Article pubs.acs.org/JPCC
Contrasting Structural Stabilities and New Pressure-Induced Polymorphic Transitions of Scheelite- and Zircon-Type ZrGeO4 Xuerui Cheng,†,‡ Yufen Ren,† Jimin Shang,† and Yang Song*,‡ †
School of Physics and Electronic Engineering, Zhengzhou University of Light Industry, Zhengzhou, Henan 450002, China Department of Chemistry, The University of Western Ontario, London, Ontario N6A 5B7, Canada
‡
ABSTRACT: As promising optical materials, the structures of scheelite- and zircon-type ZrGeO4 have been investigated by Raman spectroscopy at pressures up to 51.6 GPa using diamond anvil cells. For scheelite-type ZrGeO4, two reversible phase transitions at 16.2 and 41.2 GPa were identified from the Raman spectral profiles and pressure dependence of the Raman shift upon compression. The first transition can be interpreted as the change of the tetragonal scheelite structure to the monoclinic fergusonite structure. By comparison with a previously reported isomorphic crystal, we infer that the second high pressure phase likely has a monoclinic structure. The zircon-type ZrGeO4 initially transforms to a scheelite structure at ∼10.1 GPa, while such phase transformation is completed at around 24.6 GPa, and remains stable up to 37.6 GPa. Moreover, the zircon-to-scheelite phase transition is irreversible, and the high pressure scheelite structure cannot be recovered upon decompression. These phase transitions for different starting ZrGeO4 materials were unambiguously identified for the first time, and their contrasting structural stabilities as well as the transition mechanisms can be understood from the intrinsic characteristics of the crystal lattices. These results contribute to the understanding of the pressure behavior of very sparsely reported germinate compounds in the ABO4 family.
1. INTRODUCTION Among the large ABO4 family of crystals, zirconium germanate (ZrGeO4) either as pure or doped materials has received considerable attention due to technological applications and especially as optical materials.1−4 ZrGeO4 can crystallize in two polymorphs currently known, namely, the zircon structure (space group I41/amd) and the scheelite structure (space group I41/a). Although the scheelite- and zircon-type ZrGeO4 have related structures and are both tetragonally built from the ZrO8 polyhedra and GeO4 tetrahedra, they show quite different physical properties. It is well-known that zircon-type material shows low thermal expansion coefficients, higher bulk modulus, and high radiation stabilities, while scheelite-type material is of great interest due to its higher thermal expansion, high X-ray absorption, and UV luminescence.5−7 Besides the different physical properties, the scheelite-type ZrGeO4 is the thermodynamically stable state, which can be synthesized by the conventional solid state method, while zircon-type ZrGeO4 is metastable, which is difficult to synthesize.8 However, it has been reported recently that single-phase zircon- and scheelitetype ZrGeO4 can be selectively synthesized by controlling the pH value under hydrothermal conditions.8 Furthermore, the metastable zircon-type ZrGeO4 can be transformed to the scheelite-type structure at 1180 °C.9 In addition to temperature and pH, pressure has been extensively investigated as another approach to discover and access new structures or novel properties of materials. Moreover, under extreme high pressure conditions, it is possible to obtain more information about the structural stabilities and thus to synthesize © XXXX American Chemical Society
specific structures selectively. Several experiments and theoretical results have confirmed that the scheelite- and zircon-type compounds are rather sensitive to the external pressure, and pressure-induced phase transitions usually occur for these compounds during compression.10 For instance, scheelite-tofergusonite phase transition for CaWO4, a typical scheelite-type material, has been confirmed at around 10.0 GPa under quasihydrostatic conditions.11−13 The pressure induced phase transition from the zircon to scheelite structure has also been observed for the typical zircon structure material ZrSiO4.14,15 Many other scheelite- and zircon-structured ABO4 ternary oxides have also been extensively studied, and similar results were reported for other ABO4 materials, including tungstates, molybdates, vanadates, and phosphates.16−24 