Raman scattering study on pressure-induced phase transformation of marokite (CaMn2O4)

An in-situ Raman Spectroscopic study was conducted to explore the pressure-induced phase transformation of CaMn₂O₄ to pressures of 73.7 GPa. Group theory yields 24 Raman active modes for CaMn₂O_4, of which 20 are observed at ambient conditions. With the slight compression below 5 GPa, the pressure-induced contraction compensates the structural distortion induced by a Jahn–Teller (JT) effect, resulting in the occurrence of the zero pressure shifts of the JT-related Raman modes. Upon elevation of pressure to nearby 35 GPa, these Raman modes start to display a significant variation in pressure shift, implying the appearance of a pressure-induced phase transformation. Group factor analyses on all possible structure polymorphs indicate that the high-pressure phase is preferentially assigned to an orthorhombic structure, having the CaTi₂O₄ structure. The cooperative JT distortion is continuously reduced in the CaMn₂O₄ polymorph up to 35 GPa. Beyond 35 GPa, it is found that the JT effect was completely suppressed by pressure in the newly formed high-pressure phase. Upon release of pressure, this high-pressure phase transforms to the original CaMn₂O₄ phase, and continuously remains stable to ambient conditions.     

High-pressure x-ray diffraction and Raman spectroscopy of ice VIII

In situ high-pressure/low-temperature synchrotron x-ray diffraction and optical Raman spectroscopy were used to examine the structural properties, equation of state, and vibrational dynamics of ice VIII. The x-ray measurements show that the pressure-volume relations remain smooth up to 23 GPa at 80 K. Although there is no evidence for structural changes to at least 14 GPa, the unit-cell axial ratio ca undergoes changes at 10-14 GPa. Raman measurements carried out at 80 K show that the nu(Tz)A(1g)+nuT(x,y)E(g) lattice modes for the Raman spectra of ice VIII in the lower-frequency regions (50-800 cm(-1)) disappear at around 10 GPa, and then a new peak of approximately 150 cm(-1) appears at 14 GPa. The combined data provide evidence for a transition beginning near 10 GPa. The results are consistent with recent synchrotron far-IR measurements and theoretical calculations. The decompressed phase recovered at ambient pressure transforms to low-density amorphous ice when heated to approximately 125 K.     

Equation of state and structural changes in diaminodinitroethylene under compression

Structural changes in 1,1-diamino-2,2-dinitroethylene (DADNE, FOX-7) compressed to high pressure in diamond anvil cells were investigated using angle-dispersive x-ray diffraction analysis, Raman spectroscopy, and optical polarizing microscopy. The x-ray results show several changes above 1 GPa. When the x-ray data are indexed according to the ambient-pressure structure, DADNE shows anisotropic compression, with higher compression along the b axis than along the a or c axis. An ambient-temperature isothermal equation of state of DADNE was generated from these data. In addition, the experimentally obtained Raman spectra were matched with vibrational normal modes calculated using quantum chemistry calculations. The shifts in vibrational modes indicate changes in H-wagging vibrations with pressure.     

Pressure-induced phase transitions in LiNH2

In situ high-pressure Raman spectroscopy studies on LiNH2 (lithium amide) have been performed at pressures up to 25 GPa. The pressure-induced changes in the Raman spectra of LiNH2 indicates a phase transition that begins at approximately 12 GPa is complete at approximately 14 GPa from ambient-pressure alpha-LiNH2 (tetragonal, I) to a high-pressure phase denoted here as beta-LiNH2. This phase transition is reversible upon decompression with the recovery of the alpha-LiNH2 phase at approximately 8 GPa. The N-H internal stretching modes (nu([NH2]-)) display an increase in frequency with pressure, and a new stretching mode corresponding to high-pressure beta-LiNH2 phase appears at approximately 12.5 GPa. Beyond approximately 14 GPa, the N-H stretching modes settle into two shouldered peaks at lower frequencies. The lattice modes show rich pressure dependence exhibiting multiple splitting and become well-resolved at pressures above approximately 14 GPa. This is indicative of orientational ordering [NH2]- ions in the lattice of the high-pressure beta-LiNH2 phase.     

