High-pressure Raman Spectroscopic Study Research Articles

Raman spectroscopic insights into the transition to the post-post-perovskite phase in NaZnF3

The phase transition sequence of perovskite materials is an important subject in numerous fields ranging from geophysics to material science. The fluoride NaZnF3, adopting the distorted perovskite (Pv) structure (GdFeO3-type), is a model system for studying such transitions. Up to date compression experiments on this compound, limited to pressures of 40 GPa, have identified a perovskite to post-perovskite (pPv, CaIrO3-type structure) transition to occur above 11 GPa followed by yet another transition commencing at 25 GPa and leading to a post-post-perovskite (ppPv) phase whose structure is yet to be identified. Here we present high-pressure Raman spectroscopic study aided by Density Functional Theory (DFT) calculations to explore the phase transition sequence of NaZnF3 at pressures reaching 70 GPa. We confirm that NaZnF3 undergoes a gradual phase transition between the GdFeO3 and CaIrO3 phases starting at 12 GPa. Our results also indicate that above 31 GPa, the CaIrO3 polymorph slowly transforms to the ppPv phase which we unambiguously confirm to adopt the metastable Sb2S3-type structure despite the ground state structure above 23 GPa being La2S3. The transition into Sb2S3-type structure type instead of the La2S3 polytype is most probably due to kinetic effects which also hinder the predicted decomposition of NaZnF3.

High pressure Raman spectroscopic study of phase transformations in Zn(CN)2

High-pressure Raman investigations in Zn(CN)2 has been reported up to a pressure of 10 GPa under hydrostatic and non-hydrostatic conditions. We observe several changes in the spectra including splitting of existing modes, broadening of Raman modes and sudden changes in the slope of the frequency vs pressure curve at ≈1 and 1.6 GPa which indicate that the crystal undergoes phase transitions at these pressures. The crystal amorphizes at the highest pressure of 10 GPa. On gradual pressure reduction and total pressure release, the sample doesnot return to its original phase. This indicates that Zn(CN)2 transforms irreversibly to an amorphous phase.

Raman spectra of shortite Na2Ca2(CO3)3 compressed up to 8 GPa

ABSTRACTA rare mineral shortite, Na2Ca2(CO3)3, occurs among groundmass minerals in unaltered kimberlites, which suggests its participation in the evolution of kimberlite system. This work presents a high pressure Raman spectroscopic study of natural shortite (Udachnaya east kimberlites) compressed in KBr up to 8 GPa in a diamond anvil cell. At ambient pressure the spectrum contains two strong bands related to symmetric C-O stretching vibrations, four in-plane bending modes, and several low-frequency modes of lattice vibrations. Upon the pressure increase up to 8 GPa, almost all the bands exhibit positive shift with the rate of 1–4 cm−1/GPa for the lattice modes and 3.6 and 3.9 cm−1/GPa for the C-O stretching modes. The shifts of Raman modes are rather regular, which implies the absence of reconstructive phase transitions within the studied pressure range, similarly to the behavior of nyerereite, a related carbonate mineral. However, minor anomalies in the ν/P and FWHM/P dependences, observed at about 2 GPa, suggest some rearrangement and disordering of carbonate groups. The obtained data can be used for the estimation of residual pressure in shortite-bearing inclusions in deep-seated minerals.

Nanofluid lubrication and high pressure Raman studies of oxygen functionalized graphene nanosheets

Ultralow friction coefficient in reduced graphene oxide (rGO) nanofluid was observed at high pressure lubrication conditions. High pressure Raman spectroscopic studies of graphene oxide (GO) and rGO in a hydrostatic pressure medium in diamond anvil cell (DAC) showed an increase in G-band linewidth in GO but this value was decreased in rGO at same pressure range due to the defect relaxation in sp2 networks. Moreover, loss of recovery of G band linewidth in decompression cycles was clearly noticed in both the samples but it was significantly higher in GO due to the irreversible sp3 into sp2 planar phase.

High-pressure synchrotron x-ray diffraction and Raman spectroscopic study of plumbogummite**Project supported by the National Natural Science Foundation of China (Grant Nos. 41473056 and 41472037).