However, the report on the high pressure behavior of germinates is very limited. Recently, Raman spectroscopy indicates a possible structural phase transition occurs around 12.0 GPa for scheelite-type ZrGeO4.25 In contrast, X-ray diffraction results show scheelite-type ZrGeO4 does not undergo any phase transition up to 20 GPa, which has also been confirmed by calculation.26,27 Therefore, despite the experimental efforts made in the past, we have not yet achieved a full understanding of the high pressure structural behavior of scheelite-type ZrGeO4. Moreover, to the best of our knowledge, there have been no studies on the high pressure behavior of zircon-type germinates due to the Received: October 30, 2016 Revised: December 10, 2016 Published: December 15, 2016 A
DOI: 10.1021/acs.jpcc.6b10916 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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analysis, as shown in Figure 1. It can be seen that the structure of the samples is quite different on the basis of the different heat
metastable state. Therefore, additional research is needed to further examine the high pressure behavior of scheelite- and zircon-type ZrGeO4. In this work, scheelite-type and zircon-type ZrGeO4 were successfully synthesized using a hydrothermal method. Then, in situ high pressure Raman experiments on both types of ZrGeO4 were performed at room temperature. By analyzing the Raman spectra of different starting ZrGeO4 materials, several phase transitions were identified for both scheelite- and zircontype ZrGeO4 and possible new structures were examined comprehensively. These results contribute to a more in-depth understanding of the pressure-induced phase transition for the scheelite- and zircon-type ABO4 compounds.
2. EXPERIMENTAL SECTION 2.1. Materials Preparation. Pure scheelite-type and zircontype ZrGeO4 were synthesized using a hydrothermal method. The starting materials were GeO2 (99.99%, Aladdin Ltd., China), ZrOCl2·8H2O (99.99%, Aladdin Ltd., China), and deionized water. First, 8 mmol of ZrOCl2·8H2O was dissolved into 40 mL of deionized water with vigorous stirring for 10 min, and an aqueous solution with a Zr concentration of 0.2 mmol/mL was obtained. Then, 8 mmol of GeO2 was added to this solution with a Ge concentration of 0.2 mmol/mL. After stirring for 20 min, the suspension (40 mL) was poured into a 50 mL Teflon-lined stainless autoclave for hydrothermal treatment at 240 °C for 10 h. After reaction, the autoclave was allowed to cool to room temperature naturally. Finally, the as-prepared precipitation was separated by centrifugation, washed with deionized water and ethanol until the pH value of the rinsed water was 7.0, and dried under at 60 °C in air for 12 h. After cooling down naturally, pure zircon-type ZrGeO4 was obtained. The as-prepared powder of zircon-type ZrGeO4 was further heat treated in an alumina crucible at 900 and 1300 °C in air for 1 h, respectively. Pure scheelite-type ZrGeO4 was obtained after treatment of zircon-type ZrGeO4 at 1300 °C. The final zircon- and scheelite-type ZrGeO4 samples were obtained and characterized. 2.2. X-ray Diffraction Measurements. The crystal structures of the samples were identified by powder X-ray diffraction (XRD) using a Bruker D8 Advance diffractometer with Cu Kα radiation (1.5406 Å) over the 2θ range 10−80°. The refinements of the XRD patterns and the structural analysis were performed using GSAS.28 2.3. High Pressure Raman Measurements. The hydrostatic high pressure experiment was carried out to the sample in a diamond anvil cell (DAC) with two diamonds of 350 μm culet size. A T301 stainless gasket was preindented to a thickness of 60 μm, and a hole of 120 μm in diameter was drilled in the center of the gasket. Argon (Ar) was used as the pressure transmitting medium (PTM). The pressure was measured by the shift of the R1 photoluminescence line of ruby. Both the Raman and fluorescence measurements were performed by using a Renishaw InVia Raman microscope equipped with a charge-coupled device (CCD) detector with a 532 nm laser and edge filters that enable the Raman spectra to be collected above 100 cm−1. Raman spectra were collected in a backscattering geometry using a 2400 g/mm grating. The frequency calibration was conducted with the Raman characteristic 520 cm−1 line of silicon crystal. All measurements were carried out at room temperature.
