PDT procedure using a pharmacological preparation including a photosensitizer (D) and a nanophosphor- 3 1 converter of radiation: D* is excited photosensitizer; O2 is tissue oxygen; O2 is singlet oxygen [105].

Phosphors based on quantum dots. Nanophosphors based on ZnSe and other quantum dots are char- + 2+ 3+ 3+ – acterized by X-ray luminescence in the red spectral region: ZnS/CdS: Ag , Cu , Ga , In , Cl (λem = 2+ 3+ 650–660 nm), ZnS/CdSe: Cu , Ga (λem. = 655–680 nm) [111]. However, as is evident from elemental composition and as was stated above, such phosphors are toxic. 3+ Oxide phosphors. The luminescence spectrum of the In Y2O3:Sm phosphor exhibits intense bands in the red region of optical spectrum. Such a phosphor was synthesized by a complex combustion reaction [112]. 2+ Silicate and phosphate phosphors. Manganese-containing magnesium orthosilicate Mg2SiO4:Mn is characterized by emission at the maximum wavelength 660 nm (photo and cathode luminescence) [113]. 3+ Phosphors on the basis of magnesium orthosilicate, for example, Mg2SiO4:Tb , are used for thermolumi- nescence dosimetry and are sensitive to γ-rays [114]. A drawback of this phosphor is the complex technology of the synthesis of nanopowders. Phosphate phosphors possess high biocompatibility and are stable to radi- 2+ 2+ ation. Phosphors based on zinc phosphate Zn3(PO4)2:Mn and barium phosphate Ba3(PO4)2:Eu possess radioluminescence in region λem. = 560–720 nm with a maximum at 630 nm [105, 115, 116]. Europium- 2+ containing calcium phosphate β-Ca3(PO4)2:Eu can be used for the activation of the “Fotoditazin®” photosensitizer in the region of its “blue” absorption band (400 nm). The maximum in its luminescence spectrum is at the wavelength 412 nm [113]. The simultaneous introduction of Mn2+ and Sm3+ into the composition of this phosphate can ensure emission in the red spectral region. It should be noted that a few number of the inorganic phosphors accepted for X-PDT, in particular, those containing f elements (Eu2+, Sm3+) were described in the literature. Taking into account the specific features of the framework structure of compounds of NaZr2(PO4)3 and langbeinite families, which may contain different cations and ensure their strong binding in the structure, and also the data about the radiation [117–119], thermal [120–125], chemical [124, 126–129] stability of the known isostructural analogues, it was supposed that phosphates containing biogenous elements (K, Mg, Ca) with such structures may be of interest for the “photosensitizer+nanoluminophor” version of X-PDT. The generalization of approaches to the development of luminescent materials for the considered problems of LED equipment and biomedical applications suggests that studies based on the principles of chemical and structural similarity can be promising and systematic. Such crystal chemistry concept we are developing on an example of compounds with tetrahedral oxoanions, first of all, crystallizing in the structural types of NaZr2(PO4)3 (NZP) and langbeinite (Lb) with high possibilities of various heterovalent cation and/or anion substitutions. Modeling of compositions on the basis of such isostructural compounds, including those of necessary compositions (biogenous elements, lanthanides as activators of practically important optical properties, phosphorus, other) was described in [65–68]. This review summarized and systematizes data on the synthesis of new phosphates, study of regularities of structure formation, microstructures, and properties responsible for their application in LED technologies and biomedicine.

2. NaZr2(PO4)3 AND LANGBEINITE STRUCTURAL FAMILIES Following the known “composition–structure–property” paradigm allows a researcher to develop functional materials and control their properties. The prediction of the properties of inorganic materials consisting of octahedral–tetrahedral framework compounds is based on the dynamic crystal chemistry

REVIEW JOURNAL OF CHEMISTRY Vol. 8 No. 1 2018 PHOSPHORS BASED ON PHOSPHATES 7 theory of mixed frameworks, developed by Academician N.V. Belov and his school [130–134], and principles of crystal chemistry modeling of formula compositions, developed by our group for ortho compounds with single-nucleus tetrahedral oxoanions [65, 67, 68]. The most widespread type of structure-forming frameworks is the mixed tetraedral–octahedral frame- n− work of topological domains {T243 (XO ) } , formed by [TO6] octahedra and [XO4] tetrahedra [135]. The discreteness of these types of Pauling polyhedrons is determined by the ratio T : X = 2 : 3 [133]. Depending on the oxidation state of cations T (from +1 to +5) and X (from +4 to +6), the charge of the framework n takes values from 0 to 4. For different X (X4+, X5+, X6+), plausible compositions are similar to those presented in [65, 66] for X = P(X5+) at a shift of framework charge n to unity (towards increase for X4+ or decrease for X6+) [68]. n− Crystal chemistry analysis revealed three main polymorphic types of the framework {T243 (XO ) } with different relative positions of [TO6] and [XO4] polyhedra: NaZr2(PO4)3 (mineral kosnarite) [136], K2Mg2(SO4)3 (mineral langbeinite) [137], and Ca3Al2(SiO4)3 (mineral garnet) [138]. n− Structure-forming domains {T243 (XO ) } in these modifications are packed differently. As a result, channels and voids in the structure differ by their shapes and sizes, and the total numbers of voids for cations compensating the charge of the framework per one formula unit are also different. These structural types are genetically related to one another. Frameworks of structural types NZP and Lb possess the closest relation- ship, which is confirmed by the existence of morphotropic transitions in these systems [139–142].

2.1. NZP Family

Zirconium–sodium orthophosphate NaZr2(PO4)3 is the parent compound of the structural type cov- ering a great number of isostructural analogues and solid solutions. The first study of the crystal structure of NaZr2(PO4)3 was performed in 1968 by Swedish crystal chemists Hagman and Kierkegaard [136].

The NaZr2(PO4)3 phosphate crystallizes in the trigonal system and has space group Rc3 of rhombohedral symmetry (a = 8.8043(2) Å, c = 22.7585(9) Å, V = 1530.5 Å3, Z = 6) [136]. It is characterized by an octa- − hedral–tetrahedral framework structure with the structure-forming domain {Zr243 (PO ) } (Fig. 3a) formed by two [ZrO6] octahedra connected with three [PO4] tetrahedra via oxygen bridges. Uniform domains form clusters extended along the 3 axis. The places of intersection of channels penetrating through the structure are crystallographic voids of M1 and M2 types (Fig. 3b), differing in size, shape, and coordination environment of the cations arranged in the voids. The M1 positions are between [ZrO6] octahedra and have distorted octahedral coordination. Positions of the other type, M2 with CN 8 are between – structure-forming {[Zr2(PO4)3] }3∞ domains. Their number is three times higher, than the number of M1 positions [143, 144]. M1 and M2 positions can be occupied completely, partially, or remain unoccupied. In general, the crystal chemistry formula of compounds with the structure of NZP is as following:

[6] [8] [6] [4] n− (1)(2){[(XO)]}MM32 L 433∞ , n− where {[L243 (XO )] } ∞ is the framework of the structure (topological domain), L is the position of the framework; XO4 is tetrahedral radical anion; M1 and M2 are types of extra framework cation positions; and [4, 6, 8] are coordination numbers [144, 145]. The NZP structure was most completely investigated in phosphate systems: transition from one compound to another occurred by iso- and heterovalent isomorphous substitutions of cations in the framework and/or cavity at the preservation of the electroneutrality of the whole structure. The possible formula compositions of such phosphates were calculated and summarized in the review [65] and the known representatives were summarized in the later review [146] (though incompletely) and other works [2, 68, 124, 147].

In the known zirconium orthophosphates of the type AZr2(PO4)3, where A = Li, Na, K, Rb, and Cs, M1 positions are completely occupied by atoms of alkali elements [148]. At isomorphous substitutions involving elements in oxidation states +2 (alkaline-earth elements B) and +3 (lanthanides Ln), the symmetry of the cell is reduced: 1. Rc3 → R3: atoms B occupy positions 3b {0; 0; 1/2}, positions 3a {0; 0; 0} are vacant, glide planes c disappear [145, 149–151]; 2. Rc3 → Pc3 : Ln atoms are ordered in positions 2b {0; 0; 0 }, translation symmetry changes R → P [152–154].

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(a) (b) (c) Na K Zr P Mg O S O

M1 M2

Fig. 3. (a) A fragment of a crystal structure of NaZr2(PO4)3 (b) with designations of types of extra-framework positions M1, CN 6 and M2, CN 8 and (c) a fragment of langbeinite structure.

The data on the inclusion of lanthanum and lanthanides in the structure of NZP are of special interest in developing luminescent materials. The analysis of published data indicates that lanthanum and f element (3+, 4+) cations can enter into the composition of phosphates with NZP structure both in in-framework and extra-framework positions [155–164]. The population of L positions by Ln3+ cations depends on the ionic radius of the cation and decreases with its growth [155]. This was found on an example of the Na1 – xLnx(PO4)3 system, where Ln = Yb–Gd on decreasing x from 1 for Yb to 0.2 for Gd. Lanthanide cations of bigger size do not occupy the L position [155–157]. Complex zirconium phosphates in which trivalent element cations are compensating cations are presented by compounds of the type R0.33Zr2(PO4)3, where R = Sc, Y, Bi and Ln = La, Pr, Nd, Sm, Eu, Gd, Tb, Er, Tm, Yb, Lu [154–163]. It was found that, with an increase in the serial number of the lanthanide, the parameter c of the unit cell trends to reduce, which is due to the phenomenon of lanthanide contraction; at the same time, the parameter a increases and the volume V decreases [154].

The refinement of the structures of Ln0.33Zr2(PO4)3 (Ln = Ce, Eu, Yb) phosphates using the Rietveld method allowed us to find that the symmetry of the structures of the studied phosphates reduced to space group Pc3 [154].