PbAl3(PO4)2(OH,H2O)6, an important environmental mineral, is in-situ studied by synchrotron x-ray diffraction (XRD) and Raman scattering combined with diamond anvil cells (DACs) at pressures up to ∼11.0 GPa and room temperature. The XRD results indicate that plumbogummite does not undergo a phase transition between 0 GPa and 10.9 GPa. Moreover, the c axis is more compressible than the a axis, revealing its anisotropic behavior. The pressure-volume data are fitted to the third-order Birch-Murnaghan equation of state to yield the plumbogummite bulk modulus of 68(1) GPa and of 6.1. The [PO4]3− and [HPO4]2− Raman vibrational modes exhibit scale nearly linearly as a function of pressure. The [PO4]3− stretching modes are generally more sensitive to pressure than the bending modes. The Grüneisen parameters range from −0.07 to 1.19, with an arithmetic mean of approximately 0.39.

Evidence of iso-structural phase transition in high pressure Raman spectroscopic studies of CaCu 3 Ti 4 O 12

Abstract High pressure Raman spectroscopic studies on Polycrystalline CaCu 3 Ti 4 O 12 and SrCu 3 Ti 4 O 12 compounds are carried out in order to validate the theoretical predictions of a structural transformation near 3–4 GPa and an insulator to metal transition above 7 GPa [S. B. Fagan et al., Phys. Rev. B 72 , 014106 (2005)]. Our studies showed normal hardening and broadening of peaks with increasing pressure up to 20 GPa besides a discontinuity between 8 and 9 GPa which is attributed to an isostructural phase transition. The Ag(1) peak occurring at 442 cm −1 showed asymmetric Fano lineshape and the lineshape parameter ‘ q ’ shows an increase with increasing pressure. This is completely against the theoretical predictions of an insulator to metal transition above 7 GPa.

Pressure-induced phase transitions in the CdCr2Se4 spinel

We have conducted high-pressure x-ray diffraction and Raman spectroscopic studies on the $\mathrm{CdC}{\mathrm{r}}_{2}\mathrm{S}{\mathrm{e}}_{4}$ spinel at room temperature up to 42 GPa. We have resolved three structural transitions up to 42 GPa, i.e., the starting $Fd\overline{3}m$ phase transforms at $\ensuremath{\sim}11\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$ into a tetragonal $I{4}_{1}/amd$ structure, an orthorhombic distortion was observed at $\ensuremath{\sim}15\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, whereas structural disorder initiates beyond 25 GPa. Our ab initio density functional theory studies successfully reproduced the observed crystalline-to-crystalline structural transitions. In addition, our calculations propose an antiferromagnetic ordering as a potential magnetic ground state for the high-pressure tetragonal and orthorhombic modifications, compared with the starting ferromagnetic phase. Furthermore, the computational results indicate that all phases remain insulating in their stability pressure range, with a direct-to-indirect band gap transition for the $Fd\overline{3}m$ phase taking place at 5 GPa. We attempted also to offer an explanation behind the peculiar first-order character of the $Fd\overline{3}m(\mathrm{cubic})\ensuremath{\rightarrow}I{4}_{1}/amd$ (tetragonal) transition observed for several relevant Cr spinels, i.e., the sizeable volume change at the transition point, which is not expected from space group symmetry considerations. We detected a clear correlation between the cubic-tetragonal transition pressures and the next-nearest-neighbor magnetic exchange interactions for the Cr-bearing sulfide and selenide members, a strong indication that the cubic-tetragonal transitions in these systems are principally governed by magnetic effects.

