ISSN 00201685, Inorganic Materials, 2015, Vol. 51, No. 10, pp. 1039–1046. © Pleiades Publishing, Ltd., 2015. Original Russian Text © Yu.K. Voronko, A.A. Sobol, V.E. Shukshin, I. Gerasymov, 2015, published in Neorganicheskie Materialy, 2015, Vol. 51, No. 10, pp. 1120–1127.
Structure and Phase Transitions of RareEarth Pyrosilicates Studied by Raman Spectroscopy Yu. K. Voronkoa, †, A. A. Sobola, V. E. Shukshina, and I. Gerasymovb a
b
Prokhorov General Physics Institute, Russian Academy of Sciences, ul. Vavilova 38, Moscow, 119991 Russia Institute of Scintillating Materials, National Academy of Sciences of Ukraine, pr. Lenina 60, Kharkiv, 61001 Ukraine email:
[email protected],
[email protected] Received December 4, 2014; in final form, February 18, 2015
Abstract—The vibrational spectra of the rareearth pyrosilicates Lu2Si2O7 (C structure), Gd2Si2O7 (E struc ture), Gd2Si2O7:La (11 mol %) (A structure), and Gd2Si2O7:Ce (15 mol %) (F structure) have been studied by Raman spectroscopy. Hightemperature Raman spectroscopy has been used to investigate the A → F phase transformation of Gd2Si2O7:La (11 mol %). Based on analysis of the Raman spectra, we discuss earlier pro posed structures of the crystal lattices of the A, C, E, and Fphases of the rareearth pyrosilicates. DOI: 10.1134/S0020168515090204 †
INTRODUCTION
Ce3+doped rareearth pyrosilicates, such as Lu2Si2O7 and Gd2Si2O7, find application as radiation detectors [1, 2]. In addition to the preparation of lute tium and gadolinium silicates containing a low rare earth sensitizer (R) concentration, the synthesis of (Gd1 – xRx)2Si2O7 (R = La, Ce) mixed pyrosilicates has been the subject of recent work [3]. The rareearth pyrosilicates have a great diversity of crystal structures, which depend on the nature of the rare earth and the synthesis temperature [4]. Since not all of the rare earth pyrosilicates have been prepared in the form of perfect single crystals, an accurate analysis of their structure presents serious difficulties, and several models have been proposed in interpreting some of the structures of the pyrosilicates [4, 5]. Raman spectros copy allows one to supplement the information obtained by Xray diffraction techniques about the structure of materials. In addition, the possibility of measuring Raman spectra at high temperatures allows one to investigate phase transformations in situ, during heating and cooling of materials, which is important in the case of hightemperature synthesis. In connection with this, the purpose of this work was to gain information about the key features of the phase transformations of a number of rareearth pyro silicates using Raman spectroscopic techniques, in particular at high temperatures. In addition, since Raman spectra have been reported to date for only one rareearth pyrosilicate, Lu2Si2O7 (thortveitite structure) [6], it is necessary to analyze vibrational spectra of the other structure types † Deceased.
of the rareearth pyrosilicates. Such information will offer the possibility of compiling a library of Raman spectra that could be used as “fingerprints” of the var ious structure types of the rareearth pyrosilicates and easily identifying the phase composition of the rare earth pyrosilicates prepared under various conditions. EXPERIMENTAL Crystals for this investigation were prepared by the Czochralski technique and top seeded solution growth (TSSG). Crystals of the lowtemperature forms of the (Gd1 – xRx)2Si2O7 solid solutions were prepared by TSSG (from a hightemperature solution) [3]. The compositions, structure types, and growth techniques of the rareearth pyrosilicates are presented in Table 1, where the structure types of the crystals identified by Xray diffraction techniques are denoted by symbols like in Felshe [4]. In addition to the Raman spectra of the pyrosili cates indicated in Table 1, we studied the spectra of the Lu2SiO5 and Gd2SiO5 oxyorthosilicates prepared by the Czochralski technique and investigated previously [7] and those of LiGd9(SiO4)6O2 and Gd9 1/3(SiO4)6O2 crystals with the apatite structure. The former crystal was grown by the Czochralski process [8], and the lat ter was prepared by TSSG. The Raman spectra of these crystals were used to identify possible inclusions of the phases in question in the pyrosilicates. Raman spectra were measured on a Spex Ramalog 1403 spectrometer with a resolution of 0.8 cm–1 for polycrystalline samples at 20 and 300 K and a resolu tion of 3 cm–1 at high temperatures, up to 1783 K. The excitation source used was an ILA120 argon laser operated at an output power of 0.6 (488.0 nm line) or 0.5 W (514.5 nm line).