Figure 1. Background-subtracted XRD patterns of zircon-type and scheelite-type ZrGeO4 obtained after heat treatment at (a) 900 °C and (b) 1300 °C, respectively. Experimental data (crosses) and calculated diffraction patterns (solid lines) are shown together with the residuals of the refinement. The ticks indicate the position of the calculated reflections. The refinement results are a = b = 4.8678(2) Å, c = 10.5299(1) Å, Rwp = 18.6% and Rp = 12.7% for scheelite-type ZrGeO4, a = b = 6.6819(1) Å, c = 6.2556(1) Å, Rwp = 16.2% and Rp = 11.3% for zircon-type ZrGeO4.
treatment condition during the synthesis process. For the as-prepared sample formed under hydrothermal conditions at 240 °C for 10 h, its diffraction peaks correspond to those of the zircon-type ZrGeO4. Moreover, the crystal quality is improved by annealing, and the zircon-type ZrGeO4 can be maintained even after annealing at 900 °C, as shown in Figure 1. However, the zircon-type ZrGeO4 transforms to the scheelite-type structure by heat treatment at 1300 °C. This result is in excellent agreement with the previous literature that the zircon-to-scheelite transition occurs at higher temperature for ZrGeO4.8,9 The observation of no unindexed reflections and high quality structural refinement results suggest the successful synthesis of high purity ZrGeO4 in two distinctive known crystal structures. Figure 2 shows the representative Raman spectra of scheeliteand zircon-type ZrGeO4 measured under ambient conditions. For scheelite-type ZrGeO4 (space group: I41/a), according to the group theory analysis, 13 Raman active modes are predicted: 3Ag + 5Bg + 5Eg. In agreement with the previous report, all 13 expected Raman modes are detected, as shown in Figure 2 with the symmetry assignment summarized in Table 1.25 Out of these 13 Raman active modes, 7 are internal modes originated from GeO4 tetrahedra. The highest frequency mode at 798 cm−1 is the symmetric stretching mode (ν1) of the GeO4 unit with Ag symmetry, while the modes at 775 and 726 cm−1 are identified as the antisymmetric stretching modes (ν3) with Eg and Bg symmetries, respectively. The modes at 563 and 492 cm−1 are the asymmetric bending modes (ν4), while those at 402 and 375 cm−1 are the symmetric bending modes (ν2). The modes of the low frequency region (10.1 GPa)
ω0b −1
dw/dp (cm−1/GPa)
ω0c (cm−1)
dw/dp (cm−1/GPa)
224 335 355 436
−1.2 2.3 3.9 2.2 5.6 6.3 6.3
203 263 295 347 398 404 537 594 788 845
0.8 0.7 0.6 0.4 1.1 2.0 3.2 2.3 1.0 3.0
(cm )
972 1006
This work, collected at 0.1 GPa. bReference 15. cCollected at 15.6 GPa.
