非线性光学晶体的高压拉曼散射研究

 报道了高压下非线性光学晶体KNbO₃（KN）,KIO₃（KI）和KTiOAsO₄（KTA）的Raman散射研究结果,压力引起的Raman光谱非常丰富,在所有的样品中都观察到了压致相变。另外,在KN晶体中发现了非常强的压致振动耦合现象及高压非晶相。对于KTA,建立了Raman光谱变化与其倍频效率随压力提高之间的可能联系。在22 GPa以上,KI晶体的基本组成集团可能由IO₃变为IO₆。     

A high pressure Raman study of KTP and pressure induced phase transitions

Potassium titanate orthrophosphate KTiOPO₄ (KTP) has been studied by high pressure Raman technique to 17 GPa using a diamond cell. The Raman data reveal that two phase transitions occur in the system: one near 5.5 GPa and another near 10 GPa. The Lower-pressure transition is definitely first-order but appears to be driven by the phonon mode near 56 cm⁻¹, which exhibits marked softening. A mean field-like behavior is observed. It is hypothesized that this transition is likely to be from ferroelectric (FE) to an antiferroelectric (AF) phase. The 10 GPa transition may be due to AF-PE (paraelectric) transition driven by pressure from high temperature to room temperature. From the larger pressure responce of the PO₄ vibrational modes it is believed that the PO₄ polyhedral compression is larger than the TiO₆ polyhedral compression. leading to polyhedral tilt transitions.     

Pressure dependence of the Raman spectra of indium sulphide

Raman spectra of InS single crystals have been studied at different hydrostatic pressures up to 1.2 GPa. Mode-Grüneisen parameters have been obtained for Raman-active normal modes. It is shown that the variations observed in Raman spectra with growing pressure can be interpreted from the standpoint of the structural phase transition D¹²_2h → D¹⁷_4h in InS as the hydrostatic pressure continues to increase. The transition pressure has been evaluated at (7 ± 1) GPa.     

High‐pressure polarized Raman spectra of Gd2(MoO4)3: phase transitions and amorphization

Polarized Raman spectra of a single crystal of gadolinium molybdate [Gd₂(MoO₄)₃] were obtained between 1 atm and 7 GPa. Using a mixture of alcohols as the pressure‐transmitting medium, YY, ZZ, XY components of scattering matrices were measured. The ZZ spectra were also obtained in argon. Five phase transitions and amorphization were identified. The first and second transitions are reversible, while amorphization is not. In alcohol, amorphization is observed above 6.5 GPa. With argon as the pressure‐transmitting medium, amorphization is progressive and begins above 3 GPa. The spectral changes with pressure affect the high wavenumber bands attributed to symmetric and antisymmetric MoO₄ stretching modes as well as the very low wavenumber modes such as librations of the tetrahedra. This means that both short‐range and long‐range organizations of the tetrahedra are involved in these phase transitions. The amorphization mechanism and its dependence on the pressure‐transmitting medium are discussed, and the steric hindrance between polyhedra is believed to be the most relevant mechanism. The TO and LO low wavenumber modes of A₁ symmetry, observed in the Y(ZZ)Y and Z(YY)Z geometries, respectively, below 50 cm⁻¹, soften continuously through the first three phases when increasing pressure. The strong A₂ mode observed in the Z(XY)Z spectra exhibits the same anomalous behavior by decreasing from 53 to 46 cm⁻¹ at 2 GPa. The softening of these modes is related to the orientation change of tetrahedra observed by ab initio calculations when the volume of the cell is decreased. These orientation changes can explain the wavenumber decrease of the Mo O stretching modes above 2 GPa, which indicates an increase of Mo coordination. Copyright © 2010 John Wiley & Sons, Ltd.     