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(b) 1
2
2
3
Intensity
Intensity
νs (SiOSi) 1
3 4 5
4
6 200
400
600
800 1000 1200 Raman shift, cm–1
200
400
600
800 1000 1200 Raman shift, cm–1
Fig. 1. (a) 300K Raman spectra of the oxyorthosilicates (1) Lu2SiO5, (2) Gd9 1/3(SiO4)6O2, (3) LiGd9(SiO4)6O2, and (4) Gd2SiO5; (b) Raman spectra of the pyrosilicates (1) CLu2Si2O7, (2) FGd2Si2O7:La (11 mol %), (3) FGd2Si2O7:Ce (15 mol %), (4) АGd2Si2O7:La (11 mol %), and (5) EGd2Si2O7 at 300 K and (6) EGd2Si2O7 at 20 K. The vertical lines mark the region of characteristic νs(Si–O–Si) stretching vibrations of bridge bonds in the pyrosilicates.
The above procedure enabled luminescence lines of a number of rareearth ions present as unintentional impurities in our samples to be excluded from the Raman spectra of the rareearth pyrosilicates. To carry out a more detailed analysis of the Raman spectrum of the Eform of gadolinium pyrosilicate, its Raman spectrum was measured at 20 K using a Ley bold Heraeus cryostat. EXPERIMENTAL RESULTS Figure 1a shows the 300K Raman spectra of the Lu2SiO5 and Gd2SiO5 oxyorthosilicates and those of the LiGd9(SiO4)6O2 and Gd9 1/3(SiO4)6O2 apatite structure silicates. These silicates contain isolated [SiO4]4– groups. However, since the LiGd9(SiO4)6O2
and Gd9 1/3(SiO4)6O2 samples had an imperfect struc ture, their Raman spectra are heavily broadened in comparison with the Raman spectra of the oxyortho silicates. The 300K Raman spectra of the rareearth pyrosilicates with different structures are presented in Fig. 1b. Also shown in Fig. 1b is the 20K Raman spectrum of ЕGd2Si2O7. No Raman lines characteris tic of Lu2SiO5, Gd2SiO5, or Gd9 1/3(SiO4)6O2 were detected in the Raman spectra of the rareearth pyro silicates, indicating that these materials contained no inclusions of oxyorthosilicate phases, which might form in the synthesis process (Fig. 1). As seen in Fig. 1b, the 300K Raman spectra of the pyrosilicates of the structure types A, C, E, and F show three groups of lines in the ranges 60–600, 600–800, and 900– 1200 cm–1. The number and relative intensities of the
Table 1. Compositions, structure types, and growth techniques of the rareearth pyrosilicates Growth charge composition
Structure type
Preparation technique
Lu2Si2O7
Monoclinic, C
Czochralski pulling
Gd2Si2O7
Orthorhombic, E
TSSG
Gd2Si2O7:La (11 mol %)
Tetragonal, A
TSSG
Gd2Si2O7:Ce (15 mol %)
Triclinic, F
TSSG
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(b) F 8
5 4 3
A
Intensity
Intensity
6
7 6 5 4 3 2 1
2 1 200
400
600
800 1000 1200 Raman shift, cm–1
200
400
600
800 1000 1200 Raman shift, cm–1
Fig. 2. Raman spectra of (a) EGd2Si2O7 at (1) 300, (2) 573, (3) 773, (4) 1173, (5) 1573, and (6) 1773 K; (b) asgrown AGd2Si2O7:La (11 mol %) at (1) 300, (2) 973, (3) 1173, (4) 1723, (5) 1748 (onset of the phase transition), (6) 1748 (end point of the phase transition), and (7) 1773 K; and (8) FGd2Si2O7:La (11 mol %) after cooling.