3.2. High Pressure Raman Spectra of Scheelite-Type ZrGeO4. Starting from near-ambient pressure, Raman spectra of scheelite-type ZrGeO4 were collected upon compression to 51.6 GPa with selected spectra depicted in Figure 3. As can be seen, upon compression to 16.2 GPa, most of the characteristic Raman peaks shift toward the higher frequencies. However, the modes at 363 and 375 cm−1 gradually shift to the contrary direction and split into two well-resolved peaks under compression. At 16.2 GPa, the shoulder peak at 363 cm−1 is found to finally merge with another mode at 302 cm−1. Furthermore, the intensity of the Raman mode at 289 cm−1 gradually increases during compression to 16.2 GPa, and then decreases at higher pressure. Another notable feature is the observation of a weak shoulder peak at 398 cm−1 present at 16.2 GPa which becomes gradually enhanced with further compression. Simultaneously, the stretching modes at 775 and 798 cm−1gradually emerge into a single peak at 16.2 GPa. All of these features are indicative of a pressure-induced phase transition at 16.2 GPa, in qualitative agreement with the previous observation.25 Upon subsequent compression from 16.2 to 38.5 GPa, no other change is detected, meaning the post-scheelite phase is stable in this pressure region. However, at 41.2 GPa, several
polarized Raman spectra of ZrSiO4.29 However, here, only 7 are observed in the Raman spectrum for zircon-type ZrGeO4 at ambient pressure, similar to the previous reports of ZrSiO4 and HfSiO4 measured using the regular Raman spectroscopy.15,30 It is possible that some polarized phonon modes and some weak Raman modes cannot be discerned in our experiment. Because the zircon-type ZrGeO4 and ZrSiO4 are isomorphic crystals, the symmetry assignment for Raman active modes for zircon-type ZrGeO4 can be inferred on the basis of the data of ZrSiO4 as given in Table 2. The peak at 914 cm−1 is assigned as the ν1(A1g) mode due to the symmetrical stretching of the GeO4 tetrahedra. The peaks at 880 and 793 cm−1 correspond to ν3(Eg) and ν3(B1g) modes resulting from antisymmetric stretching of the GeO4 tetrahedra. The peaks at around 383 and 312 cm−1 arise from ν2(A1g) and ν2(B2g) modes due to the GeO4 bending. The lower frequency modes mainly come from the lattice vibrations. In addition, no extra Raman peaks are observed for both structures, which further confirms the scheelite- and zircon-type structures for the prepared ZrGeO4 samples, in agreement with the XRD results discussed above.
Table 1. Observed Raman Modes for Scheelite-Type ZrGeO4 and High Pressure Phases with Assignments and Pressure Dependence scheelite-type ZrGeO4 (0−12.6 GPa) assignment ΓZr(Bg) ΓZr(Eg) ΓGe(Eg) Γlib(Ag) ΓGe(Bg) Γlib(Eg) ν2(Ag) ν2(Bg) ν4(Eg) ν4(Bg) ν3(Bg) ν3(Eg) ν1(Ag)
a
ω0a
−1
(cm ) 168 180 251 289 302 363 375 402 492 563 726 775 798
−1
fergusonite (16.2−38.5 GPa) −1
−1
−1
post-fergusonite (>41.2 GPa)
ω0 (cm )
dω/dp (cm /GPa)
ω0 (cm )
dw/dp (cm /GPa)
ω0 (cm−1)
dω/dp (cm−1/GPa)
171 182 255 294 306 368 378 406 495 565 729 782 802
0.2 1.0 1.3 0.5 2.6 −1.1 2.5 1.4 3.5 2.6 4.0 4.0 3.7
172 193 272 299 342
0.1 0.5 1.2 0.5 1.2
400 425 549 602 790
1.5 1.8 3.4 2.5 3.5
857
3.2
174 204 302 313 341 372 439 468 493 578 636 664 877 938
0.2 0.7 0.9 0.8 1.3 1.8 0.7 1.5 2.0 1.2 1.3 1.1 1.6 3.1
b
This work, collected at 0.1 GPa. bReference 25. C
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Figure 4. Pressure dependence of the characteristic Raman modes for the scheelite-type ZrGeO4, for which I, II, and III correspond to scheelite, fergusonite, and post-fergusonite phases.