2.2. Lb Family

The crystal structure of the double potassium–magnesium sulfate K2Mg2(SO4)3, an analogue of natural mineral langbeinite, was first described by Zemann in 1957. This compound crystallizes in the cubic 3 system, space group P213 (a = 9.920 (4) Å, V = 976.2 (1) Å , Z = 4) [137, 165]. According to the data [166, 167], sulfur atoms are in the center of slightly distorted oxygen tetrahedra and occupy in the lattice common positions of C1 symmetry. K and Mg atoms are at threefold rotation axes in the centers of polyhedra of C3 symmetry. The [SO4] tetrahedra are located in the lattice so as to form slightly distorted [MgO6] octahedra of C3 symmetry around the Mg atom (Fig. 3c).

K atoms in points of local symmetry C3 are located in structure voids in two positions, surrounded by 9 or 12 oxygen atoms (Fig. 3c). The values of O–S–O angles lie in the range 107.1° –111.0° and the lengths of S–O bonds, within 1.458–1.468 Ǻ. Mg atoms are located so that two types of distorted octahedra, (1) (2) (1) [Mg O6] (dbond. = 2.057 Ǻ) and [Mg O6] (dbond. = 2.066 Ǻ) can be distinguished in the structure. K atoms are coordinated by twelve and K(2) atoms, by nine oxygen atoms: the average lengths of K(1)–O and K(2)–O bonds are equal to 3.045 and 2.992 Å, respectively [167].

The [SO4] tetrahedra and [MgO6] octahedra, bound with each other via vertexes, form the framework 2– {Mg2(SO4)3} (Fig. 3c). In the structure of K2Mg2(SO4)3, columns of octahedra are oriented along four nonintersecting directions, parallel to the main diagonals of the cube.

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Based on structural data, the crystal chemistry formula of langbeinite-like phases can be presented as:

[12] [9] [6] [6] [4] n− (MM 1) ( 2) {[( LL 1) ( 2) (X O43 ) ] } 3∞, where L1, L2 are in-framework positions and M1, M2 are extra-framework positions [67]. The most numerous group of the known compounds with langbeinite-like topology is exemplified by sulfates. The results of the generalized analysis of compounds with Lb structure we performed in [67] point to wide possibilities of manifestation of this crystal structure also in phosphates. The charges n of langbeinite frameworks can take values from 2 to 4 if both M1 and M2 positions are fully occupied. The majority of the known phosphates are characterized by frameworks with n = 2: A2[LnM(PO4)3], where A = K, Rb, Cs; M = Ti, Zr; Ln = Pr–Lu [140, 168–173]. For n = 3, the formula types are as follows: A1.5Ln0.5[MgZr(PO4)3], where A = K, Rb, Cs; Ln = Pr, Sm, Yb [124, 174]; AB[Ln2(PO4)3], where A = KBa, RbBa, CaBa, K, Cs; Ln = Dy, Ho, Er, Tm, Yb [124, 174–187]; A1.5Ln0.5[Fe2(PO4)3], where A = K, Rb, Cs; Ln = Pr, Sm, Yb [124, 174]. For n = 4, the following phosphates are known: Ba2Mg1 + 0.5(1 – x)[LnxZr0.5(1 – x)(PO4)3], where 0 ≤ x ≤ 1; Ln = Sm, Yb [178, 179].

Some phosphates with partially vacant M positions were also described: K1.822Nd0.822Zr1.178(PO4)3 and Rb2.05Ti0.81Yb1.19(PO4)3 [180]. The analysis of the published data indicates that the possibilities of the Lb structure on the inclusion of lanthanides cations in-framework and extra-framework positions are significantly wider than those of the NZP structure. The most favorable crystallographic positions occupied by f elements in the Lb structure are framework-forming L positions, and the atomic concentration of lanthanides can be increased to two in the formula composition AB[Ln2(PO4)3] [124, 174–177]. The data on the occupation of void positions by small f element cations are scanty. n– It should be also noted that, along with the formation of framework ortho compounds {T2(XO4)3} including differently charged cations and iso- and heterovalent substitutions of cations in the formation of solid solutions, there is also a possibility of the formation of both NZP and Lb isostructural phases with different oxoanions XO4 (X are cations forming tetrahedral oxoanions with oxidation numbers from +3 to +7) [68]. In [68], calculations of formula compositions of compounds with tetrahedral oxoanions in the framework structure with noninteger frameworks charges were performed (on an example of an NZP structure) and their graphical representation was given. This opens additional possibilities for “modeling” complex compositions of solid solutions on the basis of iso- and heterovalent isomorphism, also with the simultaneous participation of cations both in the cationic and anionic parts of the framework. This crystal chemistry principle can be used in the design of new materials with properties varied in the desirable direction and also with anticipated new properties, including optical ones. At the same time, the above reviews and monographs contain no systematic presentation of optical properties of NZP and Lb compounds and their application as luminescent materials. As follows from the analysis of the published data, compounds (in particular, phosphates) with the structures of NZP and Lb offer promise for the creation of “on a plan” materials because of specific features of their structures: presence of several crystallographic positions that can be occupied by various cations and anions in various combinations and ratios. Studies of a possibility of the introduction of biogenous elements and lanthanides into such crystal structures seem to be a promising direction of inorganic chemistry and materials science, favoring the solution of a number of scientific and applied problems, in particular, the design of phosphors materials for LED and biomedical technologies. In general, the analysis of the literature, as was shown above, allowed us to see the critical problems of present-day inorganic chemistry and materials science as the need in the development, preparation, and study of luminescent materials for LED and biomedical technologies. It should be also noted that the available data on inorganic phosphors for biomedical applications are fragmentary, which complicates the purposeful search and the development of new compounds. The practical implementation of the principles of crystal chemistry modeling on the basis of compounds with tetrahedrally coordinated oxoanions can successfully contribute to solving the listed problems. The “modeled” compounds containing active luminescence centers can be a basis for phosphors materials for the above technologies, in particular, for energy saving white light sources, in vitro and in vivo monitoring of physiological processes at the cell level, and for therapeutic purposes in the treatment of oncological diseases by the X-PDT method. The development of studies in this direction retains a need in new experimental data and thus opens a wide field for research.

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2.3. Crystal Chemistry Approach in the Design of New Phosphate Phosphors. Choice of Formula Compositions The analysis of a wide range of experimental data on the isomorphism in phosphates of NZP and Lb families performed in [65, 67] allows a conclusion that, in these systems, the empirical Goldschmidt and Sobolev–Povarennykh rules are met not in all cases. The difference in the radii of the cations simultaneously located in the framework can reach 68%. The charges of cations may differ by two units in voids and three units in the framework. The difference in the electronegativity of cations can be as high as 0.8 in voids and 0.6 in the framework. Cations with various electron configurations, s–s, s–d, and s–f can occupy identical positions. Cations of s, p, d, and f elements in different combinations may simultaneously be framework-forming cations [65]. The “design” of chemical compositions of compounds for the above-stated purposes the authors of this review performed on the basis of the previously calculated scheme [65, 68] of possible formula compositions of compounds with tetraedral–octahedral frameworks, using different combinations and ratios of A+, B2+, R3+, and M4+ cations responsible for the stabilization of the structure and the expected properties. This scheme with the selected formula compositions for carrying out studies in this work is shown in Fig. 4 [181]. In the choice of chemical compositions, we took into account that the compounds must be safe and biocompatible. Taking into account these properties, we proved compositions containing, first of all, sodium, potassium, magnesium, calcium, and phosphorus. For the deeper scientific understanding of the effect of compound composition on the studied functional characteristics of compounds, we also introduced atoms of chemical analogues into the structures. As activators we used manganese (Mn2+) and f elements (Eu2+, Eu3+, Sm3+, active centers in phosphates for LED technologies; Er3+, Yb3+ for bioimaging; and Eu3+, Sm3+ for X-PDT). In general, the chosen compositions of compounds contained the following cations: A+ = Na, K, Rb, Cs; B2+ = Mg, Ca, Sr, Mn, Eu2+; R3+ = Ln (Sm, Eu, Er, Yb); M4+ = Zr. In modeling the chemical formulas of compounds, we took into account both formal and crystal chemistry criteria. The chosen compounds formed series presented below. Compounds with the structure of NZP. Zirconium and alkali (Na, K, Rb, Cs) and alkaline-earth (Ca, Sr) element phosphates containing lanthanides (Eu3+, Eu2+, Sm3+, Er3+, Yb3 +) and Mn2+ 3+ A1 – 3xEuxZr2(PO4)3 (Eu ), A = Na, K, Rb, Cs; x = 0, 0.001, 0.01, 0.05, 0.1, 0.25, 0.33; 2+ Ca0.5 – xEuxZr2(PO4)3 (Eu ), x = 0, 0.001, 0.01, 0.05, 0.1, 0.2, 0.5; 2+ 3+ Ca0.2EuxSmyZr2(PO4)3 (Eu , Sm ), (x, y) = (0.067, 0.2); (0.133, 0.1); (0.167, 0.05); (0.2, 0); 2+ 2+ Ca0.5 – (x + y)MnxEuyZr2(PO4)3 (Eu , Mn ), (x, y) = (0.1, 0); (0.1, 0.2), (0.2, 0.1); 2+ Sr0.5 – xEuxZr2(PO4)3 (Eu ), x = 0, 0.1, 0.2; 3+ 3+ Ca0.5 – 1.5(x + y)ErxYbyZr2(PO4)3 (Er , Yb ), 0.02 ≤ x + y ≤ 0.33. Zirconium and calcium phosphate silicates containing lanthanides (Er3+, Yb3+)

Ca0.75 – 1.5(x + y)ErxYbyZr2(PO4)2.5(SiO4)0.5,0.1 ≤ x + y ≤ 0.4. Phosphates with the structure of langbeinite. Zirconium, magnesium, and potassium phosphates containing lanthanides (Sm3+, Er3+, Yb3+)

