Luminescence of Agrellite Specimen from the Kipawa River Locality

Article Luminescence of Agrellite Specimen from the Kipawa River Locality

Maria Czaja 1,* and Radosław Lisiecki 2

1 Faculty of Natural Sciences, Institute of Earth Sciences, University of Silesia, B˛edzi´nska60, 41-200 Sosnowiec, Poland 2 Institute of Low Temperature and Structure Research, Polish Academy of Sciences, W. Trzebiatowski Institute, Okólna 2, 50-422 Wrocław, Poland; [email protected] * Correspondence: [email protected]

Received: 29 October 2019; Accepted: 30 November 2019; Published: 3 December 2019

Abstract: Using steady-state luminescence measurements, the luminescence spectra of Ce3+, Pr3+, Nd3+, Sm3+, Eu3+, Dy3+, Er3+ and Yb3+ for the agrellite sample from the Kipawa River region have been measured. The emission spectra of Eu3+ and Dy3+ next to those of Sm3+ and Pr3+ have been measured for characteristic excitation conditions. The most eﬃcient luminescence activator in the studied sample was Ce3+. This ion was also a sensitizer of Pr3+, Sm3+, Eu3+, and Dy3+ luminescence.

Keywords: agrellite; lanthanide ion; energy transfer

1. Introduction Agrellite is a rather rare mineral and is usually found in pegmatite lenses and pods or in mafic gneisses in a regionally metamorphosed agpatic alkali rock complex. It occurs with eudialyte, britholite, aegirine, as well as miserite, vlasovite, calcite, fluorite, clinohumite, gittinsite, norbergite, zircon, biotite, phlogopite, galena, and quartz. The most known localities of it are the Sheffield Lake complex, Kipawa River, Villedieu Township, Québec (Canada), Dara-i-Pioz massif, Alai Range, Tien Shan Mountains (China), (Tajikistan), the Murun massif, southwest of Olekminsk, Yakutia, (Russia) and Wausau complex, and finally, Marathon Co. in Wisconsin (USA). Among the minerals from the Kipawa River complex, only britholite, fluorite, calcite, miserite, and zircon are predisposed to exhibit fluorescence from lanthanide ions. However, no such reports are known so far. The knowledge of agrellite luminescence comes from websites only [1–3], and from the Gorobets and Rogojine review book [4]. According to these sources, agrellite has a dull pink color or a bright pink color under shortwave ultraviolet (SW UV) or longwave ultraviolet (LW UV, respectively. The following emission centers have been observed [4]: Ce3+, Dy3+, and Sm3+ for samples from the Kipawa River; Fe3+, Mn2+, Eu2+, and presumed Nd3+ for samples from the Yakutia; and of Ce3+ and Mn2+ for specimens from the Dara-i-Pioz. The latest results of spectroscopic investigation of agrellite samples from Dara-i-Pioz (Tajikistan) and Murun massif (Russia) shows luminescence from Ce3+ and EPR spectra of Mn2+ [5]. An attempt was also made to synthesize sodium calcium silicate with an initial composition such as agrellite [6]. These materials were doped with Mn2+ (1%, 2%, and 3%), Ce3+ (0.5%), and Tb3+ (4%), but as a result of synthesis, multiphase nanoparticles composed of wollastonite 2M CaSiO3, devitrite Na2Ca3Si6O16, and cristobalite SiO2 was formed. The current paper presents the preliminary results of a study of the luminescence properties of agrellite NaCa2Si4O10F specimens from the Kipawa Alkaline Complex, Québec, Canada. Agrellite crystallized in triclinic, space group P( 1), and unit cell parameters are: a = 7.759 (2) Å, − b = 18.946 (3) Å, c = 6.986 (1) Å, α= 89.88◦, β= 116.65◦, γ= 94.34◦,Z = 4 [7]. All atoms in the agrellite lattice are present in general positions and the Wyckoff site of all atoms is denoted as 2i. The crystal