that the post-scheelite phase of ZrGeO4 is highly likely a monoclinic fergusonite phase. However, the transition pressure of 16.2 GPa in the present work is significantly higher than the pressure of 12.0 GPa reported previously.25 The reason may be attributed to the different PTMs used in the two different experiments. Ar was used as the PTM in this work, while the 4:1 methanol−ethanol mixture was used in the previous work. Furthermore, it is interesting to note that the tetrahedral coordination of Ge is retained in fergusonite structure with only a slight distortion for the GeO4 tetrahedron. Therefore, the fergusonite phase is a weakly distorted version of the original scheelite structure, and thus the scheelite-to-fergusonite phase transition is a continuous or quasi-continuous transformation, where no prominent volume collapse and/or significant change in the atomic coordinates takes place. Consequently, drastic changes are not expected in the XRD patterns. The fact that no phase transition was observed previously in high pressure XRD measurements26 could be primarily attributed to such subtle structural changes, which requires further more careful investigation using in situ X-ray diffraction and other characterization methods. Nonetheless, the second phase transition at 41.2 GPa has not been reported previously and is proposed for scheelite-type ZrGeO4 for the first time. The Raman spectra of scheelite-ZrGeO4 have been recorded upon decompression as well, as shown in Figure 5 in the spectral regions of 100−1100 cm−1. When decompressed from 51.6 GPa, the unknown high pressure phase transforms back to the fergusonite phase below 33.5 GPa. This transition is most evidenced by the drastic reduction in the intensity of the new mode observed at 335, 549, and 589 cm−1. The next back transformation, which is the fergusonite phase to the original scheelite phase, is observed below 16 GPa and most clearly visible at around 10.5 GPa, as characterized by the splitting of the modes at 335 and 850 cm−1. Therefore, all of the observed pressure-induced structural changes are completely reversible for scheelite-ZrGeO4, although with minor hysteresis, consistent with continuous or quasi-continuous transformations. 3.3. High Pressure Raman Spectra of Zircon-Type ZrGeO4. To examine the high pressure behavior of another structure, zircon-type ZrGeO4, we have also measured the Raman spectra in situ in DAC up to 37.6 GPa. It is observed that the majority of the Raman characteristic peaks shift toward the higher frequencies during compression, as shown in Figure 6. However, it is important to note that the lowest frequency mode
Figure 3. Selected Raman spectra of scheelite-ZrGeO4 collected at room temperature on compression at pressures of (a) 0.1−25.0 GPa and (b) 29.0−51.6 GPa. The pressures in GPa are labeled for each spectrum. The downward arrows denote the diminished peaks, while the upward arrows denote the new peaks.
modifications in the Raman spectra take place, as shown in Figure 3b. For instance, three extra modes emerge at 335, 549, and 589 cm−1. The intensity of these modes gradually evolves on compression and becomes very prominent at 51.6 GPa. Therefore, these observed changes can be interpreted as the occurrence of another phase transition above 41.2 GPa for scheelite-ZrGeO4. The pressure dependence of the observed Raman modes of scheelite-type ZrGeO4 is examined by plotting the vibrational frequencies as a function of pressure, as shown in Figure 4. Evidently, most Raman characteristic peaks shift toward the higher frequencies during the compression process. However, two distinctive pressure boundaries are clearly observed at 16.2 and 41.2 GPa, respectively, as indicated by the pressure dependence of several Raman modes collectively, corresponding to two phase transitions proposed for the scheelite-type ZrGeO4. The high pressure behaviors of other scheelite-structure ABO4 compounds have been studied extensively. According to the previous experiments and theoretical calculations, most scheelite-structure ABO4 compounds will transform to monoclinic fergusonite structure under high pressure.16−18 More importantly, scheelite-to-fergusonite phase transition has also been confirmed in ThGeO4 by in situ high pressure XRD.31 Given the isomorphic relationship between ThGeO4 and scheelite-type ZrGeO4, therefore, we can confidently propose D
DOI: 10.1021/acs.jpcc.6b10916 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 5. Selected Raman spectra of scheelite-ZrGeO4 during decompression. The downward arrows denote the diminished peaks, while the upward arrows denote the new peaks.