High-pressure X-ray diffraction and Raman spectroscopy of CaFe2O4-type β-CaCr2O4

In situ high-pressure synchrotron X-ray diffraction and Raman spectroscopic studies of orthorhombic CaFe2O4-type β-CaCr2O4 chromite were carried out up to 16.2 and 32.0 GPa at room temperature using multi-anvil apparatus and diamond anvil cell, respectively. No phase transition was observed in this study. Fitting a third-order Birch–Murnaghan equation of state to the P–V data yields a zero-pressure volume of V 0 = 286.8(1) A3, an isothermal bulk modulus of K 0 = 183(5) GPa and the first pressure derivative of isothermal bulk modulus K 0′ = 4.1(8). Analyses of axial compressibilities show anisotropic elasticity for β-CaCr2O4 since the a-axis is more compressible than the b- and c-axis. Based on the obtained and previous results, the compressibility of several CaFe2O4-type phases was compared. The high-pressure Raman spectra of β-CaCr2O4 were analyzed to determine the pressure dependences and mode Gruneisen parameters of Raman-active bands. The thermal Gruneisen parameter of β-CaCr2O4 is determined to be 0.93(2), which is smaller than those of CaFe2O4-type CaAl2O4 and MgAl2O4.

Comparative high pressure Raman studies on perfluorohexane and perfluoroheptane

High pressure Raman spectroscopic studies on perfluorohexane and perfluoroheptane have performed up to 12GPa. Perfluorohexane under goes two pressure induced transitions: (1) liquid–solid transition at 1.6GPa and (2) solid–solid transition at 8.2GPa. On the contrary, perfluoroheptane under goes three phase transitions, they are as follows: (1) liquid–solid transition at 1.3GPa, (2) intermediate solid I transition at 3GPa, (3) solid II transition at 7GPa. The change in slope (dω/dP) shows that the solid I transition at 3.0GPa could be the conversion of mid-gauche defect into trans conformers for perfluoroheptane. The pressure induced Raman spectra and the behavior of individual band with pressure shows that the solid phase comprises more than one conformer beyond crystallization. The intensity ratio for both the compounds shows that the high pressure phase beyond 8.2 and 7.0GPa tends to have close packing with distorted all-trans conformers.

Spectroscopic evidence of irreversible changes in cesium loaded iron phosphate glasses under pressure

Detailed Raman spectroscopic studies on 40:60 iron phosphate glasses (IPG) and cesium doped IPG under ambient conditions revealed an evident disproportionation of the pyrophosphate network beyond 18mol% Cs doping. In situ high pressure Raman spectroscopic studies were carried out on IPG and Cs doped IPG up to 40GPa for the first time in order to investigate the structural stability of these glasses under pressure. Although the parent glass is found to be robust to such high pressures, pressure quenched samples loaded with cesium beyond 18mol% concentration were observed to permanently cross link leading to new high density forms of the glass. The extent of cross linking is observed to systematically increase with the cesium content in the parent matrix.

High pressure X-ray diffraction and Raman spectroscopic studies of the phase change of D2O ice VII at approximately 11 GPa

High pressure experiments were performed on D2O ice VII using a diamond anvil cell in a pressure range of 2.0–60 GPa at room temperature. In situ X-ray diffractometry revealed that the structure changed from cubic to a low symmetry phase at approximately 11 GPa, based on the observed splitting of the cubic structure's diffraction lines. Heating treatments were added for the samples to reduce the effect of non-hydrostatic stress. After heating, splitting diffraction lines became sharp and the splitting was clearly retained. Although symmetry and structure of the transformed phase have not been determined, change in volumes vs. pressure was calculated, assuming that the low-symmetry phase had a tetragonal structure. The bulk modulus calculated for the low-symmetry phase was slightly larger than that for the cubic structure. In Raman spectroscopy, the squared vibrational frequencies of ν1 (A1g), as a function of pressure, showed a clear change in the slope at 11–13 GPa. The full width at half maxima of the O-D modes decreased with increasing pressure, reaching a minimum at approximately 11 GPa, and increased again above 11 GPa. These results evidently support the existence of phase change at approximately 11 GPa for D2O ice VII.