Minerals 2019, 9, 752; doi:10.3390/min9120752 www.mdpi.com/journal/minerals Minerals 2019, 9, 752 2 of 15 lattice are present in general positions and the Wyckoff site of all atoms is denoted as 2i. The crystal structure of agrellite consists of two different NaO8 distorted cubes polyhedra, two CaO5F Minerals 2019, 9, 752 2 of 15 octahedra (hereinafter referred to as Ca1B and Ca2A), and two CaO6F2 polyhedra (named as Ca1A Minerals 2019, 9, 752 2 of 13 and Ca2B)lattice arecoupled present with in general two different positions Si and8O20 the chains Wyckoff [7]. site The of mean all atoms of Ca is denoted–O and asCa 2i.–F The distances crystal are smallerstructure for CaO of5 F agre octahedrallite consists and equal of two for different Ca1B and NaO Ca2A,8 distorted respectively cubes 2.381 polyhedra, Å , 2.192 two Å CaO , 2.3675F Å , and 2.201structureoctahedra Å than of (hereinafter agrellite CaO6F consists2 andreferred equal of twoto foras di Ca1B ﬀCa1Aerent and NaOand Ca2A),8 Ca2B,distorted and respectively two cubes CaO polyhedra,6F 22.574, polyhedra two2.402, CaO (named 2.6155F octahedra Åas andCa1A 2.454 Å (Fig(hereinafterandure Ca2B) 1a,b). coupled referred with to as two Ca1B different and Ca2A), Si8O and20 chains two CaO[7]. The6F2 polyhedramean of Ca (named–O and as Ca Ca1A–F distances and Ca2B) are coupledsmaller for with CaO two5F di octahedraﬀerent Si 8andO20 equalchains for [7]. Ca1B The meanand Ca2A, of Ca–O respectively and Ca–F 2.381 distances Å , 2.192 are smallerÅ , 2.367 for Å , CaOand 52.201F octahedra Å than andCaO equal6F2 and for equal Ca1B for and Ca1A Ca2A, and respectively Ca2B, respectively 2.381 Å, 2.574, 2.192 Å,2.402, 2.367 2.615 Å, andÅ and 2.201 2.454 Å thanÅ (Fig CaOure6 F1a2 ,andb). equal for Ca1A and Ca2B, respectively 2.574, 2.402, 2.615 Å and 2.454 Å (Figure1a,b).

Figure 1. A sketch of agrellite structure: (a) on (100) plane; oxygen atoms—small red balls, silicon

atomsFigure—small 1. A blue sketch balls, of agrellite calcium structure: atoms— (biga) on blue (100) balls, plane; Ca1A oxygen site atoms—small—big blue redand balls, magenta silicon balls, Figure 1. A sketch of agrellite structure: (a) on (100) plane; oxygen atoms—small red balls, silicon sodiumatoms—small atoms—big blue yellow balls, ballscalcium, and atoms—big fluorine blue atoms balls,—small Ca1A black site—big balls; blue (b) and on (010) magenta plane; balls, small atoms—small blue balls, calcium atoms—big blue balls, Ca1A site—big blue and magenta balls, darksodium blue tetrahedra atoms—big in yellowSi8O20 balls,chains, and big ﬂuorine light blue atoms—small Ca–O, F and black yellow balls; (NaOb) on8 (010)polyhedra; plane; small red sodium atoms—big yellow balls, and fluorine atoms—small black balls; (b) on (010) plane; small ballsdark—oxygen blue tetrahedra atoms. in Si8O20 chains, big light blue Ca–O, F and yellow NaO8 polyhedra; small red dark blue tetrahedra in Si8O20 chains, big light blue Ca–O, F and yellow NaO8 polyhedra; small red balls—oxygen atoms. balls—oxygen atoms. 2. Materials2. Materials and and Methods Methods 2. Materials and Methods The The agrellite agrellite sample sample studied studied here here takes takes the the form form of ﬁne of lath-shaped fine lath-shaped aggregates aggregates measuring measuring from a fromfew a few toThe over to agrellite 100over mm 100 sample in mm length. in studied Itlength. has a here greenish-white It takeshas a thegreenish form color of- (Figurewhite fine 2 lathcolora) which-shaped (Fig changesure aggregates 2a) to which lavender-pink measuring changes to lavenderunderfrom- pinka mid few UVunder to (Figureover mid 1002 b).UV mm (Fig in urelength. 2b). It has a greenish-white color (Figure 2a) which changes to lavender-pink under mid UV (Figure 2b).

Figure 2. Photos of agrellite sample under white light (a) and LW UV (365 nm) (b). FigureFigure 2. Photos 2. Photos of ofagrellit agrellitee sample sample under under white lightlight ( a()a and) and LW LW UV UV (365 (365 nm) nm) (b). (b). Rare earth elements (REE) concentrations in the studied agrellite crystal were measured by the ISP-MSRare method earth elements at ACMA (REE) Lab concentrations (Canada), and in the the studied results agrellite are listed crystal in were Table measured 1. Steady by-state the Rare earth elements (REE) concentrations in the studied agrellite crystal were measured by the ISP-MSfluorescence method measu at ACMArements Lab for(Canada), ions shown and the in results the chemicalare listed in analysis Table1 . were Steady-state performed ﬂuorescence using a ISP-MS method at ACMA Lab (Canada), and the results are listed in Table 1. Steady-state measurementsJobin-Yvon (SPEX) for ions spectrofluorimeter shown in the chemical FLUOROLOG analysis 3 were-12 (Jobin performed-Yvon, using Grenoble, a Jobin-Yvon France SPEX (SPEX) at fluorescence measurements for ions shown in the chemical analysis were performed using a Jobin -Yvon (SPEX) spectrofluorimeter FLUOROLOG 3-12 (Jobin-Yvon, Grenoble, France SPEX at