Figure 7. Pressure dependence of the characteristic Raman modes for the zircon-type ZrGeO4, for which I and II correspond to zircon and scheelite phases.
at 191 cm−1 exhibits a softening behavior during compression and splits at 10.1 GPa. At the same time, several new weak peaks appear at 267, 297, 532, and 597 cm−1, while the Raman bands at around 767 cm−1 merged into a broad peak, as marked in Figure 6.
coefficient. In the pressure range 10.1−24.6 GPa, one can observe that several modes exhibit discontinuities in their pressure dependence. The disappearance of several modes in this region, as well as the discontinuities in the pressure coefficients, provides another evidence of a phase transition for zircon-type ZrGeO4. Therefore, a new high pressure phase is produced at 10.1 GPa which coexists with the original zircon phase in the pressure range 10.1−24.6 GPa. Both scheelite and zircon phases have tetragonal structures, built from the ZrO8 polyhedra and GeO4 tetrahedra. Under pressure, tetragonal zircon usually undergoes a first-order phase transition to a scheelite structure, which has also been observed for other zircon-type compounds.32,33 Recently, high pressure studies revealed a zircon to scheelite structural transition for another germinate zircon-type compound, ThGeO4.31 As an isomorphic crystal of ThGeO4, we propose that ZrGeO4 also undergoes the zircon to scheelite phase transition. As shown in Figure 7, when compressing zircon-type ZrGeO4 to 24.6 GPa, at least 10 Raman modes are observed for the new high pressure structure. As discussed above, there are 13 Raman active modes for scheelite-type ZrGeO4 at ambient pressure, as shown in Figures 2 and 3. By comparing the Raman spectra of scheelite-ZrGeO4 with that of the post-zircon phase of ZrGeO4 (see Figure 6) and especially that of the recovered material upon decompression (see discussion below), the structure of the postzircon phase can be unambiguously assigned to the scheeliteZrGeO4. Furthermore, it has been reported that the zircon-toscheelite phase transition is irreversible, and the high pressure scheelite structure can persist after decompression.32,33 Therefore, the zircon-to-scheelite phase transition for ZrGeO4 can be further confirmed by the following decompression experiment. Figure. 8 shows the selected Raman spectra of zircon-type ZrGeO4 during decompression from 37.6 GPa to ambient pressure. It is interesting to note that the characteristic peaks of the initial zircon-ZrGeO4 are not observed during decompression, while all the peaks of the high pressure phase persist. This result indicates that this phase transition is irreversible, and the high pressure phase of zircon-ZrGeO4 cannot be recovered as the pressure is released. Moreover, by comparing the Raman spectra of ZrGeO4, it is evident that the Raman pattern of the high pressure phase of zircon-ZrGeO4 is similar to that of the scheeliteZrGeO4. Consequently, this result further confirms that the new
Figure 6. Selected Raman spectra of zircon-ZrGeO4 collected at room temperature upon compression up to 37.6 GPa. The downward arrows denote the diminished peaks, while the upward arrows denote the new peaks. For comparison purposes, the Raman spectrum of scheeliteZrGeO4 at 12.6 GPa (gray curve) is plotted below that of zircon-ZrGeO4 at 15.6 GPa as a reference to show the similarities.