K2 – xSmxMg0.5 + xZr1.5 – x(PO4)3 (–4 ≤ n ≤ –2), 0 ≤ x ≤ 1;

K2Mg0.5 – 0.5(x + y)ErxYbyZr1.5 – 0.5(x + y)(PO4)3 (n = –2), 0 ≤ x ≤ 1; The following isomorphous substitutions were found in the series of the studied compounds: (1) isovalent in voids B2+ ↔ Eu2 + (B = Ca, Sr); (2) heterovalent in voids with the formation of cationic vacancies (h) A+ ↔ 1/3Eu3+ + 2/3 h (A = Na, K, Rb, Cs), 3Ca2+ ↔ Er3+ + Yb3+ + h; with interstitial cations 4 P5+ + h ↔ 5Si4+;

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Framework Framework charge cations (n) (T) CM Interstitial cations C R 0 3/2 1/2 C5/3B1/3 C7/4A1/4 CR C4/3B1/3 1 C3/2A1/2 AABB1/21/2 R1/31/3 MMMM C1/2R3/2 CB B C5/4A3/4 2 MR A2 M3/23/2 B1/2 M5/3A2/3 C2/3B4/3 CA 3 MB AB M4/3A2/3 RR C1/4B5/3 C3/4A5/4 4 M1/2B3/2 AR MA RB A, B, R, M, C are elements in oxidation states +1, +2, +3, +4, +5, respectively

NaZr2(PO4)3 structural type Langbeinite structural type

Fig. 4. Calculated formula compositions of compounds with NaZr2(PO4)3 and langbeinite structures.

with the substitution of cationic couples with equal total charges (coupled isomorphism) K+ + M4+ ↔ Sm3+ + Mg2+, M = Ti4+, Zr4+, 1/2Mg2+ + 1/2M4+ ↔ R3+ (R = Er3+, Yb3+).

3. SYNTHESIS 3.1. Brief review of Methods for the Preparation Orthophosphates of NZP and Lb Families The majority of anhydrous framework orthophosphates of NZP and Lb families are obtained by reac- tions proceeding in the solid phase and aqueous solutions (sol–gel and hydrothermal technologies) and, more rarely, in melts (alkali metal chlorides and nitrates, boron oxide). In the preparation of phosphates by the solid-phase method, the initial reagents are salts easily decom- posing on heating (mainly nitrates, carbonates), also chlorides or oxides of elements forming the cationic part of the compound, and ammonium hydrogen and dihydrogen phosphates. A mixture of stoichiometric amounts of the initial reagents is subjected to consecutive thermal treatment at a number of temperatures with intermediate dispersing [146, 182–184]. The drawback of the solid-phase method is the need in high calcination temperatures (T > 950°C) for the formation of the final monophase product and the careful dispersing of powders at each stage of heating. In recent years, much attention has been paid to the improvement of the sol–gel method of synthesis based on gel-formation in water sols [146, 185–188]. The method consists in mixing stoichiometric amounts of aqueous salt solutions of elements forming the cationic part with a phosphorus-containing reagent followed by the coagulation of the formed sol. This gives a gel of zirconium hydrogen phosphate according to the scheme:

⎯⎯⎯⎯HPO34→↓ ZrOCl242 Zr(HPO ) [189]. In the formation of colloidal particles of zirconium hydrogen phosphates, metal ions are uniformly distributed between the formed micelles, which is assisted by the continuous stirring of the solution.

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The obtained gel is dried at T = 90–100°C and subjected to consecutive heat treatment at a number of temperatures under the conditions of free air access. The scheme [190] of the formation of NaZr2(PO4)3 is presented on the following example: ⎯⎯⎯→T,C°° ⎯⎯⎯⎯⎯→ NaCl,700C Zr(HPO42 ) ZrP 2 O 7 NaZr 2 (PO 43 ) [190]. The advantages of solution technologies are in the simplicity of the preparation and control of the composition of solutions and the size and morphology of powder particles, in the achievement of high degree of homogeneity and purity of products, and in the reduction of the time and temperature of their synthesis in comparison with the conditions of interaction in the solid-phase. Except for the conditions of sol–gel of synthesis indicated above, which have become classical, there are various procedures to a certain degree replacing or supplementing some stages of the process: introduction of salting-out agents (for example, alcohols) [191, 192], organic complexants, and the Pechini method [193–196]. The Pechini method is based on the ability of tricarboxylic acids (in particular, citric acid) to form chelate complexes with many cations and to enter polycondensation (esterification) reac- tions with polyatomic alcohols (ethylene glycol) (Fig. 5) [193, 196]. The hydrothermal method allows the researchers to synthesize crystal phosphates at low T = 200– 350°C and often opens a possibility of obtaining well-shaped crystals suitable for X-ray diffraction analysis. Two versions of this method are known [197–202]. In the first version, the initial reagents (hydrox- ides, oxides, metal salts, phosphoric acid) in certain molar ratios are mixed as aqueous solutions and stored at 200–350°C in fluoroplastic-lined autoclaves for a certain time. An important factor for obtaining phases of a specified composition is the initial pH of the solution (1 < pH < 2). In the second version, stoichiometric amounts of aqueous metal salt solutions are mixed at room temperature, and then a phosphoric acid solution is added. The obtained gel is dried at 80°C, filled with water again, and kept under hydrothermal conditions at T = 200–350°C. This method of synthesis has a number of shortcomings. It can hardly or even cannot give phosphates of complex composition. In addition, the conditions of preparation strongly affect the possibility of formation of the target product. Therefore, the application of the hydrothermal method requires substantial time and resources for revealing the specific features of the processes of the formation of solid products, i.e., the choice of the optimum reagent ratio, temperature, and pressure.

3.2. Synthesis Methods Used in the Synthesis of Phosphors In this section and below we present a summary of data on the synthesis of phosphors collected by the authors. In general, for the preparation phosphor materials, we widely use the sol–gel method, including the version with the introduction of additional organic reagents into the reaction system, as was noted above. In the synthesis version with a salting-out agent (Fig. 5, Scheme 1), we used stoichiometric ratios for the initial solutions of salts and initial solutions of metal salts and precipitants. The obtained gel was heated at 90°C for dehydration. The dry residue was dispersed in an agate mortar (till 30 min) and then consecutively heated at 600, 800, and 900°C. In some experiments, temperature was raised to 1100°C. After each step of isothermal storage, the samples also carefully dispersed in ethyl alcohol (till 30 min). In the version of the Pechini citrate method (Fig. 5, Scheme 2), initial salt solutions were mixed with citric acid (CA) in a stoichiometric ratio, and then a 1 M NH4H2PO4 solution and ethylene glycol (EG) were added dropwise to the obtained mixture under continuous stirring. The molar ratio of CA to the n metal was CA : M = 15 : 1, where M = ∑ ν (Met) , and the ratio CA : EG was 1 : 4. The obtained gel i=1 i was heated to 130–350°C to remove water; the formed dry residue was dispersed in an agate mortar within 30 min and then successively heated to 600–1100°C at a step of 100 K. As phosphors must meet the requirements of microstructure uniformity and optical “purity,” the con- formity with these requirements must be taken into account in the course of synthesis. Uniform powder material can be introduced at the synthesis stage. For example, the effects of the temperature of gel formation and ultrasonic treatment on the microstructure of Ca0.5–1.5(x + y)ErxYbyZr2(PO4)3 phosphates, 0.02 ≤ x + y ≤ 0.33 were studied in [203]. The size distribution of particles found for phosphate samples was close to the normal distribution and depended on the synthesis conditions (Table 2). Tem- perature increase from 0 to 80°C at the stage of gel formation led to an increase in the average size of crys- tallites, while the use of dispersing between the stages of heating and ultrasonic treatment favored their reduction. The minimum average size of particles, 40 ± 10 nm (Tgel form = 20°C using dispersing and ultra-

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Scheme 1 Scheme 2

Addition of Addition of precipitant NH4H2PO4 and HOCH2CH2OH solutions

Mixture of salt solutions + Mixture of salt solutions + (HOOCCH ) C(OH)COOH C2H5OH 2 2

Gel formation, stirring, Complex formation, stirring, T = 20°C T = 80°C

T = 90°C, τ = 5−20 h T = 90−350°C, τ = 20 h

Dispersing in C2H5OH, 30 min

Control by X-ray T = 90−350°C, τ = 20 h powder diffraction

Fig. 5. Schemes of colloidal chemical synthesis of powders.

sound) and their maximum size, 110 ± 50 nm (Tgel form = 80°C, without dispersing) differed more than 2.5-fold (Table 2). To meet the second requirement in the preparation substances, one should completely exclude their contact with other substances containing activators. In [181], powder precursors containing europium 2+ were subjected to preliminary dehydration and decontamination at T = 400°C for 20 h. Then they were placed in a quartz container, which was placed in a quartz tube and heated in situ in a tubular furnace in a reducing argon–hydrogen atmosphere (Ar + 5% H2) at T = 800°C for 3–20 h. The qualitative analysis of substances for the presence of Eu2+ and Eu3+ cations was performed by their irradiation with an UV Wood lamp. Under UV irradiation, the samples containing Eu3+ emitted pink light and those containing Eu2+, blue light. Samarium-containing phosphates had red–pink colors of different intensity. After the experiment, samples were stored in the atmosphere of Ar + 5% H2 in vials with paraffinized stoppers.