Minerals 2019, 9, 752 3 of 13 spectroﬂuorimeter FLUOROLOG 3-12 (Jobin-Yvon, Grenoble, France SPEX at room temperature using a 450 W xenon lamp, a double-grating monochromator, and a Hamamatsu 928 photomultiplier (Hamamatsu Photonics, Shizuoka, Japan). The wavelength range for emission and excitation spectra was from 250 to 900 nm, and the resolution no lower than 1 nm. For such conditions, the emission and excitation spectra of Ce3+, Pr3+, Sm3+, Eu3+, Dy3+, and Er3+ in the UV–Vis range were recorded. The emission spectra of Nd3+, Yb3+, and Er3+ also in the NIR range were measured utilizing a Dongwoo Optron DM711 monochromator (MaeSan-Ri, Opo-Eup, GwangJu-Si, Gyeonggi-Do, Korea) coupled with a Hamamatsu 928 photomultiplier (Hamamatsu Photonics, Shizuoka, Japan), and an InGaAs detector (Hamamatsu Photonics, Shizuoka, Japan) or PbS detector (Hamamatsu Photonics, Shizuoka, Japan) depending on the spectral region. An InGaAs diode laser (Appolo Instruments, Irvine, USA) emitting infrared radiation at 975 nm and an AlGaAs diode laser (CNI, Changchun, China) emitting at 808 nm were used as continuous wave excitation sources.

Table 1. Concentration of some transition elements in the studied agrellite sample.

Contents (ppm) Y La + Lu Ce Pr Nd Sm Eu Gd >2000 >2000 >2000 1174 >2000 1034 136 1265 Contents (ppm) Tb Dy Ho Er Tm Yb Mn Fe 221 1419 282 772 93 458 1454 <2000

3. Results In the studied agrellite specimen, the minimum content of lanthanide ions as a potential luminescence activator can be estimated as 1.0% (Table1). If the distribution of these ions is uniform, then these ions are present in every 2.5 unit cell. The maximum distance among 4f ions is not less than 17.5 Å and not more than 47.25 Å. Using steady-state ﬂuorescence measurements, the emission of the following lanthanide ions were measured: Ce3+, Pr3+, Nd3+, Sm3+, Eu3+, Er3+, and Yb3+. Despite the signiﬁcant Mn content, the emission spectra of Mn2+ were not measured. Unlike the [4] data, no emission from Fe3+ was obtained.

3.1. Nd3+, Yb3+, and Er3+ Luminescence The emission of Nd3+ was measured as a group of three multiples (Figure3). The most intense was measured at 1046, 1060, 1064, 1078, and 1092 nm and corresponds to the 4F 4I transition. 3/2 → 11/2 Another group of lines at 879, 885, 893, 905, and 917 nm is associated with the 4F 4I , transitions 3/2 → 9/2 and the last with lines at 1313, 1335, and 1340 nm-with the 4F 4I transitions. The emission 3/2 → 13/2 spectrum of Yb3+ was not very intense (inset of Figure3) and band 979 nm corresponds to the transition 2F 2F . 5/2 → 7/2 It was expected that the measurement of distinct emission lines of Er3+ would be possible due to the significant content of this ion and its high luminescence efficiency. However, the emission bands of Er3+ corresponding to the 2H 4I , 4S 4I and 4F 4I transitions and usually 11/2 → 15/2 3/2 → 15/2 9/2 → 15/2 measured at a 500–600 nm range were not clearly visible for steady-state luminescence measurements using a xenon lamp excitation, contrary to time-resolved measurements [8]. It has previously been verified [9] that for steady-time measurements using a xenon lamp, the most convenient excitation for Er3+ is λ = 377 nm. However, for the studied agrellite specimen, only a weak emission band at 543 nm was measured (Figure4). In the NIR range, the distinct emission band at 1533 nm (1554 nm, 1512 nm, and 1488 nm) was measured as a result of the 4I 4I transition (Figure5). Using laser excitation, 13/2 → 15/2 emission bands of 2H 4I , 4S 4I and 4F 4I transitions were also obtained (inset 11/2 → 15/2 3/2 → 15/2, 9/2 → 15/2 in Figure5). However, the intensities of these bands at 520 and 550 nm were very weak, not only in comparison with the Er3+ emission in the NIR range but also with the Sm3+ and Pr3+ emission lines, Minerals 2019, 9, 752 4 of 13

although the samarium or praseodymium concentration was not much higher than the Er content Minerals(Figure 20196). , 9, 752 4 of 15

Minerals 2019, 9, 752 5 of 15 FigureFigure 3.3. Luminescence spectraspectra ofof NdNd33++ andand Yb Yb3+3+ inin agrellite agrellite sample. sample.

It was expected that the measurement of distinct emission lines of Er3+ would be possible due to the significant content of this ion and its high luminescence efficiency. However, the emission bands of Er3+ corresponding to the 2H11/2 → 4I15/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions and usually measured at a 500–600 nm range were not clearly visible for steady-state luminescence measurements using a xenon lamp excitation, contrary to time-resolved measurements [8]. It has previously been verified [9] that for steady-time measurements using a xenon lamp, the most convenient excitation for Er3+ is λ = 377 nm. However, for the studied agrellite specimen, only a weak emission band at 543 nm was measured (Figure 4). In the NIR range, the distinct emission band at 1533 nm (1554 nm, 1512 nm, and 1488 nm) was measured as a result of the 4I13/2 → 4I15/2 transition (Figure 5). Using laser excitation, emission bands of 2H11/2 → 4I15/2, 4S3/2 → 4I15/2, and 4F9/2 → 4I15/2 transitions were also obtained (inset in Figure 5). However, the intensities of these bands at 520 and 550 nm were very weak, not only in comparison with the Er3+ emission in the NIR range but also with the Sm3+ and Pr3+ emission lines, although the samarium or praseodymium concentration was not much higher than the Er content (Figure 6).