High pressure Raman spectroscopic study of phase transformation in TaO2F

High pressure Raman spectroscopic measurements on nearly zero thermal expansion material TaO2F are carried out up to 19GPa. Earlier report of high pressure X-ray diffraction studies shows two phase transitions, one at 0.7 and the other at 4GPa with rhombohedral (R-3c) structure above 4GPa, but the structure between 0.7GPa and 4GPa remained unclear. In high pressure Raman measurements, a reversible, cubic to rhombohedral phase transformation onsets around 0.8GPa and gets completed at 4.4GPa with all four predicted normal modes corresponding to R-3c phase and retaining the structure up to 19GPa. A mixture of cubic and rhombohedral phases is observed between 0.8 and 4.4GPa. Optically silent modes in the ambient cubic structure exhibit strong, broad Raman bands due to anionic (O/F) disorder in TaO2F altering the local symmetry and allowing for first order Raman scattering. On compression, these disorder induced first order Raman bands gradually decrease in intensity and disappear around 4.4GPa due to inhibition of local distortion caused by anions, and the modes corresponding to the rhombohedral phase appear. This is a clear evidence of disorder-free rhombohedral single phase exists above 4.4GPa in agreement with the reported HPXRD results. Temperature dependent Raman measurements reveal that the intensities of Raman bands remain almost unchanged with rise in temperature indicating static disorder in TaO2F. Disorder-induced first order Raman modes at 176, 212, 381 and 485cm−1 soften with increase in pressure whereas the other modes show low positive Gruneisen parameter. The thermal expansion coefficient calculated using these Gruneisen parameters (−2.91ppmK−1) is in fair agreement with the reported values (−1 to +1ppmK−1). On the other hand, all four modes of disorder-free rhombohedral phase show the usual hardening behavior with increase in pressure contributing to positive thermal expansion.

Hingeless negative linear compression in the mechanochromic gold complex [(C6F5Au)2(μ-1,4-diisocyanobenzene)

The compression of a crystalline material under hydrostatic pressure always results in a reduction in volume of the solid, with the observed reduction typically occurring in all crystallographic cell parameters.1 However, there are a number of exceptional materials that have been shown to expand in a specific direction upon hydrostatic compression while maintaining a positive volume compression. The phenomenon of uni-or biaxial expansion under compression is known as negative linear compressibility (NLC).1 Rare examples of NLC have attracted attention recently owing to the potential applications in a range of materials, including body armor, artificial muscle actuators, and pressure sensors.2

High-pressure resonance Raman spectroscopic study of ultramarine blue pigment

The resonance Raman spectrum of ultramarine has been studied as a function of pressure up to 5GPa. The overtone progressions of the ν1 fundamental and the combination bands of S3- and the ν1 band of S2- were measured and the pressure dependences were determined. The symmetric stretching mode of S3- is nearly harmonic. No phase transition was observed over the measured pressure range.

High-pressure Raman spectra of Sr-substituted γ-Ca3−x Sr x (PO4)2

By using diamond anvil cell (DAC), high-pressure Raman spectroscopic studies of Sr-substituted γ -Ca3−x Sr x (PO4)2 (x=0.5, 1, 2) were carried out up to 30.7, 32.4 and 31.4 GPa, respectively. No pressure-induced phase transformation was observed in the present studies. The behaviors of the phosphate modes in γ -Ca3−x Sr x (PO4)2 (x=0.5, 1, 2) below 14.4 GPa were quantitatively analyzed since a methanol:ethanol:water (16:3:1) mixture was used as the pressure medium in DAC. The Raman wavenumbers of all PO4 vibrations increased linearly and continuously with pressure for γ -Ca3−x Sr x (PO4)2 (x=0.5, 1, 2). With the increasing content of Sr, the corresponding pressure coefficients α changed, and the v i0 shifts to higher wavenumbers except that for ν2 bending vibration.

High-pressure and high-temperature Raman spectroscopic study of hydrous wadsleyite (β-Mg2SiO4)

In situ Raman spectra of hydrous wadsleyite (β-Mg2SiO4) with ~1.5 wt% H2O, synthesized at 18 GPa and 1,400°C, have been measured in an externally heated diamond anvil cell up to 15.5 GPa and 673 K. With increasing pressure (at room temperature), the three most intense bands at ~549, 720 and 917 cm−1 shift continuously to higher frequencies, while with increasing temperature at 14.5 GPa, these bands generally shift to lower frequencies. The temperature-induced frequency shifts at 14.5 GPa are significantly different from those at ambient pressure. Moreover, two new bands at ~714 and ~550 cm−1 become progressively significant above 333 and 553 K, respectively, and disappear upon cooling to room temperature. No corresponding Raman modes of these two new bands were reported for wadsleyite at ambient conditions, and they are thus probably related to thermally activated processes (vibration modes) at high-pressure and temperature conditions.