4. FEATURES OF PHASE FORMATION IN SYSTEMS OF LANTHANIDE-CONTAINING PHOSPHATES WITH NZP AND LANGBEINITE STRUCTURES 4.1. Phase Formation The obtained substances were fine white powders. Erbium-containing phosphates were colored in light pink or pink, depending on the concentration of erbium in the samples. The temperature conditions of synthesis were chosen based on the data of differential thermal analysis (DTA) and X-ray powder diffraction on an example of batch mixtures in the synthesis of Na0.25Eu0.25Zr2(PO4)3, Ca0.3Eu0.2Zr2(PO4)3 phosphates with the expected structure of NZP and of K1.5Sm0.5MgZr(PO4)3 phosphate with the expected structure of Lb. For representatives of both structural types, irreversible processes accompanied by thermal effects at T = 90–160°, 230–260°, 610–730°C were found in the temperature range 20–900°C. These effects can be due to the removal of crystallization water, ethanol, and gaseous products of reaction and chemical interaction, respectively [181]. The conditions of formation of highly crystalline phases (temperature, duration of heat treatment) were chosen and optimized in special experiments and were as follows: T = 800–900°C, τ = 20 h in air, τ = 3 – 20 h in an atmosphere of Ar + 5% H2. The chemical composition and agreement with the calculated composition were confirmed by energy- dispersive X-ray spectroscopy (EDX). X-ray diffraction studies. X-ray diffraction patterns of samples were characterized by reflexes typical for compounds of NZP (hexagonal system) and Lb (cubic system) structural families, no asymmetry of dif-

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Table 2. AFM data. Samples of Ca0.35Er0.05Yb0.05Zr2(PO4)3 obtained under different conditions Average particle size, nm Tgel, °C without dispersing with dispersing 0 90 ± 30 60 ± 20 20 100 ± 60 50 ± 20 20 (USD) 90 ± 10 40 ± 10 80 110 ± 50 90 ± 20 USD is ultrasonic dispersing. fraction line profiles and diffusion halo of amorphous phases and, in general, pointed to the preparation the monophase products. As examples, Fig. 6 presents X-ray diffraction patterns of phosphates from different space groups. X-ray diffraction patterns were indexed by comparison with the known analogues for different structural types and space groups:

(1) phosphates with the structure of NZP analogues of NaZr2(PO4)3 (space group Rc3 ) [136],

KZr2(PO4)3 (space group Rc3 ) [204, 205], RbZr2(PO4)3 (space group Rc3 ) [206], CsZr2(PO4)3 (space group Rc3 ) [207], Eu0.33Zr2(PO4)3 (space group Pc3 ) [154], Eu0.5Zr2(PO4)3 (space group R3) [208],

Ca0.5Zr2(PO4)3 (space group R3) [209, 210];

(2) phosphates with the structure of Lb analogues of K2Mg2(SO4)3 [137, 165], K2YZr(PO4)3 [175], K2Mg0.5Zr1.5(PO4)3 [211], (space group P213). The analysis of X-ray diffraction patterns of phosphates with the structure of NZP revealed a series of morphotropic transitions due to the ordering of rare-earth element cations of in the structure positions. Thus, in the series A1–3xEuxZr2(PO4)3 (A = Na, K, Rb) phosphates at x ∈ (0; 0.1) crystallized in the space group Rc3 , while representatives of the same series with higher concentrations of europium (x = 0.25) were superstructures of the space group Pc3 . Morphotropic transitions also occurred in the series

B0.5 ‒ xEuxZr2(PO4)3 (B = Ca, Sr): R3→ R3 at x = 0.2; Ca0.5–1.5(x + y)ErxYbyZr2(PO4)3: R3 → Pc3 at x = 0.2 (Table 3). For phosphates with the structure of Lb, the morphotropism phenomenon was not observed. Limited isomorphous mixibility was noted for representatives of both structural families, namely, for the series Cs1 – 3xEuxZr2(PO4)3 (NZP) and K2 – xSmxMg0.5 + xZr1.5 – x(PO4)3 (Lb). For the series Cs1–3xEuxZr2(PO4)3 with cesium, it was found that solid solutions occurred in the range 0 ≤ x ≤ 0.1 [212]. A sample of Cs0.25Eu0.25Zr2(PO4)3 was a mixture of phases, which was confirmed in repeated experiments. The formation of monophase products in the series K2 – xSmxMg0.5 + xZr1.5 – x(PO4)3 was limited by the compositions 0 ≤ x ≤ 0.5 [213] (Table 3). The parameters of unit cells for all of the obtained phosphates were calculated by the results of index- ation of X-ray diffraction patterns by the least-squares technique (Table 3); they changed with the growth of x. In the series of phases with the structure of NZP, parameters c and V increased and parameter a decreased; in the series of phases of the Lb family, parameters a and V increased. n− The considered isomorphism illustrates an important property of {T243 (XO ) } frameworks, their “elasticity” at high strengths, which determined a wide range of possible substitutions in different groups of cations without the rupture or cardinal reorganization of the initial motif.

Data of IR spectral analysis. For A1 – 3xEuxZr2(PO4)3 systems (A = Na, K, Rb, Cs), two types of IR spectra were observed depending on the concentration of europium (x) [212]; their typical views are presented in Fig. 7 as examples. Factor group analysis for AZr2(PO4)3 phosphates (A = Na, K, Rb, Cs) with 3− ν the space group R3c predicts six active regions of valence vibrations of PO4 groups in IR spectra: 1 – Eu; ν3 – 2A2u + 3Eu and 7 bending vibrations: ν2 – 2Eu; ν3 – 2A2u + 3Eu [214, 215]. The ordered distribution of lanthanide atoms, in particular, of europium, in M1 positions of the NaZr2(PO4)3 structure results in the reduction of symmetry to the space group Pc3 [154]. The results of factor group analysis for the com- ν 3− pounds Ln0.33Zr2(PO4)3 where Ln = Ce–Lu indicated 14 active IR modes of 3 vibrations of PO4 groups:

REVIEW JOURNAL OF CHEMISTRY Vol. 8 No. 1 2018 PHOSPHORS BASED ON PHOSPHATES 15 310 321 311 320 211 111 210 331 432 221 422 431 410 332 a 110 113 116 104 024 012 300 226 214 128 119 208 306 134 018 b 110 , rel. units , rel. 116 113 I 217 104 024 300 221 012 401 214 128 134 208 119 018 306 003 101 112 004 016 114 c 110 113 104 116 024 300 012 214 226 128 134 101 306 003 02.10 015 119 208 018 d

10 15 20 25 30 35 40 45 50 2θ, deg

Fig. 6. X-ray diffraction data of (a) K1.5Sm0.5MgZr(PO4)3, phosphates, Lb type, cubic system, space group P213; (b)Na0.7Eu0.1Zr2(PO4)3, NZP type, trigonal system, space group Rc3 ; (c) Na0.25Eu0.25Zr2(PO4)3, NZP type, trigon. system, space group Pc3 ; (d) Ca0.3Eu0.2Zr2(PO4)3, NZP type, trigon. system, space group R3.

5A2u + 9Eu; and 4 vibrational ν1 modes: A2u + 3Eu [216]. The IR spectra of Na0.25Eu0.25Zr2(PO4)3 phosphates (as of analogous potassium and rubidium phosphates) were similar to the spectra of

Ln0.33Zr2(PO4)3 and can also be assigned to space group Pc3 . Note that alkali metal atoms can occupy not only M1 but also/or M2 positions of the NZP structure, or reaction products can be mixtures of compounds with space groups Rc3 and Pc3 . In this case, vibrational bands of the rhombohedral phase will overlap with vibrational bands of compound with the space group Pc3 . Based on the fact that the band of interaction P–O/Zr–O at 1205 cm–1, characteristic for IR spectra of phosphates of the space group Rc3 , was not observed in IR spectra of A0.25Eu0.25Zr2(PO4)3 phosphates, we can suppose that the obtained products are NZP phases in which alkali metal cations occupy M2 positions. 3+ 3+ Similar data were obtained for the morphotropic series Ca0.5 – 1.5(x + y)ErxYbyZr2(PO4)3 (Er , Yb ) [181, 203]. In general, the data of IR spectroscopy confirmed the functional composition of phosphates, and pointed to their crystallization in space groups Rc3 , R3, Pc3 (NZP phosphates), and P213 (Lb phosphates) and also to the absence of X-ray amorphous impurities in the samples.

4.2. Structural Data To reveal specific features of the structures of the synthesized and characterized compounds, and also to study the distribution of cations between the structural positions, structural studies for some compounds were performed by methods of full-profile analysis by the data of X-ray powder diffraction (Riet- veld method) and EXAFS. Rietveld method. Crystal structures of two representatives of the NZP family containing europium, i.e., Na0.7Eu0.1Zr2(PO4)3 and Na0.25Eu0.25Zr2(PO4)3, were studied as an example. These phosphates are mem- bers of the same series, in which the ratio of Na to Eu was changed, as a result of which, a morphotropic transition with a change of space group from Rc3 to Pc3 was observed. As basic models for the refinement

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Table 3. Crystallographic data for compounds Space Compound series x a, Å c, Å V, Å3 group Phosphates with the structure of NZP ≤ ≤ Na1 – 3xEuxZr2(PO4)3 0 x 0.1 Rс3 8.757(5)– 22.832(2)– 1522(1)– 0.25 ≤ x ≤ 0.33 8.819(1) 23.00(2) 1542(2) Pс3 K1 – 3xEuxZr2(PO4)3 8.710(1)– 23.34(7)– 1534(2)– 8.739(8) 24.04(4) 1588(2)