3+ FigureFigure 4. 4.PhotoluminescencePhotoluminescence spectra of of Er Er3+from from the the agrellite agrellite sample. sample.

Figure 5. Emission spectra of Er3+ from agrellite samples measured under laser excitations.

Minerals 2019, 9, 752 5 of 15

Minerals 2019, 9, 752 Figure 4. Photoluminescence spectra of Er3+ from the agrellite sample. 5 of 13

Minerals 2019, 9, 752 6 of 15 Figure 5. Emission spectra of Er3+ from agrellite samples measured under laser excitations. Figure 5. Emission spectra of Er3+ from agrellite samples measured under laser excitations.

3+ 3+ 3+ FigureFigure 6. 6.Emission Emission spectra spectra of of Sm Sm3+,, PrPr3+,, and and Er Er3+ inin Vis Vis range range measured measured under under laser laser excitation. excitation. 3.2. Ce3+ Fluorescence 3.2. Ce3+ Fluorescence The cerium content in the studied agrellite sample was high. Electric-dipole 4f–5d transitions are The cerium content in the studied agrellite sample was high. Electric-dipole 4f-5d transitions even and spin allowed with the oscillator strength at half-width and short luminescence decay time. are even and spin allowed with the oscillator strength at half-width and short luminescence decay time. Cerium Ce3+ is often added to synthetic crystals because of its well-known properties as an efficient sensitizer of luminescence. Luminescent materials doped with Ce3+ can efficiently absorb excitation energy. A very intensive emission band at 388 nm has been measured (Figure 7). The zero phonon line (ZPL) position was designated in the point of spectral overlap of the excitation and emission curves, i.e., at λ = 337 nm (29,673 cm−1). The Stokes shift ∆S, as the energy difference between absorption/excitation and emission maxima of transition between the lowest 5d and the 4f ground states, is equal to 1567 cm−1. This value is not very high, probably owing to the high coordination number around Ce3+ and the long Ca–O distance [10,11]. For the lowest Raman frequency of agrellite 331 cm−1, the Huang-Rhys parameter S is equal to S = 2.8, so electron-phonon coupling may be recognized as intermediate.

Minerals 2019, 9, 752 6 of 13

Cerium Ce3+ is often added to synthetic crystals because of its well-known properties as an efficient sensitizer of luminescence. Luminescent materials doped with Ce3+ can efficiently absorb excitation energy. A very intensive emission band at 388 nm has been measured (Figure7). The zero phonon line (ZPL) position was designated in the point of spectral overlap of the excitation and emission curves, i.e., 1 at λ = 337 nm (29,673 cm− ). The Stokes shift ∆S, as the energy difference between absorption/excitation and emission maxima of transition between the lowest 5d and the 4f ground states, is equal to 1567 1 3+ cm− . This value is not very high, probably owing to the high coordination number around Ce and 1 the long Ca–O distance [10,11]. For the lowest Raman frequency of agrellite 331 cm− , the Huang-Rhys parameterMinerals 2019S, 9is, 752 equal to S = 2.8, so electron-phonon coupling may be recognized as intermediate.7 of 15

3+ FigureFigure 7.7. PhotoluminescencePhotoluminescence spectraspectra ofof CeCe3+ inin agrellite agrellite sample. sample. On the excitation spectra, two bands at 284 nm and 321 nm were recorded, so the crystal-field On the excitation spectra, two bands at 284 nm1 and 321 nm were recorded, so 3the+ crystal-field splitting 10 Dq of 5d level was estimated as 4059 cm− . The upper excited level of Ce is T2g and the −1 3+ 2g splitting 10 Dq of 5d level was estimated 3as+ 4059 cm . The upper excited level of Ce is1 T and the lower is Eg. The 10 Dq parameter for Ce usually has a value in the 5000–10,000 cm− range. For lower is Eg. The 10 Dq parameter for Ce3+ usually has a value in the 5,000–10,000 cm−1 range. For the the studied crystal it was slightly smaller because the Ca–O length in agrellite is larger than in other studied crystal it was slightly smaller because the Ca–O length 3in+ agrellite is larger than in other Ca–minerals, for example fluor-apatite. The emission band of Ce 3+ is nearly symmetric, although it Ca–minerals, for example fluor-apatite. The emission band of Ce1 is nearly symmetric,1 2 although it was fitted to two Gaussian components with maxima at 28,023 cm− −1and 25,819 cm−−1(R 2 = 0.998) that was fitted to two Gaussian components with maxima at 28,023 cm and 25,819 cm2 (R =2 0.998) that can be discerned, (Figure8), which correspond to the transition terminated on the F5/2 and F7/2 levels. can be discerned, (Figure 8), which correspond to the transition terminated on the 2F5/2 and 2F7/2 The intensity of the Ce3+ emission band at 388 nm measured for λ = 284 nm was almost levels. exc 1.5 times higher than those observed for λexc = 321 nm, despite the power of the xenon lamp is lower at λ = 284 nm than at 321 nm. This may indicate that a significant part of the excitation energy at λexc = 321 nm in the studied mineral transferred to other luminescence centers. In [5] for the agrellite sample from Dara-i-Pioz, the emission and excitation spectra of Ce3+ have been presented. The Ce-content in this sample was 1160–1263 ppm, distinctly less than for the Kipawa River (Table1). The emission band of Ce 3+ was measured at 370 nm as a rather symmetrical band, while on the excitation spectrum, the following bands have been measured: 190, 220, 245, 281, and 317 nm. The differences in emission and excitation bands for the sample from Dara-i-Pioz (Tajikistan) and the Kipawa River are certainly due to the change in the Ce–O bonding length in both samples.