Raman lines in each of these regions depend on the structure type of the materials. In contrast to the Raman spectra of the pyrosilicates, the spectra of the silicates containing isolated [SiO4]4– groups have no lines in the range 600–800 cm–1 (νs(SiOSi) (Fig. 1b). Table 2 presents the 300K Raman frequencies in the various structure types of the pyrosilicates studied here. For Gd2Si2O7, Table 2 gives both the 300 and 20K Raman frequencies. Figure 2 illustrates the behavior of the Raman spec tra of the EGd2Si2O7 and AGd2Si2O7:La (11 mol %) samples on heating from 300 to 1773 K. The Raman spectra of the hightemperature, Еphase of Gd2Si2O7 demonstrated a severe broadening of its lines and a shift of their frequencies to lower frequen cies, which is characteristic of the behavior of the Raman spectra of materials on heating (Fig. 2a). No new lines were detected in Raman spectra of ЕGd2Si2O7 on heating, which suggested that it underwent no phase transformations and that no other phases precipitated in the temperature range exam ined. After heating to 1723 K and subsequent cooling to 300 K, the Raman spectrum of the ЕGd2Si2O7 sample was identical to that of the asgrown crystal. Other trends were detected in Raman spectra of the lowtemperature pyrosilicate АGd2Si2O7:La (11 mol %) (Fig. 2b). At a temperature of 1723 K, a wing of a new line (F) emerged in the frequency range 700–750 cm–1, on the shortwavelength side of a line in the spectrum of the Aphase. As the temperature was raised to 1748 K, the intensity of the F line was well seen to increase, whereas the intensity of the A band dropped. Variations in the relative intensities of the A and F lines at 1748 K continued for 40 min. On further heating to 1773 K after 10 min, the A line completely disap peared. The 300K Raman spectrum of the Gd2Si2O7:La (11 mol %) sample after cooling from INORGANIC MATERIALS
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1773 K showed only lines of the hightemperature, Fphase of the rareearth pyrosilicates (Fig. 2b, spec trum 6). Thus, the Raman spectra in Fig. 2b illustrate the A → F phase transformation of the Gd2Si2O7:La (11 mol %) pyrosilicate. The transformation tempera ture determined by us for the sample under consider ation lies in the range 1730–1770 K, which approaches the maximum temperature of the A → F phase transition, which is observed according to Felshe [4] in europium pyrosilicate. DISCUSSION Since the number and frequencies of lines in Raman spectra are related to crystal lattice symmetry, consider the key features of the above Raman spectra of the rareearth pyrosilicates of different structure types in relation to available data on their structure. The pyrosilicates contain [Si2O7]6– anions with strong covalent bonding in the form of two cornersharing silicon–oxygen tetrahedra. This allows one to treat the vibrational spectrum of such complexes in terms of an approximation applied in the case of molecular crys tals. The Raman and IR absorption spectra for the internal vibrations of the [Si2O7]6– anion in crystals lie in the highfrequency region and reflect structural fea tures of the crystal lattice and their changes upon phase transformations. The vibrational spectra of the [Si2O7]6– anion were considered in detail in Lazarev [10] and Lazarev et al. [11]. It follows from a group theoretical analysis of a “free [Si2O7]6– anion” of D3d symmetry that its total vibrational spectrum can be represented as Γ = 3A1g + A2g + 4Eg + A1u + 4A2u + 5Eu. In the internal vibration spectrum of the [Si2O7]6– anion, it is convenient to single out the characteristic