Study of the P-T phase diagram of pyridinium perchlorate by X-ray diffraction and Raman spectroscopy

The crystal structure and vibrational spectra of deuterated pyridinium perchlorate (d ₅PyH)ClO₄ (C₅D₅NHClO₄) are studied by means of neutron diffraction in ambient conditions, X-ray diffraction at high pressures up to 3.5 GPa in the temperature range 297–420 K, and Raman spectroscopy at high pressures up to 5.7 GPa. Deuterated pyridinium perchlorate at ambient conditions has rhombohedral structure with the R3m symmetry (paraelectric phase I). Over the pressure range of 0.5–1.2 GPa, the phase II with monoclinic symmetry Cm exists. At pressure P ～ 1.2 GPa, the phase transition to monoclinic phase III with the Pm symmetry is observed at ambient temperature. The lattice parameters, unit cell volume, and frequencies of internal vibrational modes as functions of pressure are obtained for different phases of deuterated pyridinium perchlorate. The P-T phase diagram of (d ₅PyH)ClO₄ over the extended pressure and temperature range is discussed.     

Raman spectroscopy of natural cordierite at high water pressure up to 5 GPa

The high‐pressure behaviour of cordierite, a widespread ring aluminosilicate with channels incorporating fluid compounds (H₂O, CO₂), is characterized by the absence of phase transitions up to 2.5 GPa. However, the distortion of the ring tetrahedra observed previously at 2.3 GPa is supposed to introduce a phase transition at higher pressure, which has not been checked so far. This work presents a high‐pressure Raman spectroscopic study of natural cordierite compressed in water medium up to 4.7 GPa in a diamond anvil cell. At P > 4 GPa, a disordering of both the framework and intrachannel H₂O subsystem is apparent from significant broadening of Raman peaks and the evolution of short‐range order parameters. This is followed by abrupt shifts of the framework and O–H stretching modes at about 4.5 GPa, indicating a first‐order phase transition. Its reversibility is seen from the recovery of the initial spectrum at P < 3 GPa. The shift amplitudes of different framework modes indicate the predominance of distortion over contraction of the framework polyhedra upon this transition. The disordering of the H₂O subsystem in the high‐pressure phase is likely a consequence of distortion of the channel‐forming framework elements, which is supposed to be a driving force of this transition. Copyright © 2011 John Wiley & Sons, Ltd.     

Formation of the high pressure graphite and BC8 phases in a cold compression experiment by Raman scattering

We use Raman scattering to study phase transition in the graphitic g‐BC₈ phase and graphite at high pressure up to 84 GPa. The E_2g Raman active mode of graphite (G peak) can be detected up to 84 GPa. We demonstrate that there is (1) a phase transition in g‐BC₈ and in graphite at 35 GPa and (2) that above 35 GPa, the g‐BC₈ and graphite transform under high pressure to possibly fully sp³‐bonded, disordered hp‐BC₈, and hp‐C phases. Below the phase transition, a polynomial fit to the G peak position versus pressure data yielded a quadratic relation; above the phase transition, it demonstrates linear behavior. The phase transition at high pressure in BC₈ system and graphite is reversible. Quenched hp‐BC₈ and hp‐C phases have the Raman spectrum typical to that of the graphitic phases. Copyright © 2013 John Wiley & Sons, Ltd.     

Raman spectroscopy of melamine at high pressures up to 60 GPa

In this work, the Raman scattering of melamine was studied under high pressure up to 60 GPa. The behavior of the most intensive peaks of the Raman spectrum of melamine, 677 cm^?1 and 985 cm^?1 modes, and their line widths do not show any phase transition or indication of formation of sp ³ bonds. Comparing the behavior of the line width of the Raman peaks of graphite under pressure and that of melamine leads us to conclude that the s-triasine (C–N) ring is more rigid than the C–C graphite ring. High pressure results with melamine suggest that the direct phase transition g-C₃N₄ to dense C₃N₄ phase should occur above 60 GPa.     