Minerals 2019, 9, 752 7 of 13

It can be concluded that the crystal-ﬁeld strength for the Kipawa River specimen is less than for the sample from Dara-i-Pioz. The possibility of Ce4+ presence and Ce4+ charge transfer bands on the absorption spectrum at 400 nm, which may be due to the gray color of agrellite crystals, was also Minerals 2019, 9, 752 8 of 15 assumed [5].

3+ 2 2 Figure 8. The deconvolution of emission band of Ce Eg(4f5d)3+ 4f ( F , F ) into2 two2 components. Figure 8. The deconvolution of emission band of Ce →Eg(4f5d5)/ 2→ 74f/2 ( F5/2, F7/2) into two components. For the studied agrellite sample, the energy transfer between Ce3+ and Dy3+, Eu3+, Sm3+, and Pr3+ were found. Energy transfer in phosphors with sensitizer-activator ion pairs means that part of The intensity of the Ce3+ emission band at 388 nm measured for λexc = 284 nm was almost 1.5 the excitation energy of the sensitizer ion transfers to activator ion through a non-radiative process and times higher than those observed for λexc = 321 nm, despite the power of the xenon lamp is lower at it subsequently enhances or generates the emission of the activator. The energy transfer among Ce3+ λ = 284 nm than at 321 nm. This may indicate that a significant part of the excitation energy at λexc = and other 4f ions is well known and used to synthesize phosphors with expected properties [12–18]. 321 nm in the studied mineral transferred to other luminescence centers. Gd3+ is another activator with intense absorption and emission lines in the UV range. Its excitation In [5] for the agrellite sample from Dara-i-Pioz, the emission and excitation spectra of Ce3+ have line is usually at 275 nm and corresponds to 8S 6I transition, while emission could occur been presented. The Ce-content in this sample was7/2 → 11607/–21263 ppm, distinctly less than for the either from the 6I or the 6P level as a 312 nm band. The Gd3+ emission was rarely measured Kipawa River (Table7/2 1). The emission7/2 band of Ce3+ was measured at 370 nm as a rather symmetrical for mineral samples. However, the Gd3+ luminescence was found in some scheelite, anhydrite, band, while on the excitation spectrum, the following bands have been measured: 190, 220, 245, 281, apatite, and ﬂuorite specimens [19]. Moreover, it was found for CaF [19] that due to energy transfer and 317 nm. The differences in emission and excitation bands for2 the sample from Dara-i-Pioz (6I -6P )(Gd3+)-(3F ,3H -3H )(Pr3+), the emission at 275 nm is weakened, whereas at 312 nm it is (Tajikistan)7/2 7/2 and the Kipawa2 6 River4 are certainly due to the change in the Ce–O bonding length in enhanced. After using laser induction [4], time-resolved measurements of the Gd3+ ﬂuorescence for both samples. It can be concluded that the crystal-field strength for the Kipawa River specimen is zircon, anhydrite, and hardystonite have been prepared. The energy transfer among Gd3+ and many less than for the sample from Dara-i-Pioz. The possibility of Ce4+ presence and Ce4+ charge transfer RE3+ have been analyzed [10–27]. For the studied agrellite sample, the energy transfer from Gd3+ to bands on the absorption spectrum at 400 nm, which may be due to the gray color of agrellite RE3+ could be excluded. The validity of this thesis is based on the fact that: crystals, was also assumed [5]. For the studied agrellite sample, the energy transfer between Ce3+ and Dy3+, Eu3+, Sm3+, and Pr3+ (a) for all substances known from existing literature, the Gd2O3 content was about 25 mol% or more. w(b)ere found.the excitation Energy andtransfer emission in phosphors lines of Gd with3+ didsensitizer not appear-activator in the ion agrellite pairs means spectra that at all,part although of the excitationcertain energy narrow of the lines sensitizer should ion be clearly transfers visible. to activator ion through a non-radiative process and it subsequently enhances or generates the emission of the activator. The energy transfer among Ce3+ and3.2.1. other Energy 4f ions Transfer is well Ce known3+-Dy and3+ used to synthesize phosphors with expected properties [12–18]. 3+ Gd is another activator with3+ intense absorption4 6 and emission lines in the UV range. Its The intense emission of Dy related to F9/2 H13/2 transition at 575 nm was measured for excitation line is usually at 275 nm and corresponds→ to 8S7/2 → 6I7/2 transition, while emission could the most suitable excitation at λ = 438 nm (solid olive line in Figure9). However, for λexc = 321 nm, occur either from the 6I7/2 or the 6P7/2 level as a 312 nm band. The Gd3+ emission was rarely measured for mineral samples. However, the Gd3+ luminescence was found in some scheelite, anhydrite, apatite, and fluorite specimens [19]. Moreover, it was found for CaF2 [19] that due to energy transfer (6I7/2-6P7/2)(Gd3+)-(3F2,3H6-3H4)(Pr3+), the emission at 275 nm is weakened, whereas at 312 nm it is enhanced. After using laser induction [4], time-resolved measurements of the Gd3+ fluorescence for zircon, anhydrite, and hardystonite have been prepared. The energy transfer among Gd3+ and many