Rb1 – 3xEuxZr2(PO4)3 8.661(2)– 23.76(4)– 1554(1)– 8.692(4) 24.48(2) 1594(1) ≤ ≤ Cs1 – 3xEuxZr2(PO4)3 0 x 0.1 Rс3 8.580(4)– 24.91(2)– 1588(2)– 8.604(9) 25.02(7) 1604(3) ≤ ≤ Ca0.5 – xEuxZr2(PO4)3 0 x 0.2 R3 8.696(5)– 22.767(3)– 1517(1)– 0.2 ≤ x ≤ 0.5 R3 8.772(2) 23.418(2) 1534(1) Sr0.5 – xEuxZr2(PO4)3 8.681(4)– 23.352(3)– 1529(1)– 8.694(3) 26.281(5) 1715(1) ≤ ≤ Ca0.2EuxSmyZr2(PO4)3 0 x 0.2 R3 8.737(1)– 22.42(2)– 1499(2)– 0.067 ≤ y ≤ 0.2 8.786(7) 22.97(6) 1519(8) ≤ ≤ Ca0.2EuxMnyZr2(PO4)3 0.1 x 0.2 R3 8.811(6)– 22.87(4)– 1540(1)– 0.1 ≤ y ≤ 0.2 8.832(6) 22.91(6) 1545(1) ≤ ≤ Ca0.5 – 1.5(x + y)ErxYbyZr2(PO4)3 0 x 0.2 R3 8.770(5)– 22.48(3)– 1513(0)– 0.2 ≤ x ≤ 0.33 8.840(5) 22.783(4) 1525(1) Pс3 ≤ ≤ Ca0.75 – 1.5(x + y)ErxYbyZr2(PO4)2.5(SiO4)0.5 0.02 х + y 0.4 R3 8.773(5)– 22.70(2)– 1512(2)– 8.791(12) 22.76(4) 1523(3) Phosphates with the structure of Lb

K2 – xSmxMg0.5 + xZr1.5 – x(PO4)3 0 ≤ x ≤ 0.5 P213 10.259(9)– – 1079(7)– 10.325(2) 1100(2)

K2Mg0.5 – 0.5(x + y)ErxYbyZr1.5 – 0.5(x + y)(PO4)3 0 ≤ x ≤ 1 P213 10.161(7)– – 1049(6)– 10.320(5) 1099(6)

of structures of the above phosphates, coordinates of atoms of NaZr2(PO4)3 (space group Rc3 ) [136] and

Eu0.33Zr2(PO4)3 (space group Pc3 ) [154] phosphates, respectively, were used. More complete crystallographic information, including the found coordinates, isotropic thermal parameters of atoms, and bond lengths and bond angles was presented in [181, 212]. The obtained data indicate that the crystal structures of Na0.7Eu0.1Zr2(PO4)3 and Na0.25Eu0.25Zr2(PO4)3 phosphates are isotypic. They formed by topological invariants [Zr2(PO4)3], which consisted of isolated Zr octahedra and PO4 tetrahedra bound via common vertexes. Each two Zr octahedra were connected with three PO4 tetrahedra. In the structure of the Na0.7Eu0.1Zr2(PO4)3 phosphate, the bond length in tetrahedra and octahedra varies in narrow ranges: 1.53(5)–1.55(5) Å (P–O) and 2.01(4)–2.06(4) Å (Zr–O). The bond angles O–P–O and O–Zr–O also corresponded to tetrahedral and octahedral coordination, respectively [212]. In the structure of the Na0.25Eu0.25Zr2(PO4)3 phosphate, the framework-forming polyhedra were characterized by a wider dis- persion of interatomic distances: 1.92(2)–2.22(3) Å (Zr–O) and 1.50(3)–1.60(3) Å (P–O). Sodium and europium atoms occupied extra-framework positions. Thus, in the structure of the Na0.7Eu0.1Zr2(PO4)3 phosphate (space group Rc3 ), these atoms occupied M1 positions of the symmetry 6b, and, in the

Na0.25Eu0.25Zr2(PO4)3 phosphate (space group Pc3 ), they were distributed between two positions, M1 and M2, of the symmetry 2b and 6f, respectively [212].

EXAFS method. Phosphates of the series Ca0.5 – xEuxZr2(PO4)3 were investigated by the EXAFS method [217]. An approximation of an EXAFS spectrum was obtained based on the data on the 2+ 2+ Ca0.5Zr2(PO4)3 structure [209]. It was taken into account that Eu substitutes for Ca , forming a Eu–O bond in the first coordination sphere and a Eu–Zr bond in the second coordination sphere. Figure 8 pres-

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(a) (b) 578 866 556 573 640

423 1 648

1 1205 1210 574 553 463 575 1114 1112 640 646 1063 1018 1017 1032 424 2 1030 2 1205 1207 577 555 466 517 553 644 1103 573 1021 1017 1103 1028 639 1065 939

1042 3 1204 947 424

Transmittance, % Transmittance, 3 Transmittance, % Transmittance, 1207 582 1097 583 1064 1015 1033 554 1139 565 554 640 640 428 428 1045 1233 567 1227 4 4 938 973 970 946 991 1131 1133 1025 1092 1049 1024 1066 1079 1058

1400 1200 1000 800 600 400 1400 1200 1000 800 600 400 ν, cm−1 ν, cm−1

Fig. 7. IR spectra of A1–3xEuxZr2(PO4)3 phosphates, where A = (a) Na, (b) K; x = 0 (1), 0.05 (2), 0.1 (3), space group Rc3 ; 0.25 (4) space group Pc3 .

2 ents an example of a k -weighed LIII absorption edge of europium and average interatomic distances Eu–O for Ca0.5 – xEuxZr2(PO4)3. According to the results of EXAFS, with the growth of europium concentration, the bond lengths in M1 polyhedra increased because of the larger ionic radius of Eu2+ in comparison with that of Ca2+ (1.17 and 1.00 Å according to Shannon [218], respectively). This agrees with crystallographic parameters found by us and those presented above (an increase in cell volume with the growth of x), and also with the results of work [219], where the structure was refined by the Rietveld method. Summarizing the experimental data presented in Sections 3 and 4, let us note that we have demonstrated the application of crystal chemistry principles to the formation of crystal materials with the expected NZP and Lb structures. All of the predicted compounds with the expected structures were synthesized. The analysis of their composition and structure validated the chosen approach. The majority of individual compounds and solid solutions were obtained for the first time. A possibility of the formation of substitution solid solutions shown in this work for a series of framework phosphates points to the stability of the total structural motif of the framework octahedral–tetrahedral construction and opens ways to the control of useful properties of these substances in the design of monophase phosphate materials.

5. LUMINESCENT PROPERTIES The luminescent properties of the obtained phosphates were studied within three tasks stated above: LED technologies, intracellular bioimaging, and X-PDT of oncological diseases.

5.1. Phosphate Phosphors for LED Technologies 3+ 3+ AZr2(PO4)3:Eu system (A = Na, K, Rb, Cs). Eu luminescence in phosphates of the series A1 – 3xEuxZr2(PO4)3, A = Na, K, Rb, Cs, 0.001 ≤ x ≤ 0.25 at λexcit = 210 nm was observed in the regions λem = 370–410 nm and λem = 580–620 nm [212]. The optical properties of europium in the oxidation state +3 are due to 4f–4f transitions. According to the Laporte rule, these transitions are forbidden, but, because of vibrations of the polyhedron, substantial peaks of these transitions can be observed even if their intensities are relatively diffuse. According to the data presented in [220], the 4f–4f transitions of europium (3+) in the emission mode in the visible spectral region consist of the following components:

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0.6 (a)2.8 (b) k2

2.7 ) k ( O, Å χ

0 − 2 k Eu 2.6

−0.6 2.5 0 2 4 6 8 10 12 14 0 0.1 0.2 0.3 0.4 0.5 k, Å−1 x

Fig. 8. Data of EXAFS spectroscopy of Ca0.5 – xEuxZr2(PO4)3 phosphates: (a) normalized EXAFS function extracted from the experimental absorption spectrum and the result of its approximation; (b) interatomic distance Eu−O as a function of Eu concentration.

5 7 (1) D0 → F0 at apprx. 570–580 nm. This transition is singlet, which means that peak intensity is directly related to the number of crystallographic positions occupied by Eu3+; 5 7 (2) D0 → F1 at apprx. 585–600 nm. This transition belongs to a magnetic dipole. The number of the peaks relating to one crystallographic position is equal to three. Splitting is proportional to crystal field strength. This transition is due to the localization of europium in an environment with centers of inver- sion; 5 7 (3) D0 → F2, also named “hypersensitive transition,” at apprx. 600–625 nm, relates to an electric dipole. This transition is due to the localization of europium in an environment without a center of inver-

I 57− sion. The ratio DF02 is used to characterize the centrosymmetric character of europium position. I 57 DF01− Figures 9a–9d present emission spectra of phosphates of the series with sodium, Na1 – 3xEuxZr2(PO4)3, x = 0.001, 0.01, 0.1, 0.25 [212]. The intensity of bands in the more short-wavelength region increased with the growth of x and reached the maximum value at x = 0.25. The spectra also contained wide low-intensity bands at apprx. 400–450 nm, pointing to the presence of trace amounts of Eu2+. The emission bands corresponded to the expected ones. Such changes in the spectra can be due to the transition of europium from an M1 to an M2 position in the structure with CN values of 6 and 8, respectively (Fig. 3). This transition results in the weakening of the crystal field and the shift of emission to the short-wavelength region. For systems with potassium and rubidium, changes in the spectra were similar (Figs. 9e–9g) [212]. 2+ B0.5Zr2(PO4)3:Eu system (B = Ca, Sr). For compounds of the series B0.5 – xEuxZr2(PO4)3, B = Ca, Sr, x = 0.001, 0.01, 0.05, 0.1, 0.2, 0.5, photoluminescence was measured on excitation by laser sources with λ = 350 and 400 nm (UV and blue regions, respectively). Figure 10 presents normalized emission spectra of phosphates [217]. They are characterized by wide excitation bands in the region 250–420 nm and emission in the region 400–700 nm with wide shoulders in the long-wavelength region and correspond to europium transitions from the excited state 4f 65d1 to the ground state 4f 7. Low intensity of emission at 610 nm, present in some spectra, is due to europium in the oxidation state +3 (trace amounts after storage of samples in the reducing atmosphere).