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Table 2. Vibrational frequencies in the Raman spectra of some pyrosilicates ν, cm–1 20 K
300 K EGd2Si2O7
40
74 81 85 93 97 104 112 119 126 132 138
38 49 62 72 79 85 92 101 109 116 125 134 143 158
AGd2Si2O7
FGd2Si2O7
62 75
57 65 74
85 94
85 93
110 119 125 133 140 148 154 165 172 176 185 194 201 210 215 230
108 117 123 129 141 148 154 164
162 167 170 182 193 197 209 215
205 211
234 240 248
228 237 245
270 277 292 299
265 272 288
313 324
312 323
323
345
339
334
170 180 189
244 267 277 286 293
356 371 374 381
415 429
372 380
413 431
467
467
502
502
Assignment
93
148 161
177 181 199 204 211 νdef + νexternal
229 249 258 280
283
301 312 323 327 335 340 358 371 376
375 388 405 418 427 435 442 460 471 489 498
Lu2Si2O7
380 387 410 420 430
419 442
470 479
489
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Table 2. (Contd.) ν, cm–1 20 K
300 K EGd2Si2O7
Assignment
AGd2Si2O7
FGd2Si2O7
Lu2Si2O7
517 527
525
585
508 525 537 546 559 570 578
588 602
598 602
669 681
627
626
706
842
840
903
843 850 859 876 883 902
949
915 944
516 522 530 545 561 574
904 947
515 519 546 561
νdef + νexternal
549 573 580 670 740 826 833 862 874
νs(SiO3)
900 910 939
926
951 963
975
972
1044
+ νas(SiO3) +
967
1005 1028
νs(SiOSi)
962
1000 1022
ν 's (SiO3)
972
968
+
982
996
ν'as (SiO3)
1003 1017
1006
+ νas(SiOSi)
1036 1043 1048 1061 1072 1086
1040
958
1077 1097
νs(SiOSi and νas(SiOSi) stretching modes of the bridge bonds between the tetrahedra and vibrations of the ter minal (SiO3) groups: νs(SiO3), ν 's (SiO3), νas(SiO3), and ν 'as (SiO3) [11].
tion with available Xray diffraction data on their structure.
Consider the Raman spectra of the observed struc ture types of the rareearth pyrosilicates in conjunc
Lu2Si2O7 should correspond to this structure type. Grouptheoretical analysis allows one to obtain the
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(1) Structure type C, sp. gr. C 2/m (C23h ), two formula units per unit cell (Z = 2) (thortveitite structure) [4].
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νs(SiOSi) 664
νs' (SiO3)
νas(SiOSi) 1
Intensity
903
626 598 585
1000 963 949 972 1022 1040
840
706 681
859
944 1003 883 972 1017 915
826
700
1061
3
996
740
600
2
800
900 874
900
968
1006
939
1000
1077 1097
4
1100 Raman shift, cm–1
Fig. 3. 300K Raman spectra of different structure types of the rareearth pyrosilicates in the highfrequency region: (1) Lu2Si2O7, (2) EGd2Si2O7, (3) AGd2Si2O7:La (11 mol %), (4) FGd2Si2O7:La (11 mol %). The notations νs(SiOSi), ν's (SiO3), νs(SiO3), ν'as (SiO3), νas(SiO3), and νas(SiOSi) correspond to the characteristic vibrational frequencies of “free” Si2O7 anions.