Minerals 2019, 9, 752 9 of 15

RE3+ have been analyzed [10–27]. For the studied agrellite sample, the energy transfer from Gd3+ to RE3+ could be excluded. The validity of this thesis is based on the fact that: (a) for all substances known from existing literature, the Gd2O3 content was about 25 mol% or more. (b) the excitation and emission lines of Gd3+ did not appear in the agrellite spectra at all, although certain narrow lines should be clearly visible.

3.2.1. Energy Transfer Ce3+-Dy3+

Minerals 2019 9 3+ 4 6 The ,intense, 752 emission of Dy related to F9/2 → H13/2 transition at 575 nm was measured 8for of 13the most suitable excitation at λ = 438 nm (solid olive line in Figure 9). However, for λexc = 321 nm, not 3+ 3+ notonly only was was Ce Ce 3emission+ emission at at 388 388 nm nm measured, measured, but but so was aa DyDy3+ emissionemission bandband (solid(solid red red lines lines in in FigureFigure9), 9), and and luminescence luminescence intensity intensity was was stronger stronger than than for the for previous the previous excitation. excitation. Moreover, Moreover, on the on the excitation spectrum at λem = 575 nm corresponded to4 the 4F69/2 → 6H13/2 transition (dash-dot olive excitation spectrum at λem = 575 nm corresponded to the F9/2 H13/2 transition (dash-dot olive line line in Figure 9), while the excitation band at 321 nm prevails→ on the Dy3+ band. It appears that the in Figure9), while the excitation band at 321 nm prevails on the Dy 3+ band. It appears that the Ce3+ Ce3+ excitation band overlapped on 3the+ Dy9 3+8 4f9-4f85d broadband or that it is transferred to4 the 4K15/2, excitation band overlapped on the Dy 4f -4f 5d broadband or that it is transferred to the K15/2, 17/2 17/2 excited Dy3+ level. excited Dy3+ level.

FigureFigure 9. 9.The The photoluminescence photoluminescence spectra spectra of Dy of 3 Dy+, Sm3+, Sm3+,3+ and, and Pr 3 Pr+ and3+ and Ce 3 Ce+.3+ Solid. Solid lines—emission lines—emission spectra:spectra: black— blackλ—excλ=exc284 = nm,284 red— nm,λ exc red=—321λexc nm, =olive 321 line— nm, λexc olive= 348 line nm.—λ Dash-dotexc = 348 lines—excitation nm. Dash-dot spectra:lines— darkexcitation olive line—spectra:λem dark= 575 olive nm, line black—λem line— = 575λem nm,= 388black nm. line—λem = 388 nm.

3.2.2. Energy Transfer Ce3+-Sm3+, Pr3+, and Eu3+ 3.2.2. Energy Transfer Ce3+-Sm3+, Pr3+, and Eu3+ The eﬃcient emission of Sm3+ and Pr3+ was also found for agrellite crystal. In the spectral range The efficient emission of Sm3+ and Pr3+ was also found for agrellite crystal. In the spectral range ofof 550–700 550–700 nm, nm, the the emission emission lines lines of theseof these two two ions ions are locatedare located close close to each to each other other and are:and at are: 562 at nm 562 4 6 3+ 3+ 4 6 and 569 nm as G5/2 H5/2 transition of Sm ; at 601 nm and 607 nm of both Sm G5/2 H7/2 and nm and 569 nm as 4G5/2 → 6H5/2 transition of Sm3+; at 601 nm and 607 nm of both Sm3+ 4G5/2 → 6H7/2 3+ 1 3 3 → 3 3+ 3 → 3 Pr D2 H4 + P0 H6 transitions; and at 648 nm mainly for Pr transition P0 F2. As it and Pr3+→ 1D2 → 3H4 +→ 3P0 → 3H6 transitions; and at 648 nm mainly for Pr3+ transition 3P→0 → 3F2. As it was found earlier [9], the most convenient excitation for Sm3+ is the 399-402 nm line, while for Pr3+ it was found earlier [9], the most convenient excitation for Sm3+ is the 399-402 nm line, while for Pr3+ it isis 480 480 nm nm (Figure (Figure 10 10).). 3+ 3+ 3+ For λexc = 402 nm excitation, beside of Sm and Pr emission lines, the Eu emission at 613 nm 5 7 attributed to transition D0 F2 have been measured as well (Figure 11). 2+ → 3+ No emission of Eu was found. The emission of Eu as a characteristic line at 613 nm was obtained for the most convenient excitation λexc = 393 nm (Figure 12). However, this excitation also caused Sm3+ and Pr3+ emission. Minerals 2019, 9, 752 10 of 15