The analysis of emission spectra of phosphates of the series B0.5 – xEuxZr2(PO4)3, B = Ca, Sr, 0.001 ≤ x ≤ 0.5 at different x (λexcit = 350, 400 nm) indicates that the intensity of emission increased with the growth of x in range 0.001 ≤ x ≤ 0.2 and decreased at x ≥ 0.2. The last effect may be due to concentration quenching. With the growth of x, the bands shifted to the region of higher energies because of the reduction of crystal field strength on the replacement of the Ca2+ cation by the larger Eu2+ cation, as in the sub- 2+ 2 + stitution of Ca by Sr (2+ = 1.00 Å, 2+ = 1.17 Å, 2+ = 1.18 Å, CN 6, according to Shannon [218]). rCa rEu rSr The shape of spectra also indicates that emission is not characterized by only one band. As a result of the mathematical processing of spectra, it was found that each emission band was presented by two Gauss- ians. The spectrum of the Ca0.3Eu0.2Zr2(PO4)3 phosphate as an expansion of an emission band into two

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(a) (b) 5 7 60000 60000 D0− F2 50000 Eu2+ 50000 λem = 610 nm λ 40000 em = 592 nm 40000 λ λexc = 210 nm λem = 610 nm exc = 210 nm 30000 30000 Eu3+ 5 7 D0− F1

Intensity, rel.u. Intensity, 20000 rel.u. Intensity, 20000 Eu2+ Eu2+ 10000 10000 5 7 D0− F0 0 0 200 300 400 500 600 700 200 300 400 500 600 700 Wavelength, nm Wavelength, nm (c) (d) 1200000 5 7 100000 D0− F2 1000000 λ 80000 λexc = 210 nm em = 588 nm λem = 584 nm 800000 λem = 592 nm λ = 592 nm λ em em = 610 nm 5 7 60000 D0− F1 600000 λexc = 230 nm λ 40000 exc = 300 nm

400000 rel.u. Intensity,

Intensity, rel.u. Intensity, 5 7 D0− F2 5 7 20000 D0− F1 200000 5 7 D0− F0 5 7 D0− F0 0 0 200 300 400 500 600 700 200 300 400 500 600 700 Wavelength, nm Wavelength, nm (e) (f) (g) 100000 100000 100000 Rb0.7Eu0.1Zr2(PO4)3 C0.85Eu0.05Zr2(PO4)3 K0.25Eu0.25Zr2(PO4)3 80000 80000 80000 5 7 D0− F2 60000 60000 60000 λ Rb Eu Zr (PO ) λexc = 208 nm em = 585 nm 0.97 0.01 2 4 3 λ λem = 585 nm 5 7 λexc = 213 nm λ K0.97Eu0.01Zr2(PO4)3 λ exc = 225 nm D0− F1 40000 em = 592 nm 40000 λ em = 585 nm 40000 λem = 591 nm λexc = 230 nm exc = 218 nm λ = 590 nm λem = 610 nm λ em λem = 610 nm exc = 225 nm λem = 610 nm 5 7 20000 20000 20000 D0− F0 0 0 0 200 300 400 500 600 700 200 300 400 500 600 700 200 300 400 500 600 700 Wavelength, nm Wavelength, nm Wavelength, nm

Fig. 9. Photoluminescence spectra of phosphates of the series Na1 – 3xEuxZr2(PO4)3, x = (а) 0.001, 0.01 (b), 0.1 (c), 0.25 (d) and A1 – 3xEuxZr2(PO4)3 phosphates, where A = K (e), Rb (f), Cs (g).

Gaussians, high-energy g2 (Fig. 11, curve 3) and low-energy g1 (Fig. 11, curve 4), is shown in Fig. 11 as an example. As the smaller Stokes shift is characteristic for crystallographic positions of a larger volume, one can suppose that the g2 component corresponds to europium occupying the M2 position (octagon, CN 8), and the g1 component, to the M1 position (trigonal antiprism, CN 6) (Fig. 3) [217]. The contribution of the high-energy Gaussian increased with the growth of x. Its intensity increased, which points to an increase in the percentage of occupied M2 positions (Fig. 12a). Because of the growth of population of some or other position with an increase in x of the cation with the larger radius (europium), emission must shift to the short-wavelength region (for both components g1 and g2), as was found from the obtained experimental data (Fig. 12b). For the deeper understanding and description of Gaussian components, Glorieux et al. investigated photoluminescence of the Ca0.3Eu0.2Zr2(PO4)3 phosphate at 80 K [217]. The spectra at 80 K and room temperature were similar. As one would expect, the width of emission band decreased with decreasing temperature. At 80 K, the width of the Gaussian decreased, which was due to the weakening of the vibra- tion processes. The decrease in temperature favored the strengthening of high-energy components, which, probably, reflects the weakening of the channel of nonradiative relaxation between the cations, whereas configuration quenching did not play an important role. The observed very weak red shift was due to the contraction of the lattice at low temperatures, and, therefore, the strengthening of the crystal field of the environment of the europium cation because of the reduction of the Eu–O bond length. This strengthening led to the splitting of the 5d level [221, 222] and, as a result, to the lowering of the level of the excited state responsible for the red shift. With an increase in excitation wavelength, a small red shift due to a transition to a low-energy state was observed.

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I 1.0

1 0.5 6 7 2 3 4 5 0 350 400 450 500 550 600 650 700 λ, nm

Fig. 10. Normalized emission spectra of phosphates of the series Ca0.5 – xEuxZr2(PO4)3, x = (1) 0.001, (2) 0.01, (3) 0.05, (4) 0.1, (5) 0.2, (6) 0.5, and (7) Sr0.3Eu0.2Zr2(PO4)3.

The process of luminescence decay was described by two components: the first one with the longer (>71 ns) and the second one with the shorted (from 16 to 39 ns) decay time [217]. With the growth of emission wavelength, the values of τ1 and τ2 decreased; their ratio τ1/τ2 increased. The intensity of the short- wavelength components in this case decreased and that of the long-wavelength component increased. A similar effect was observed with an increase in excitation wavelength at a constant emission wavelength. Therefore, the short-lived component can be assigned to low-energy emission. However, both decay processes always appeared simultaneously in all experiments; therefore, it was impossible to accurately extract one process. It was only clear that each process was not associated with only one crystallographic position. In this case, as was also noted above, we could distinguish environments of two types: the first of them determined high-energy emission with the long lifetime and the second, low-energy emission with the short lifetime because of the influence of the process of nonradiative relaxation. The calculated ICE (International Commission on Energy) color coordinates for all of the studied compounds B0.5 – xEuxZr2(PO4)3 (B = Ca, Sr) changed almost linearly from {0.32; 0.41} to {0.18; 0.17} with an increase in the amount of europium irrespective of the excitation wavelength [217]. To search for the compositions of phosphates within the studied family with color coordinates most close to standard NTSC (National Television Standard Committee) coordinates of white luminescence

I λexcit = 350 nm λem = 480 nm

2 1 5

250 300 350 400 450 500 550 600 650 700 λ, nm

Fig. 11. Photoluminescence of Ca0.3Eu0.2Zr2(PO4)3 phosphate: (1), excitation spectrum, λexcit = 350 nm; (2), photoluminescence spectrum, λem = 480 nm; (3) and (4), Gaussian components (g1 and g2, respectively); (5), total component.

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1.00 600 (a) (b)

0.75 550 rel g1 I

0.50 , nm

g em 2 λ 500 g2 0.25

g1 450 0 0.1 0.2 0.3 0.4 0.5 0 0.1 0.2 0.3 0.4 0.5 xx

Fig. 12. Dependences of (a) relative intensity and (b) emission wavelengths on composition x for phosphates of the series Ca0.5 – xEuxZr2(PO4)3.

{0.33; 0.33}, we studied compounds simultaneously containing europium and manganese and europium and samarium [181, 217]. 2+ 2+ Ca0.5Zr2(PO4)3:Eu , Mn system. For the phosphate system containing manganese and europium Ca0.5 – (x + y)MnxEuyZr2(PO4)3 (x, y) = (0.1, 0); (0.1, 0.2); (0.2, 0.1), emission was observed in the blue spectral region (λem. = 475 nm) on excitation with an UV source (λexcit = 350 nm). The Ca0.4Mn0.1Zr2(PO4)3 phosphate was characterized by red luminescence of low intensity. However, at the simultaneous presence of Mn2+ and Eu2+, a sharp increase in emission intensity was noted in comparison with the emission of the sample containing no manganese (Fig. 13) [223]. 2+ 3+ Ca0.5Zr2(PO4)3:Eu , Sm system. For the phosphate system simultaneously containing europium and samarium, CaxSmyEuzZr2(PO4)3 (x, y) = (0.067, 0.2); (0.133, 0.1); (0.167, 0.05); (0.2, 0), photoluminescence spectra were registered at λexcit = 400 nm (Fig. 14), which corresponds to the most sensitive tran- 3+ 6 4 sition of Sm , H5/2 → K11/2, and also the wavelength of an UV excitation source InxGa1 – xN [8]. The 2+ observed emission bands were characteristic for these cations, λem. = 400–700 nm (Eu ) and λem = 550– 670 nm (Sm3+), and were also due to 4f–4f transitions (Fig. 14). The best color coordinates (close to white light) were obtained for the Ca0.2Sm0.133Eu0.1Zr2(PO4)3 phosphate at λexcit = 400 nm. They were {0.27, 0.34} [217]. The data on luminescent properties for phosphates with NZP structure, known from the recent review are presented in Table 4. Among these phosphates, special attention should be paid to the series 2+ B0.5Zr2(PO4)3:Eu (B = Ca, Sr, Ba) [219]. At the isomorphous transition Ca → Sr → Ba, a shift of emission maxima to the left was observed, which was also due to the influence of the crystal environment on the activator cation.