following set of vibrational modes for lutetium pyrosil icate: Γ = (8Ag + 7Bg)(Raman) + (6Au + 9Bu)(IR) + (Au + 2Bu)(acoust). Since this structure has a center of inversion, according to the rule of mutual exclusion the Raman spectrum should have only 15 vibrational modes, another 15 modes are IRactive, and three species are acoustic modes. It follows from Table 2 that 14 lines were detected in the Raman spectrum of Lu2Si2O7, which is consistent with the vibrational spectrum cal culated for the space group C2/m (C23h ). There was also agreement in finer details of the Raman spectrum of Lu2Si2O7 in the frequency range of the internal vibra tional modes νs(SiOSi), νas(SiOSi), νs(SiO3), ν 's (SiO3) and νas(SiO3), and ν 'as (SiO3) (Fig. 3, spectrum 1). As pointed out by Lazarev et al. [11], the internal vibra tion spectrum of [Si2O7]6– in the thortveitite structure almost completely reflects all the distinctive features of the spectrum of a “free” [Si2O7]6– anion. In connec
tion with this, the Raman spectrum of Lu2Si2O7 has a very simple form in the region of internal vibrations of the bridge bond, νs(SiOSi) (Ag line), and in the region of highfrequency vibrations of the terminal groups, νs(SiO3) (Ag line) and νas(SiO3) (Ag + Bg band) (Fig. 3, spectrum 1). Spectrum 1 in Fig. 3 indicates the corre spondence of the lines observed in the Raman spec trum of Lu2Si2O7 to the vibrational spectrum of a “free” [Si2O7]6– anion in the highfrequency region. The vibrational frequencies νas(SiOSi), ν 's (SiO3), and ν 'as (SiO3), which are Ramanforbidden but were detected in the IR spectrum of a thortveitite crystal [11], are also represented in spectrum 1 in Fig. 3. Analysis of survey Raman spectra and the internal vibration spectrum of the [Si2O7]6– anion in the Lu2Si2O7 crystal indicates that these spectra are com pletely consistent with the crystal structure of thortvei tite determined by Xray diffraction. (2) For the structure type E, there is no unique result as to the structure of the rareearth pyrosilicates, including Gd2Si2O7. Based on Xray structure analysis INORGANIC MATERIALS
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results, Felshe [4] considered two orthorhombic E structures for this phase, with space groups Pnam ( D216h ) and Pna21 (C29v ), Z = 4. For space group Pna21 ( C29v ), Z = 4, Felshe [4] determined the positional parameters of all the atoms: the R cations and [Si2O7]6– anions were shown to occupy sites with the lowest symmetry C1. Based on these data, we carried out a grouptheo retical analysis of the vibrational spectrum of the E structure for C29v lattice symmetry. Since the C29v struc ture has no center of inversion, the Raman spectrum of such a crystal has 129 vibrational modes: Γ = 32A1 + 33A2 + 32B1 + 32B2. Its internal vibration spectrum should correspond to Γ = 27A1 + 27A2 + 27B1 + 27B2. Since species of A1, B1, and B2 symmetries are both Raman and IRactive, the number of Raman lines with these symmetries may be doubled owing to the LO–TO splitting effect. In Raman spectra of a crystal with the E structure, we detected at most 55 lines, that is, half the number suggested by calculation for the C29v structure. Accordingly, in the νs(SiOSi) region of vibrations of the bridge bond, we observed three lines, instead of the four predicted by calculation, and in the region of νas(SiOSi), νs(SiO3), and ν 's (SiO3) vibrations we observed nine lines instead of the 28 predicted by calculation (Fig. 3, spectrum 2; Table 2). Thus, the number of lines observed in the Raman spectra of the rareearth pyrosilicate crystals with the E structure differs markedly from that in the C29v model proposed for this structure. Given this, we calculated vibrational spectra for 16 D2h symmetry as a model for the E structure. In this structure, with four formula units per primitive unit cell (Z = 4), the cations should occupy C1 sites and the [Si2O7]6– anions should reside in sites of Cs or Сi sym metry. Since the D216h lattice has a center of inversion, some vibrations are Ramanactive and some are IR active. Moreover, there are vibrations that are both Raman and IRinactive: Au. In the case of Ci site sym metry, the total vibrational representation for the [Si2O7]6– anion has the form ( inactive )