High-pressure Raman spectroscopic studies on orthophosphates Ba 3(PO 4) 2 and Sr 3(PO 4) 2

By using diamond anvil cell (DAC), high-pressure Raman spectroscopic studies of orthophosphates Ba 3(PO 4) 2 and Sr 3(PO 4) 2 were carried out up to 30.7 and 30.1 GPa, respectively. No pressure-induced phase transition was found in the studies. A methanol:ethanol:water (16:3:1) mixture was used as pressure medium in DAC, which is expected to exhibit nearly hydrostatic behavior up to about 14.4 GPa at room temperature. The behaviors of the phosphate modes in Ba 3(PO 4) 2 and Sr 3(PO 4) 2 below 14.4 GPa were quantitatively analyzed. The Raman shift of all modes increased linearly and continuously with pressure in Ba 3(PO 4) 2 and Sr 3(PO 4) 2. The pressure coefficients of the phosphate modes in Ba 3(PO 4) 2 range from 2.8179 to 3.4186 cm −1 GPa −1 for ν 3 , 2.9609 cm −1 GPa −1 for ν 1 , from 0.9855 to 1.8085 cm −1 GPa −1 for ν 4 , and 1.4330 cm −1 GPa −1 for ν 2 , and the pressure coefficients of the phosphate modes in Sr 3(PO 4) 2 range from 3.4247 to 4.3765 cm −1 GPa −1 for ν 3 , 3.7808 cm −1 GPa −1 for ν 1 , from 1.1005 to 1.9244 cm −1 GPa −1 for ν 4 , and 1.5647 cm −1 GPa −1 for ν 2 .

High-pressure Raman and infrared spectroscopic studies of [formula omitted

High-pressure Raman and mid-infrared spectroscopic studies were carried out on ZrP 2O 7 to 23.2 and 13 GPa respectively. In the pressure range 0.7–4.3 GPa the lattice mode at 248 cm −1 disappears, new modes appear around 380 and 1111 cm −1 and the strong symmetric stretching mode at 476 cm −1 softens, possibly indicating a subtle phase transition. Above 8 GPa all the modes broaden, and all of the Raman modes disappear beyond 18 GPa. On decompression from the highest pressure, 23.2, to 0 GPa all of the modes reappear but with larger full width at half maximum. Lattice dynamics of the high temperature phase of ZrP 2O 7 were studied using first principles method and compared with experimental values.

High-pressure Raman study on the [formula omitted] series

High-pressure Raman spectroscopic studies of the BaSO 4–SrSO 4 binary series are carried out. The variations of the Raman spectra show 1–2 bonding-state transitions. The first pressure transition may be in relation to the re-arrangement of Ba–O/Sr–O or S–O, while the second one may indicate an indistinct structural transition leading to some structural degeneracies. It is observed that the transition occurs at a lower pressure with increasing BaSO 4 molar ratio. Moreover, the radius of the cations is the main factor in the transformations of the sulfates with pressures. Barite (BaSO 4), celestine (SrSO 4), and their solid solutions have the same transitional behavior, and their transitions with pressures are all regarded as reversible.

High pressure raman spectroscopic study of low-cristobalite GaPO4

Gallium orthophosphate of low-cristobalite structure has been studied by Raman spectroscopy up to 28.5 GPa. Three phase transitions have been identified: 1.6, 3.1 and ∼7 GPa. Raman spectra (200 to 700 cm-1) acquired above 3 GPa show signs of six-coordinated gallium ions, in agreement with earlier studies that the orthorhombic C2221 structure transforms into the orthorhombic Cmcm structure. As pressure increases above 8 GPa, the Raman peaks associated with the low-cristobalite framework structure diminish gradually and replaced by peaks that are possibly related to isolated phosphate units. The spectra also show that the structure of the quenched sample retains that of the high-pressure phase prior to decompression.