5.2. Phosphate Phosphors for Bioimaging Erbium and ytterbium phosphates and phosphates silicates with NZP and Lb structures of the types Ca0.5 – 1.5(x + y)ErxYbyZr2(PO4)3, Ca0.75 – 1.5(x + y)ErxYbyZr2(SiO4)0.5(PO4)2.5, and K2Mg0.5 – 0.5(x + y)ErxYbyZr1.5 – 0.5(x + y)(PO4)3 were studied as phosphors for bioimaging [203, 231]. Erbium and ytterbium in the composition of the studied compounds were present in different concentrations and ratios. The spectra of compounds of both structural families exhibited two emission regions, in the visible and near infrared (NIR) spectral regions on excitation with an IR source, λexcit = 977 nm. Luminescence spectra were identical for all compounds. As an example, Fig. 15 presents spectra for Ca0.5 – 1.5(x + y)ErxYbyZr2(PO4)3, 0.02 ≤ x + y ≤ 0.33; x : y = 1 : 1 (NZP) and K2Mg0.5 – 0.5(x + y)ErxYbyZr1.5 – 0.5(x + y)(PO4)3, 0.25 ≤ x + y ≤ 1.0, x : y = 1 : 9 (Lb) phosphates. The spectra in the visible region (Fig. 15) demonstrated two bands at the wavelength apprx. 525 and apprx. 625 nm. The first band had a higher intensity. The positions and shapes of these bands differed from the known ones for the case of anticipated upconversion of Er3+.

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I Excitation Emission λ = 350 nm λ = 475 nm

3 3

2 2

1 1

250 300 350 400 450 500 550 600 650 700 λ, nm

Fig. 13. Luminescence excitation and luminescence spectra of phosphates (1) Ca0.4Mn0.1Zr2(PO4)3, (2) Ca0.2Mn0.2Eu0.1Zr2(PO4)3, and (3) Ca0.3Eu0.2Zr2(PO4)3.

For the Er0.165Yb0.165Zr2(PO4)3 phosphate, a shift of emission maximum to the region of shorter waves was observed (Fig. 15), with was also due to a morphotropic transition in the series Ca0.5 – 1.5(x + y)ErxYbyZr2(PO4)3 (similarly to the cases described above).

The highest emission intensity was observed for compounds of the composition NaZr2(PO4)3 at x : y = 1 : 4. The spectra in the NIR region (Fig. 15) exhibited a luminescence band at 1550 nm, characteristic for 3+ 4 4 Er and corresponding to the transition I13/2 → I15/2, and a set of narrow bands due to the Stark splitting 4 of the I15/2 level in this transition. Lanthanide concentration x + y affected luminescence intensity; however, no monotonous dependence was observed.

The highest intensity was obtained for phosphates of the series Ca0.5 – 1.5(x + y)ErxYbyZr2(PO4)3 at x : y = 1 : 10 for all x + y values.

I 1 Excitation 2 3 4 5 9/2 H 6 7/2

Emission H 6 →

5/2 → 5/2

G H 4 5/2 6 G 4 →

5/2 G 4

250 300 350 400 450 500 550 600 650 700 λ, nm

Fig. 14. Photoluminescence of CaxSmyEuzZr2(PO4)3 phosphates; x, y, z = (1) 0.2, 0.2, 0; (2) 0.2, 0.167, 0.05; (3) 0.2, 0.133, 0.1; (4) 0.2, 0.067, 0.2; (5) 0.3, 0, 0.2.

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Table 4. Positions of maxima (or range) of luminescence bands in the visible region and spectra of luminescence excitation for the known phosphate phosphors with the structure of NZP Positions of band maxima, nm Phosphor Space group Reference excitation emission

3+ Na3Al2(PO4)3:Eu Rc3 243 593; 615 [224] 2+ 288 515 Na3Al2(PO4)3:Mn Mn0.5Zr2(PO4)3 R3 400 550−615 [225]

Eu0.5Zr2(PO4)3 R3 254 460 [226] 2+ Ca0.5Zr2(PO4)3:Eu R3 350 484 [219] Sr Zr (PO ) :Eu2+ 341 459 0.5 2 4 3 335 435 2+ Ba0.5Zr2(PO4)3:Eu 2+ CaZr4(PO4)6:Eu R3 305 485 [227] 3+ Ca0.5Fe1 – xEuxSb(PO4)3:Eu R3 395 590−650 [228] La1/6Pb1/3Zr2(PO4)17/6(SiO4)1/6 R3 254 580−625 [229]

Eu0.33Zr2(PO4)3 Pс3 1 360; 393 611 [230]

The time of luminescence decay was ~3 ms (for λem = 1550 nm) and decreased with an increase in the total concentration of Er3+ and Yb3+ in all samples. No correlation was observed between the intensity and decay time and the growth of the value of x + y. This means that concentration quenching does not play an important role in luminescence relaxation mechanisms for the studied materials.

As a whole, obtained data on the luminescence of the studied phosphates of the types Ca0.5 – 1.5(x + y)ErxYbyZr2(PO4)3 and K2Mg0.5 – 0.5(x + y)ErxYbyZr1.5 – 0.5(x + y)(PO4)3 and phosphate silicate of the type Ca0.75 – 1.5(x + y)ErxYbyZr2(PO4)2.5(SiO4)0.5 suggest that they are characterized by luminescence in the visible spectrum region with intensity sufficient for detection by the naked eye, on excitation with a source of IR radiation (in the region of “therapeutic transparency window”). Therefore, the studied substances meet the requirements to materials for bioimaging by their optical properties.

5.3. Phosphate Phosphors for X-PDT To activate a number of the known photosensitizers “KillerRed®”, 610 nm; “Gematoporfirin®”, 620 ± 5 nm; “Foskan®”, 652 ± 5 nm; “Fotoditazin®”, 662 ± 5 nm; and “Fotosens®”, 670 ± 5 nm in the region of their “red” absorption bands, we investigated phosphors on the basis of phosphates containing Eu3+ and Sm3+ cations [181, 232].

The test samples were phosphates forming the series Na1 – 3xEuxZr2(PO4)3, x = 0.001, 0.01, 0.05, 0.1 (NZP structure) (described above and investigated as phosphors on excitation with an UV source) and K2 – xSmxMg0.5 + xZr1.5 – x(PO4)3, x = 0.25, 0.5 (Lb structure).

For europium-containing phosphates of the series Na1 – 3xEuxZr2(PO4)3, 0.001 ≤ x ≤ 0.1, we observed emission in the region 575–700 nm on excitation with Eexcit = 40–50 keV (Fig. 16a). The positions of emission bands remained at the same wave numbers as on excitation with an UV source. These spectra were characterized by the presence of only bands corresponding to 4f–4f intraconfiguration transitions of the Eu3+ ion.

For Sm-containing phosphates, a series of characteristic bands was observed in the region λem. = 550– 675 nm (Fig. 16b). It is clear that the observed emission was in the region of absorption of the known photosensitizers, which is a necessary condition for the initiation of their photochemical transformations [181, 232].

In general, the obtained data on the luminescence of Na1 – 3xEuxZr2(PO4)3 (x = 0.001, 0.01, 0.05, 0.1) and K2 – xSmxMg0.5 + xZr1.5 – x(PO4)3 (x = 0.25, 0.5) phosphates suggest that they are characterized by emission in the visible spectral region on excitation with an UV source. It was also found for the first time that 2 + 3+ the simultaneous presence of lanthanide Eu and Sm cations in the Ca0.5Zr2(PO4)3 matrix ensures

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I (a)

1 2 3 4

450 500 550 600 650 700 1400 1450 1500 1550 1600 1650 λ, nm λ, nm

I (b)

5 3 6 7 8 2

450 500 550 600 650 700 1400 1450 1500 1550 1600 1650 λ, nm λ, nm

Fig. 15. Luminescence spectra in the visible and NIR ranges of phosphates of the series: (a), Ca0.5 – 1.5(x + y)ErxYbyZr2(PO4)3, x:y = 1 : 1, x + y = (1) 0.02, (2) 0.1, (3) 0.2, (4) 0.33; (b) K2Mg0.5 – 0.5(x + y)ErxYbyZr1.5 – 0.5(x + y)(PO4)3, x : y = 1 : 9; x + y = (5) 0.25, (6) 0.5, (7) 0.75, (8) 1.0.

3+ 3+ emission close to white light by color coordinates; Er and Yb cations in Ca0.5 – 1.5(x + y)ErxYbyZr2(PO4)3 and Ca0.75 – 1.5(x + y)ErxYbyZr(PO4)2.5(SiO4)0.5 favor emission in the green region at λ = 525 nm with an acceptable intensity on excitation with an IR source. 3+ 3+ Ca0.2EuxSmyZr2(PO4)3 phosphates bearing cations being sources of “red” emission (Eu , Sm ) possess emission in the region 550–700 nm (Eexcit = 40–50 keV), necessary for the initiation of photochemical transformations of the known photosensitizers on excitation with an X-ray source. Therefore, the studied substances meet the requirements imposed on phosphor materials for LED technologies, bioimaging, and X-PDT by their optical properties. The regulation of optical properties in some of the studied systems was attained by changing the influence of the crystal field force of the ligand on changing the coordination environment of the emitting ion. As was shown in Sections 4 and 5, an increase in the concentration of lanthanide ions in NZP phosphates led to morphotropic transitions, which manifested themselves in luminescence spectra and were spectroscopic confirmations of crystal lattice distortions. Therefore, we can believe with confidence that the experimental results presented in this review will favor a success in the preparation luminescent materials with optimum and controlled optical characteristics.