Γ = 15(Ag + B1g + B2g + B3g)(Raman) + 14 A u + (18B1u + 13B2u + 18B3u)(IR)
and implies that the [Si2O7]6– anion has 14 (Ag + B1g + B2g + B3g) Ramanactive internal vibrational modes. Comparison of the Raman spectra calculated for the 16 D2h model of the E crystal structure with measured spectra lends support to this model for the E structure. It follows from Table 2 that the measured Raman spec trum of the Ephase shows 55 vibrational modes (instead of the 60 predicted by calculation). In the νs(SiOSi) spectral region, we observed three lines (four INORGANIC MATERIALS
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bands in the calculated spectrum). Accordingly, spec trum 2 in Fig. 3 has eight lines in the region of νas(SiOSi), νs(SiO3), ν 's (SiO3), and νas(SiO3) vibra tions (12 bands in the calculated spectrum). (3) The structure of the lowtemperature phase A (Gd2Si2O7:La (11 mol %) sample) also cannot be iden tified unambiguously. Two space groups were proposed for the A structure, P4122 ( D43 ) and P41 (C 42 ), with Z = 8 [4]. Both structures have no center of inversion, which suggests that their Raman spectra contain as well some of the IRactive vibrations, except for acoustic modes. A grouptheoretical analysis of the total vibrational spectrum of the D43 structure, with C1 site symmetry for both the cations and [Si2O7]6– anions, was carried out with the representation Γ = 33(A1 + A2 + B1 + B2 + 2E). The Raman spectrum would then have at least 164 lines, Γ(Raman) = 33(A1 + B1 + B2) + 65E, whereas only 67 lines were detected in our experiments (Table 2). In the crystal lattice of the Aphase in the case of the C 42 structure, only one site symmetry (С1) is possible for both the cations and [Si2O7]6– anions [4]. The total vibrational spectrum then has the form Γ = 33(A + B + E). Grouptheoreti cal analysis suggests that the C 42 structure has four for mula units per unit cell (Z = 4). Felshe [4] substanti ated the existence of eight formula units per unit cell by the presence of two types of [Si2O7]6– anions, dif fering in structure. The Raman spectrum of the A structure with C 42 symmetry should then contain 196 lines, 65A + 66B + 65E, in marked contrast with the present experimental data. (4) Two crystal lattices were proposed for the high temperature, triclinic phase F: P1 (C11) and P 1 (C i1) [4]. The former structure is noncentrosymmetric and has one formula unit per primitive unit cell. Accord ingly, its total vibrational spectrum has a simple form: Γ = 33A. The Raman spectrum should then have 30 lines of A symmetry. The measured Raman spec trum of the phase in question had almost twice as many lines: 54 (Table 2). The C11 structure with Z = 4 proposed by Felshe [4] is possible if the C11 lattice con tains four [Si2O7]6– groups differing in structure. The number of observed Raman lines (129) should then be twice the number of bands in the measured Raman spectrum. If we consider the other F structure, of P 1 (C i1) symmetry, a grouptheoretical analysis of its vibrational spectrum suggests that it has two formula units per primitive unit cell and C1 site symmetry for both the cations and anions. The vibrational spectrum of such a structure has the form Γ = 33(Ag + Au). Since it has a center of inversion, the 33 Ag vibrational modes should be Ramanactive. The C i1 structure of the Fphase may have four formula units per unit cell, as
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proposed by Felshe [4], if there are two types of [Si2O7]6– groups differing in structure. Raman spectra should then have 66 Ag lines, of which 54 characterize internal vibrations of the anions. This model agrees well with the observed Raman spectrum of the Fphase, as exemplified by Gd2Si2O7:Ce (15 mol %) (Table 2). In the Raman spectrum of this sample, we detected 54 lines (66 lines predicted by calculation). In the region of highfrequency vibrations (Fig. 3, spec trum 4), we also obtained satisfactory agreement between the calculated and measured spectra. We detected one νs(SiOSi) band (two lines in the calcu lated spectrum) and 13 bands in the frequency range of νas(SiOSi), νs(SiO3), ν 's (SiO3), ν 'as (SiO3), and νas(SiO3) vibrations (14 lines predicted by calculation).
2.
3.
4. 5.