Minerals 2019, 9, 752 9 of 13 Minerals 2019, 9, 752 10 of 15

Figure 10. Excitation spectra characteristic for Pr3+ (black line) and Sm3+ (red line).

For λexc = 402 nm excitation, beside of Sm3+ and Pr3+ emission lines, the Eu3+ emission at 613 nm attributed to transition 5D0 → 7F2 have been measured as well (Figure 11). 3+ 3+ FigureFigure 10. 10. ExcitationExcitation spectra spectra characteristic for for Pr Pr3+(black (black line) line) and and Sm Sm(red3+ (red line). line).

For λexc = 402 nm excitation, beside of Sm3+ and Pr3+ emission lines, the Eu3+ emission at 613 nm attributed to transition 5D0 → 7F2 have been measured as well (Figure 11).

FigureFigure 11. 11.The The photoluminescence photoluminescence spectra spectra of of Pr Pr3+3+,, Sm Sm3+3+,, andand Eu Eu3+3+ inin agrellite. agrellite. Emission Emission spectrum spectrum measuredmeasured atatλ λexcexc == 402 nmnm excitation—magentaexcitation—magenta line,line, excitationexcitation spectrumspectrum atatλ λemem == 613 nm—bluenm—blue line,line, excitationexcitation spectrumspectrum atatλ λemem == 601 nm nm—red—red line, and excitation spectrum at λemem == 648648 nm nm—black—black line. line.

Figure 11. The photoluminescence spectra of Pr3+, Sm3+, and Eu3+ in agrellite. Emission spectrum

measured at λexc = 402 nm excitation—magenta line, excitation spectrum at λem = 613 nm—blue line,

excitation spectrum at λem = 601 nm—red line, and excitation spectrum at λem = 648 nm—black line.

Minerals 2019, 9, 752 11 of 15

No emission of Eu2+ was found. The emission of Eu3+ as a characteristic line at 613 nm was Mineralsobtained2019, for9, 752 the most convenient excitation λexc = 393 nm (Figure 12). However, this excitation also10 of 13 caused Sm3+ and Pr3+ emission.

FigureFigure 12. 12.Excitation Excitation andand emissionemission spectra of Eu Eu3+3 +inin agrellite. agrellite.

3+ 3+ 3+ UnderUnder excitation excitationλexc λexc= =321 321 nm, nm, notnot onlyonly thethe emissionemission ofof DyDy3+ (574(574 nm) nm) but but also also of of Sm Sm3+ andand Pr Pr3+ (562(562 nm, nm, 569 569 nm, nm, 601 601 nm, nm, 607607 nm),nm), was measured measured (Fig (Figureure 9).9). On On the the excitation excitation spectra spectra monitored monitored at at562 nm nm,, not not only only the the most most convenient convenient bands bands for forSm3+ Sm (and3+ (andPr3+) were Pr3+) measured were measured but an intense but an band intense bandof Ce of3+ Ce was3+ wasrecorded recorded too. In too. the In excitation the excitation spectra spectra of Sm3+ of, Pr Sm3+, 3and+, Pr Eu3+3+, and, the EuCe33++ ,band the Ceat 3321+ band–323 at 321–323nm always nm always appeared appeared and it andwas itintense. was intense. (Figure (Figure13). This 13 same). This effect same has eﬀ beenect has measured been measured when whenluminescenceMinerals luminescence 2019, 9, 752was wasexcited excited into the into Ce the3+ emission Ce3+ emission band, i.e. band,, 388 i.e., nm 388(Fig nmure (Figure14). 14). 12 of 15

3+ 3+ FigureFigure 13. 13.The The excitation excitation spectra spectra of of Pr Pr3+ andand Sm3+ emissionemission in in agrellite. agrellite.

Figure 14. The emission spectra of Pr3+ and Sm3+ under Ce3+ excitations.

The presence of the Ce3+ excitation band in the PLE spectrum of Dy3+, Sm3+, Pr3+, and Eu3+ indicated the occurrence of Ce3+ → (Dy3+, Sm3+, Pr3+, and Eu3+) energy transfer process. Upon UV irradiation, electrons from the 2F5/2 ground state of Ce3+ are excited into the 5d excited state. Some of these electrons return to the ground states (2F7/2 and 2F5/2) of Ce3+ ions, resulting in the violet-blue

Minerals 2019, 9, 752 12 of 15

Minerals 2019, 9, 752 11 of 13 Figure 13. The excitation spectra of Pr3+ and Sm3+ emission in agrellite.