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(a) (b) II1 1 2 2 3 4

500 550 600 650 700 750 500 550 600 650 700 750 λ, nm λ, nm

Fig. 16. Luminescence spectra of phosphates: (a) Na1 – 3xEuxZr2(PO4)3, x = (1) 0.001, (2) 0.05, (3) 0.01, (4) 0.1; (b) K2 – xSmxMg0.5 + xZr1.5 – x(PO4)3, x = (1) 0.25, (2) 0.5. The lines correspond to absorption bands of photosensitizers.

6. OTHER PROPERTIES: BEHAVIOR ON HEATING, CHEMICAL STABILITY, BIOCOMPATIBILITY In the development of the crystal chemistry concept of the formation of crystalline phosphor materials, a necessary stage is the study of properties characterizing their stability on heating and in water, and also biocompatibility.

6.1. Behavior on Heating

Phosphates of NZP family. The majority of the known phosphates similar to NaZr2(PO4)3 are characterized by high thermal stability: compounds do not melt and do not decompose on heating to temperatures of 1000−1600°C. Some of them possess low and ultralow (down to (1−2) × 10–6 °C–1) heat expansion, regulated in the series, and can withstand repeated thermal “shocks” on varying temperature in a wide range [120–125]. The behavior on heating with the determination of coefficients of linear heat expansion (αa and αc) was studied on an example of some model calcium-containing phosphates Ca0.5Ti2(PO4)3, Ca0.5Zr2(PO4)3, Ca0.75Zr2(PO4)2.5(SiO4)0.5 and CaMg0.5Zr1.5(PO4)3 in [233] by high-temperature X-ray diffraction by the procedure described in [234]. It was found that, with an increase in the concentration of calcium in the compounds, and, therefore, the percentage of occupied interstitial positions, a trend to the growth of αa values was noted (absolute values decreased). The αc values decreased in the region of low temperatures (to 170°C). As for other temperature intervals, the αс values were virtually independent of the composition. Because of the anisotropy of heat expansion, which tends to reduce in going from Ca0.5Zr2(PO4)3 to Ca0.75Zr2(PO4)2.5(SiO4)0.5 and CaMg0.5Zr1.5(PO4)3, the average values are –6 –1 –6 –1 characterized by very low values of αav, min × 10 °C : +0.3 and –0.3, αav, min × 10 °C : –2.2 and 2.3. Anisotropy was the lowest for the CaMg0.5Zr1.5(PO4)3 phosphate. X-ray powder diffraction was also used to obtain data on the upper boundaries of phosphate stability. Samples were placed in alundum crucibles and consecutively kept at T = 1000, 1100, and 1200°C for 20 h, with an intermediate control by X-ray powder diffraction at each step. It was found that, at T = 1200°C, phosphates with NZP structure retained their phase composition. The data obtained well agreed with the data published for calcium-containing phosphates [235–237]. Phosphates of Lb family. Phosphates with Lb structure, as phosphates of NZP type, do not undergo phase transformations on heating to 1100°C [67, 124, 171, 173, 175, 213, 238]. Some of them, containing lanthanides, K2ErZr(PO4)3, A1.5Sm0.5Fe2(PO4)3 (A = K, Rb, Cs), and K1.5Sm0.5MgZr(PO4)3, underwent partial thermal destruction at T > 1100°C, accompanied by the appearance of monazite-like phases (simple phosphates of the type LnPO4) [171, 173, 175, 213, 238]. Phosphates with this structure expand along all crystallographic directions, which is characteristic for substances with the cubic lattice.

6.2. Chemical Stability Phosphates of NZP and Lb families retain their phase composition after being in long contact with water [65, 67, 124, 126–129]. Hydrolytic tests in the batch mode (T = 20°C, τ = 21 day) were performed on an example of some phosphates, Ca0.5Zr2(PO4)3, Ca0.2Er0.1Yb0.1Zr2(PO4)3, Er0.165Yb0.165Zr2(PO4)3, and Ca0.75Zr2(PO4)2.5(SiO4)0.5. The calculated rates of calcium leaching decreased with time and, in the 21st

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2 –1 –6 –8 day, RCa, g cm day were 5 × 10 for Ca0.5Zr2(PO4)3 (relative density ρ = 85%) and 1 × 10 for Ca0.75Zr2(PO4)2.5(SiO4)0.5 (ρ = 99%). The REr values for Ca0.2Er0.1Yb0.1Zr2(PO4)3 (ρ = 89%) and –8 2 –1 Er0.165Yb0.165Zr2(PO4)3 (relative density ρ = 88%) phosphates were lower than 2.5 × 10 g cm day (the limit of detection for erbium is 0.5 μg/mL) [181]. According to the data of X-ray power diffraction, phase composition remained unchanged during the experiment. The results obtained were compared with similar data on leaching calcium and strontium from Ca9Ln(PO4)7, Ln = Sm, Eu, Gd (structural type of β-whitlockite) [239] and Sr0.5Zr2(PO4)3 (structural type of NZP) [127] phosphates. According to these data, the –5 2 –1 –7 2 –1 rates of leaching calcium and strontium were RCa = 1.49–1.56 × 10 g cm day , RSr = 4.86 × 10 g cm day , respectively. Therefore, the studied compounds were highly stable in water systems.

6.3. Biocompatibility

Biocompatibility of NZP phosphates was first studied on an example of the series Ca0.5 – 1.5(x + y)ErxYbyZr2(PO4)3, where x, y = 0.01, 0.05, 0.10 [203]. Studies were performed in vitro using a culture of neutrophilic granulocytes, according to the procedure described in [240, 241]. The number of living cells in the beginning of the experiment according to the test with Trypane blue was not lower than 99%. According to the data of the test, the percentage of unstained cells was 95 ± 0.5% in incubation with phosphates. In the check experiment, the percentage of unstained cells was 98.5 ± 0.5%. A comparison of samples with the reference sample using the Student test did not reveal statistically significant distinctions. Therefore, the studied calcium phosphates containing Er and Yb satisfy the criteria of safety and nontoxicity. For comparison, the viability of cells in the presence of erbium- and ytterbium-containing phosphors based on glasses was estimated at 27.0 ± 6.6% [240].

7. CONCLUSIONS 1. An analysis of the status of works in the world on the study of luminescent materials for LED and biomedical appointments was done. The critical analysis of compositions of the developed phosphors is presented. The high promise of studying phosphates, in particular, with NaZr2(PO4)3 and langbeinite structures for these purposes is shown. Their important feature is a possibility of wide isomorphism. As a result, the variation of their properties, including optical ones, in the required direction becomes possible on changing their compositions. 2. Using the crystal chemistry approach, chemical compositions of compounds containing biogenous elements (Na, K, Mg, Ca, Si, P), and also elements being a source of luminescence (Mn, Sm, Eu, Er, Yb) with the expected structures of NZP and Lb were calculated and chosen for the study. Compounds were obtained as powders by the sol–gel by technique using ethyl alcohol as a salting-out agent. The conditions of the preparation of nanopowders were optimized: precipitation at T = 20°C, also with the use of ultrasound. 3. It was found that the synthesized compounds crystallized in structural types of NZP (space groups Rc3 , R3, R3, Pc3 ) and Lb (space group P213). The unit cell parameters of the obtained compounds were calculated. 4. Compounds did not decompose on heating to 845–1100°C. According to the found characteristics of heat expansion (high-temperature X-ray diffraction), they were assigned to the class of low-expanding compounds. –8 –2 The rate of calcium leaching was 1 × 10 g cm day for Ca0.75Zr2(SiO4)0.5(PO4)2.5 and rate of erbium –8 –2 –1 leaching, lower than 2.5 × 10 g cm day for Ca0.2Er0.1Yb0.1Zr2(PO4)3 and Er0.165Yb0.165Zr2(PO4)3. 5. The luminescent properties of the studied compounds were determined. 5.1. For LED technologies. Emission and its intensity depended on substance composition and concentration of emitting ions in it. The character of spectra was determined by the presence of an optically active ion in different crystallographic positions of the structure. Compounds B0.5 – xEuxZr2(PO4)3 (B = Ca, Sr) demonstrated blue photoluminescence, which exhibited a shift towards higher energies with the growth of europium concentration and with the introduction of strontium, which was due to the longer Eu–O and Sr–O bonds in comparison with the Ca–O bond and, correspondingly, the weakening of crystal field strength. Luminescence was characterized by color coordinates {0.27; 0.34} at the simultaneous presence of Eu2+ and Sm3+. 5.2 For bioimaging. The presence of Er and Yb lanthanides in the composition of compounds Ca0.5 – 1.5(x + y)ErxYbyZr2(PO4)3, K2Mg0.5 – 0.5(x + y)ErxYbyZr1.5 – 0.5(x + y)(PO4)3, and Ca0.75 – 1.5(x + y)ErxYbyZr2(PO4)2.5(SiO4)0.5

REVIEW JOURNAL OF CHEMISTRY Vol. 8 No. 1 2018 PHOSPHORS BASED ON PHOSPHATES 27 ensured emission at λ = 525 nm (in the visible region) with acceptable intensity on excitation with IR radiation.

5.3. For photodynamic therapy. Compounds with the structures of NaZr2(PO4)3 and K2Mg2(SO4)3, 3+ 3+ containing Eu and Sm , of the composition Na1 – 3xEuxZr2(PO4)3 and K2 – xSmxMg0.5 + xZr1.5 – x(PO4)3 possessed luminescence in the region 550–700 nm on excitation with X-rays (the range necessary for the initiation of photochemical transformations of the known photosensitizers). 6. The behavior of neutrophilic granulocytes in the presence of phosphates was investigated. The found viability was 95%. 7. The developed concept of the “on a plan” design of materials with controlled optical properties used in this work can be recommended as a basis for the design of new ecologically safe and biocompatible phosphors for LED technologies, intracellular bioimaging, and photodynamic therapy. At the same time, the found characteristics can be improved using the developed crystal chemistry approaches, which may be done in future studies.

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