Raman study of datolite CaBSiO4(OH) at simultaneously high pressure and high temperature

Using an in situ method of Raman spectroscopy and resistance‐heated diamond anvil cell, the system datolite CaBSiO₄(OH) – water has been investigated at simultaneously high pressure and temperature (up to Р ~5 GPa and Т ~250 °С). Two polymorphic transitions have been observed: (1) pressure‐induced phase transition or the feature in pressure dependence of Raman band wavenumbers at P = 2 GPа and constant T = 22 °С and (2) heating‐induced phase transition at T ~90 °С and P ~5 GPа. The number of Raman bands is retained at the first transition but changed at the second transition. The first transition is mainly distinguished by the changes in the slopes of pressure dependence of Raman peaks at 2 GPa. The second transition is characterized by several strong changes: the wavenumber jumps of major bands, the merging of strong doublets at 378 and 391 cm⁻¹ (values for ambient conditions), the splitting of the intermediate‐intensity band at 292 cm⁻¹, and the transformation of some low‐wavenumber bands at 160–190 cm⁻¹. No spectral and visual signs of overhydration and amorphization have been observed. No noticeable dissolution of datolite in the water medium occurred at 5 GPa and 250 °С after 3 h, which corresponds to typical conditions of the ‘cold’ zones of slab subduction. Copyright © 2014 John Wiley & Sons, Ltd.     

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

A 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.     

Hydrostatic pressure-induced phase transitions in RbMnCl<Subscript>3</Subscript>: Raman spectra and lattice dynamics

Raman scattering spectra of RbMnCl₃ are measured at room temperature under high hydrostatic pressure. The results are interpreted based on first principles lattice dynamics calculations. The experimental data obtained correlate with the calculations in the low frequency domain but disagree slightly in the region of high-frequency vibrations. The transition from the hexagonal to the cubic perovskite phase observed earlier (near 0.7 GPa) was confirmed, and new transitions to lower symmetry distorted phases were discovered (at 1.1 and 5 GPa).     

Phase transition of cadmium fluoride under high pressure

We investigated the high pressure phases of CdF₂ by a joint theoretical and experimental study. The structural and electronic properties of CdF₂ were extensively explored to high pressure by ab initio calculations based on the density functional theory. A structural phase transition from the fluorite-type  (Fm-3m, Z=4) structure to the cotunnite-type (Pnma, Z=4) structure was estimated below 8 GPa, and this phase transition was examined by the high pressure experiments up to 35 GPa at room temperature. Both high pressure angle dispersive X-ray diffraction and Raman spectroscopy experiments provided convincing evidence to verify the phase transition. Our work makes clear pressure-induced phase transitions and structural information of CdF₂ under high pressure.     

Vibrational spectroscopy at very high pressures—XX Raman and far-IR study of the phase transition in paratellurite

Raman and far-IR spectra have been obtained up to ca. 45 kbar [4.5 GPa] for paratellurite (Te0₂) in a diamond anvil cell. At the D₄⁴ to D₂⁴ phase transition ca. 9 kbar an E symmetry mode initially at 122cm^?1 was found to split into two components in both IR and Raman spectra: their coincidence in both types of spectra shows that one component in the Raman spectrum cannot be an LO mode. No other E modes were seen to split. A further phase transition claimed ca. 30 kbar was not confirmed.Paratellurite is exceptionally sensitive to the presence of non-hydrostatic stresses, which greatly influence the phonon frequencies. This behaviour is illustrated by Raman spectra obtained under varying degrees of shear stress.     

Structural and magnetic phase transitions in Pr<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> at high pressures

The crystal structure and Raman spectra of Pr_0.7Ca_0.3MnO₃ manganite at high pressures of up to 30 GPa and the magnetic structure at pressures of up to 1 GPa have been studied. A structural phase transition from the orthorhombic phase of the Pnma symmetry to the high-pressure orthorhombic phase of the Imma symmetry has been observed at P ∼ 15 GPa and room temperature. Anomalies of the pressure dependences of the bending and stretching vibrational modes have been observed in the region of the phase transition. A magnetic phase transition from the initial ferromagnetic ground state (T _C = 120 K) to the A-type antiferromagnetic state (T _N = 140 K) takes place at a relatively low pressure of P = 1 GPa in the low-temperature region. The structural mechanisms of the change of the character of the magnetic ordering have been discussed.     

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 (A_1g), 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.     

