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 1 2 KANUNOV, ORLOVA 1. INTRODUCTION The need in new “on a plan” functional materials, including those favoring the better quality and lon- ger 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 evi- dent 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 solu- tions) 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

Load more