This indicates the onset of a structural transition for zircon-type ZrGeO4. Upon further compression, the Raman modes of the original zircon phase gradually vanish, especially the antisymmetric stretching modes of the GeO4 tetrahedra at 880 and 918 cm−1, which are finally unresolvable at 24.6 GPa. On the contrary, the new peaks that emerged at 10.1 GPa become more prominent at higher pressures. These changes in the Raman spectra indicate a structural transformation initiated at 10.1 GPa and finally completed at 24.6 GPa. Then, upon further compression to 37.6 GPa, no obvious changes in the Raman spectra are observed, but most Raman peaks become gradually broadened and weakened. The pressure dependences of the characteristic Raman bands that further corroborate the above phase transformations are compiled in Figure 7 for zircon-type ZrGeO4. All of the modes, except the lowest frequency one, exhibit the positive pressure E
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dodecahedra along the c axis, with the chains joined along the a axis by edge-sharing ZrO8 dodecahedra. In both zircon and scheelite structure, the Ge−O distance is around 1.7 Å, while the Zr−O distance is different (2.4 Å for zircon and 2.2 Å for scheelite). Therefore, the Ge−O distances are likely more rigid than the Zr−O distances to external compression. As a result, the GeO4 tetrahedra are less compressible than the ZrO8 bidisphenoids, which makes zircon-ZrGeO4 anisotropic with the c axis less compressible than the a axis. However, in the scheelite structure, the GeO4 tetrahedra are aligned along the a axis, whereas the ZrO8 dodecahedra are intercalated between the GeO4 tetrahedra along the c axis. Therefore, this structure exhibits the opposite behavior that the a axis is the less compressible axis.34 During the zircon-to-scheelite transformation, the [110] direction of zircon is converted to the [001] direction of scheelite by a simple shearing mechanism, resulting in a distortion of the GeO4 tetrahedrons because of pressure-induced contraction.35,36 The disappearance of the highest frequency vibration at 914 cm−1 above 24.6 GPa in Figure 6, which corresponds to the symmetric stretching of the GeO4 tetrahedra, reflects a high degree of distortion of the GeO4 tetrahedra. Consequently, pressure induces a rearrangement of the GeO4 and ZrO8 units and the symmetry reduction during phase transition, resulting in the increase in the number of Raman modes. The reduction of the Zr−O bond in scheelite structure indicates a volume collapse for this transition, in agreement with the previous results that zircon-to-scheelite transition exhibits a large volume collapse around 10%.37 More interestingly, this transformation exhibits a large hysteresis. It is reported that the energetic barriers associated with this transition can not be overcome during decompression unless heated up to 1273 K for ZiSiO4.38 Therefore, zircon is not recoverable from scheelite at ambient pressure for ZrGeO4, as shown in Figure 8. In contrast, the scheelite-to-fergusonite transition is more subtle than the zircon-to-scheelite transition. In the fergusonite structure, the tetrahedral coordination of Ge is retained, with only a slight distortion of the GeO4 tetrahedron. The scheelite− fergusonite transition is a second-order and displacive transition with no volume collapse and no change in the atomic coordination.39 Therefore, drastic changes are not expected in the Raman spectra during scheelite−fergusonite transition. Upon compression to above 41.2 GPa, a new post-fergusonite structure appears, which was also observed in other scheelite-type tungstates and vanadates.40,41 However, the exact crystal structure for this phase for different materials is still controversial. According to ab initio calculations, the post-fergusonite phase is predicted to be the orthorhombic Cmca structure for CaWO4.11 Contrastingly, a monoclinic P21/m structure was recently proposed to be the post-fergusonite phase in CaWO4.42 For SrWO4, although the calculations indicate the fergusonite phase becomes unstable against both the Cmca and the monoclinic LaTaO4-type (P21/c) structures, LaTaO4-type structure composed of TaO6 octahedron and LaO8 polyhedra is reported to be more consistent with the XRD and Raman results.43 The fergusonite− LaTaO4 transition is a first-order reconstructive one, resulting in symmetry reduction during phase transition. If the monoclinic P21/c structure is the new high pressure phase of ZrGeO4, then it would involve an increased coordination for Ge from tetrahedral to octahedral and likely an increase in the Ge−O distances, which will result in the larger number of internal modes. As observed in our experiments, indeed, at least three new Raman active modes were observed during the second phase transition for scheeliteZrGeO4. Therefore, it is highly likely that the second high
Figure 8. Selected Raman spectra of zircon-type ZrGeO4 during decompression. For comparison, the Raman spectra of the scheelite- and zircon-ZrGeO4 measured under ambient conditions are also plotted at the bottom.