Raman scattering study of temperature and hydrostatic pressure phase transitions in Rb2KTiOF5 crystal

Raman spectra of Rb₂KTiOF₅ crystal were obtained and analyzed in the temperature range from 77 to 297 K and under hydrostatic pressure up to 4.2 GPa (at T = 295 K). The experimental results were compared with quantum‐chemical simulation of TiOF₅ pseudo‐octahedron. To interpret effects of lattice ordering, phonon spectra of several ordered phases of Rb₂KTiOF₅ were calculated within ab initio generalized Gordon–Kim model, and ordering of TiOF₅ molecular groups were simulated within Monte Carlo approach. The spectra exhibited orientation disordering in the cubic phase under ambient conditions. Cooling below the phase transition temperature (215 K) leads to partial ordering of the structure. The isotropic perovskite‐like phase was found to undergo first‐order transition into a low‐symmetry anisotropic phase at about 1 GPa. Further compression up to 4.1 GPa did not show any effects associated with phase transitions. Copyright © 2011 John Wiley & Sons, Ltd.     

Pressure‐induced phase transitions in L‐leucine crystal

Raman spectra of a crystal of L ‐leucine, an essential amino acid, were obtained for pressures between 0 and 6 GPa. The results show anomalies at three pressure values, one between 0 and 0.46 GPa, another between 0.8 and 1.46 GPa, and a third at P ∼ 3.6 GPa. The first two anomalies are characterized by the disappearance of lattice modes (which can indicate occurrence of phase transitions), the appearance of several internal modes, or the splitting of modes of high wavenumbers. The changes of internal modes are related to CH and CH₃ unit motions as well as hydrogen bonds, as can be inferred from the behavior of bands associated with CO₂⁻ moieties. The third anomaly is a discrete change of the slopes of the wavenumber versus pressure plots for most modes observed. Further, decompression to ambient pressure generates the original Raman spectrum, showing that the pressure‐induced anomalies undergone by L ‐leucine crystals are reversible. Copyright © 2008 John Wiley & Sons, Ltd.     

Raman scattering study of pressure-induced phase transitions in AIIB2IIIC4VI defect chalcopyrites and spinels

A^IIB₂^IIIC₄^VI defect chalcopyrites (DC) and spinels were investigated by Raman scattering spectroscopy under hydrostatic pressure up to 20 GPa. All these compounds were found to undergo a phase transition to a Raman inactive defect NaCl-type structure. The phase transition is reversible for spinels and irreversible for DC. From the analysis of the pressure behavior of Raman-active modes, it was concluded that the phase transition from spinel to NaCl-type structure is direct in MnIn₂S₄ and CdIn₂S₄, while it occurs via an intermediate LiVO₂-type NaCl superstructure in MgIn₂S₄. The observed differences in the pressures and the paths of the pressure-induced phase transitions in A^IIB₂^IIIC₄^VI compounds are discussed.     

High-pressure Raman and infrared spectroscopic studies of ZrP2O7

High-pressure Raman and mid-infrared spectroscopic studies were carried out on ZrP₂O₇ 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 ZrP₂O₇ were studied using first principles method and compared with experimental values.     

Pressure-induced amorphization of Gd2(MoO4)3: A high pressure Raman investigation

High pressure Raman spectroscopic studies on Gd₂(MoO₄)₃(GMO) have been carried out at ambient temperature in the diamond cell to 10 GPa hydrostatic pressure. These experiments have revealed pressure-induced phase transitions in GMO near 2 GPa and 6.0 GPa. The first transition is from Pba2(β′) phase to another undetermined crystalline phase, designated as phase II, and the second transition is to an amorphized state. On releasing pressure there is a partial reversion to the crystalline state. The Raman data indicate that the amorphization is due to disordering of the MoO₄ tetrahedral units. Further, it is inferred from the nature of the Raman bands in the amorphized material that the Mo-O bond lengths and bond angles have a range of values, instead of a few set values. The results of the present study as well as previous high pressure-high temperature quenching experiments strongly support that pressure-induced amorphization in GMO is a consequence of the kinetically impededβ toα phase transition. The system in frustration becomes disordered. The rare earth trimolybdates crystallizing in theβ′ structure are all expected to undergo similar pressure-induced amorphization.