Figure 14. The emission spectra of Pr3+ and Sm3+ under Ce3+ excitations. Figure 14. The emission spectra of Pr3+ and Sm3+ under Ce3+ excitations. The presence of the Ce3+ excitation band in the PLE spectrum of Dy3+, Sm3+, Pr3+, and Eu3+ indicatedThe presence the occurrence of the of Ce Ce3+3 +excitation(Dy3+ , band Sm3+ in, Pr the3+, and PLE Eu spectrum3+) energy of transfer Dy3+, Sm process.3+, Pr3+, Uponand Eu UV3+ 2 3+ →→ 3+ 3+ 3+3+ 3+ irradiation,indicated the electrons occurrence from of the CeF5 /2 ground (Dy , stateSm of, Pr Ce, andare Eu excited) energy into thetransfer 5d excited process. state. Upon Some UV of 2 2 3+2 3+ theseirradiation, electrons electrons return from to the the ground F5/2 ground states state ( F7/ 2ofand Ce F are5/2) excited of Ce intoions, the resulting 5d excited in the state. violet-blue Some of emissionsthese electrons of Ce 3return+ due toto thethe 5d ground4f transition. states (2F7/2 At a thend same2F5/2) of time, Ce3+ owing ions, toresulting the nonradiative in the violet resonant-blue → energy-transfer, a portion of excited electrons can be transferred into the 4K excited levels of 15/2, 17/2 3+ 4 6 Dy and then relax, mainly as F9/2 H13/2 transition. Similarly, another portion of excited electrons → 4 3 3+ 3+ can be transferred into excited levels of K11/2 or P2 of Sm or Pr and subsequently, the electrons can relax to their ground levels.

4. Conclusions In the studied agrellite crystal, the lanthanide ions form a complex arrangement of luminescence centers. The f-f emission of many ions has been distinctly excited by the excitation band of Ce3+. Generally, the mechanism of energy transfer from donor to acceptor ions can be attributed to an 3+ exchange interaction or electric multipolar interaction. The average distance Rc between the Ce donors and Sm3+ or Pr3+ acceptors was estimated as a value according to the below equation where C is the total concentration of 4fn ions in the studied sample, N is the coordination number, and V is the cell volume: r 3 3 V Rc = 2 · 4π C N · · when this value was determined to be 19.38 Å and it is greater than 5 Å, the energy transfer process would take place via electric multipolar interaction. The direct conﬁrmation of this conclusion could be obtained by measuring the decay lifetimes of Ce3+, Dy3+, Sm3+, and Pr3+ emissions, as well as the changes in luminescence intensities of these ions for samples containing their variable content. It is necessary to carry out measurements for other specimens of agrellite, diﬀering in the content of these ions. Minerals 2019, 9, 752 12 of 13

Contrary to the information on the website [3], the luminescence of Mn2+ has not been observed, although it was stated at 580 nm; similarly, no emission band of Eu2+ which was notified at 410 nm was observed. Additionally, the emission lines of Dy3+ at 478 nm or of Er3+ at 522 nm were not clear. It was observed that the measured emission spectra in the 400–560 nm range show intensive scattering of incident radiation not related to the effect of light reflection from the crystal surface. It can be 4 assumed that it is due to the presence of numerous defects on [Si4O10] − and will be confirmed after femtosecond excitation measurements. Some differences have been observed between the emission spectra of agrellite specimens for various localities presented in [4] or [5] and our results. The main reason for these differences is certainly the difference in REE content, although the data contained in [1,2] are incomplete. The agrellite specimen from the Kipawa River studied in the current research contains more Ce and a comparable amount of Mn and Fe as a sample from Dara-i-Pioz, and it is more than the sample from Yakutia [5]. However, contrary to [1] but similar to [5], we did not measure the emission of Mn2+ as was stated 3+ 2+ for the specimen from Dara-i-Pioz, and also the emission of Fe tetra and Eu as was shown for the specimen from Yakutia. Compared to previous luminescence results for specimens from the Kipawa River [4], the emissions from Er3+, Pr3+, and Eu3+ were also measured in the current research. The most important result of the present research is the demonstration of the phenomenon of effective energy transfer between Ce3+ as a donor and Pr3+, Sm3+, and Dy3+ as the acceptors. For this reason, a synthetic material based on agrellite, subsidized with the right amount of these ions, can be an efficient white light emitter.

Author Contributions: Conceptualization, M.C.; Investigation/Measurements, M.C. and R.L.; Resources, M.C.; Original Draft Preparation, M.C.; Writing—Review and Editing and Visualization, M.C. and R.L. Funding: This research project was supported by the Polish National Science Centre (Grant DEC-2011/03/B/ ST10/06320) and by statutory funding from the Faculty of Natural Sciences, Institute of Earth Sciences, University of Silesia. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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