CONCLUSIONS Analysis of the Raman spectra of the C, E, A, and Fphases of the rareearth pyrosilicates allowed us to correct the models proposed previously for their crystal structures. The Raman spectrum of the Сphase completely confirms that this phase has a monoclinic structure of C2/m (C23h ) symmetry. The Raman spectra of the E and Fphases are more con sistent with centrosymmetric structures: orthorhom bic symmetry Pnam ( D216h ) for the Ephase and triclinic symmetry P 1 (Ci1) for the Fphase. The Raman spec trum of the tetragonal phase A differs significantly from the spectra calculated in the two models pro posed for this structure. Additional structural studies are needed to resolve this issue. The A → F phase tran sition of Gd2Si2O7:La (11 mol %) occurs in the tem perature range 1723–1773 K according to the present Raman spectroscopy data and took 40 min in our experiments. ACKNOWLEDGMENTS This work was supported by the Russian Founda tion for Basic Research, project no. 130200707 a. REFERENCES 1. Pidol, L., KahnHarari, A., Viana, B., Ferrand, B., Dorenbos, P., de Haas, J.T.M., van Eijk, C.W.E., and
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Virey, E., Scintillation properties of Lu2Si2O7:Ce3+, a fast and efficient scintillator crystal, J. Phys.: Condens. Matter, 2003, vol. 15, pp. 2091–2102. Haruna, J., Kaneko, J.H., Higuchi, M., Kawamura, S., Saeki, S., Yagi, Y., Ishibashi, H., Fujita, F., and Homma, A., Response function measurement of Gd2Si2O7 scintillator for neutrons, IEEE Nuclear Sci ence Symposium Conference Record N24210, 2007, pp. 1421–1425. Gerasymov, I., Sidletskiy, O., Neicheva, S., Grinyov, B., Baumer, V., Galenin, E., Katrunov, K., Tkachenko, S., Voloshina, O., and Zhukov, A., Growth of bulk gadolin ium pyrosilicate single crystals for scintillators, J. Cryst. Growth, 2011, vol. 318, pp. 805–808. Felshe, J., The Crystal Chemistry of the RareEarth Sili cates, New York: Springer, 1973. Bondar’, I.A., Vinogradov, N.V., Dem’yanets, L.N., et al., Soedineniya redkozemel’nykh elementov. Silikaty, germanaty, fosfaty, arsenaty, vanadaty (RareEarth Compounds: Silicates, Germanates, Phosphates, Arsenates, and Vanadates), Moscow: Nauka, 1983. BretheauRaynal, F., Dalbiez, J.P., Drifford, M., and Blanzat, B., Raman spectroscopic study of thortveitite structure silicates, J. Raman Spectrosc., 1979, vol. 8, pp. 39–42. Voron’ko, Y.K., Sobol’, A.A., Shukshin, V.E., Zagu mennyi, A.I., Zavartsev, Y.D., and Kutovoi, S.A., Spontaneous Raman spectra of the crystalline, molten and vitreous rareearth oxyorthosilicates, Opt. Mater., 2011, vol. 33, pp. 1331–1337. Voron’ko, Yu.K., Sobol’, A.A., Shukshin, V.E., Zagu mennyi, A.I., Zavartsev, Yu.D., and Kutovoi, S.A., Structural transformations in LiGd9(SiO4)6O2 and Ca2Gd8(SiO4)6O2 crystals containing isolated [SiO4] complexes: Raman spectroscopic study, Phys. Solid State, 2012, vol. 54, no. 8, pp. 1635–1642. Voron’ko, Yu.K., Kudryavtsev, A.B., Osiko, V.V., and Sobol’, A.A., Hightemperature Raman scattering study of the melt structure and crystallization pro cesses, Rost Krist., 1988, vol. 16, pp. 178–195. Lazarev, A.N., Kolebatel’nye spektry i stroenie silikatov (Vibrational Spectra and Structure of Silicates), Lenin grad: Nauka, 1968. Lazarev, A.N., Mirgorodskii, A.P., and Ignat’ev, I.S., Kolebatel’nye spektry slozhnykh okislov (Vibrational Spectra of Multicomponent Oxides), Leningrad: Nauka, 1975.
Translated by O. Tsarev
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