high pressure phase of zircon-ZrGeO4 has a scheelite structure. Therefore, our Raman measurements confirmed that an irreversible transition from the zircon to scheelite structure occurs above 10.1 GPa for zircon-ZrGeO4. 3.4. Discussion. Both polymorphic forms of ZrGeO 4 (scheelite- and zircon-type) undergo phase transitions when rendered to high pressure conditions. Two reversible phase transitions occur for scheelite-ZrGeO4 in the pressure range 0− 51.6 GPa, while an irreversible phase transition takes place for zircon-ZrGeO4 up to 37.6 GPa. Although both structures are composed of two basic sublattices, GeO4 tetrahedra and ZrO8 polyhedra, the difference in the high pressure behavior of zirconand scheelite-ZrGeO4 can be associated with the different ordering of ZrO8 dodecahedra and GeO4 tetrahedra. Figure 9
Figure 9. Schematic representation of the unit cells of the zircon, scheelite, fergusonite, and monoclinic structures of ZrGeO4 compounds. Blue balls represent the Zr atoms, yellow balls correspond to the Ge atoms, and the small red balls are the O atoms. The Zr−O bonds and Ge−O bonds are also shown in the form of polyhedra. The pressures of the phase transitions during compression and decompression sequences are also indicated.
shows the unit cells corresponding to different crystal structures of ZrGeO4 compounds. The zircon structure can be considered as chains of alternating edge-sharing GeO4 tetrahedra and ZrO8 F
DOI: 10.1021/acs.jpcc.6b10916 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
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pressure phase of scheelite-ZrGeO4 may be a monoclinic structure, similar to that of CaWO4 or SrWO4.41−43 Ultimately, in situ high pressure synchrotron XRD studies are needed to accurately identify the crystal structure of the post-fergusonite phase of ZrGeO4. Furthermore, it is of great interest to examine the possible new polymorphs of zircon-ZrrGeO4 in an extended pressure−temperature region, especially at high pressure and high temperature simultaneously in situ.
4. CONCLUSIONS Pure scheelite- and zircon-type ZrGeO4 are successfully synthesized using the hydrothermal method. The high pressure structural stabilities of both polymorphs have been investigated by using Raman scattering methods at room temperature. For scheelite-type ZrGeO4, the prominent changes in Raman spectra indicate two reversible pressure-induced phase transitions upon compression up to 51.6 GPa followed by decompression. The first phase transition at 16.2 GPa can be interpreted as the scheelite-to-fergusonite transformation, while the second transition at 41.2 GPa leads to the observation of a new, previously unreported phase. By comparison with another isomorphic crystal, we infer that the new high pressure of ZrGeO4 possibly possesses a monoclinic structure. Additionally, for zircon-ZrGeO4, substantial changes in Raman spectra indicate a pressure-induced zircon-toscheelite transition initiated at 10.1 GPa, but completed only in the pressure region up to 24.6 GPa. In contrast, this phase transition is irreversible, indicating that scheelite phase is the most stable phase among all known polymorphs of ZrGeO4. The contrasting structural stabilities and transformation mechanisms of different polymorphs of ZrGeO4 have been examined in terms of their intrinsic crystal structures. Further experimental and theoretical investigations are needed for a more in-depth understanding of high pressure behaviors for the scheelite- and zircon-type ABO4 compounds.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Yang Song: 0000-0001-7853-3737 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge the financial support of National Natural Science Fund of China (No. 11404292), Fund for Young Teachers of Universities in Henan Province (No. 2014GGJS-085), and the overseas visiting scholarship from China Scholarship Council (No.201509440001).
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DOI: 10.1021/acs.jpcc.6b10916 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.jpcc.6b10916 J. Phys. Chem. C XXXX, XXX, XXX−XXX