Synthesis, structural transformation, thermal stability, valence state, and magnetic and electronic properties of PbNiO3 with perovskite- and LiNbO3-type structures

We synthesized two high-pressure polymorphs PbNiO(3) with different structures, a perovskite-type and a LiNbO(3)-type structure, and investigated their formation behavior, detailed structure, structural transformation, thermal stability, valence state of cations, and magnetic and electronic properties. A perovskite-type PbNiO(3) synthesized at 800 °C under a pressure of 3 GPa crystallizes as an orthorhombic GdFeO(3)-type structure with a space group Pnma. The reaction under high pressure was monitored by an in situ energy dispersive X-ray diffraction experiment, which revealed that a perovskit-type phase was formed even at 400 °C under 3 GPa. The obtained perovskite-type phase irreversibly transforms to a LiNbO(3)-type phase with an acentric space group R3c by heat treatment at ambient pressure. The Rietveld structural refinement using synchrotron X-ray diffraction data and the XPS measurement for both the perovskite- and the LiNbO(3)-type phases reveal that both phases possess the valence state of Pb(4+)Ni(2+)O(3). Perovskite-type PbNiO(3) is the first example of the Pb(4+)M(2+)O(3) series, and the first example of the perovskite containing a tetravalent A-site cation without lone pair electrons. The magnetic susceptibility measurement shows that the perovskite- and LiNbO(3)-type PbNiO(3) undergo antiferromagnetic transition at 225 and 205 K, respectively. Both the perovskite- and LiNbO(3)-type phases exhibit semiconducting behavior.     

On the effects of high temperature and high pressure on the hydrogen solubility in rhenium

In situ x-ray diffraction experiments on rhenium hydride compressed up to 46 GPa reveal a hydrogen solubility (x) significantly larger than the previously assumed saturation limit of x ~ 0.38(4). In the layered anti-CdI(2)-type structure of rhenium hydride, the hydrogen solubility was found to increase to x ~ 0.5 at 15 GPa over time. When heated to temperatures above 420 K at pressures above 23 GPa, rhenium hydride undergoes an isomorphous phase transition into the NiAs-type structure accompanied by an increase in hydrogen solubility to x ~ 0.85. The formation of fully stoichiometric rhenium hydride is discussed.     

Structural and chemical properties of the nitrogen-rich energetic material triaminoguanidinium 1-methyl-5-nitriminotetrazolate under pressure

The structural and chemical properties of the bi-molecular, hydrogen-bonded, nitrogen-rich energetic material triaminoguanidinium 1-methyl-5-nitriminotetrazolate C(3)H(12)N(12)O(2) (TAG-MNT) have been investigated at room pressure and under high pressure isothermal compression using powder x-ray diffraction and Raman and infrared spectroscopy. A stiffening of the equation of state and concomitant structural relaxation between 6 and 14 GPa are found to correlate with Raman mode disappearances, frequency discontinuities, and changes in the pressure dependence of modes. These observations manifest the occurrence of a reversible martensitic structural transformation to a new crystalline phase. The onset and vanishing of Fermi resonance in the nitrimine group correlate with the stiffening of the equation of state and phase transition, suggesting a possible connection between these phenomena. Beyond 15 GPa, pressure induces irreversible chemical reactions, culminating in the formation of a polymeric phase by 60 GPa.     

Vibrational dynamics and stability of the high-pressure chain and ring phases in S and Se

The high-pressure phases of group-VI elements sulfur and selenium in their spiral chain and ring structures are examined by in situ Raman and x-ray diffraction techniques combined with first principles electronic structure calculations. The S-II, S-III, Se-I, and Se-VII having spiral chain structures and S-VI with a molecular six-member ring structure are studied in a wide P-T range. The square spiral chain structure of S-III and Se-VII is characterized by seven Raman modes that harden with increasing pressure. The calculations reproduce the observed frequencies and allow the authors to make the mode assignment. The "p-S" and "hplt" phases of sulfur reported by previous Raman studies are identified as S-II and S-III with the triangular and square spiral chain structures, respectively. The phase relations obtained by the x-ray and Raman measurements show that the high-pressure high-temperature phases of sulfur, observed by x-ray, can be induced by laser illumination at room temperature.     

Pressure-induced structural transition in n-pentane: a Raman study

Pressure-induced Raman spectroscopy studies on n-pentane have been carried out up to 17 GPa at ambient temperature. n-Pentane undergoes a liquid-solid transition around 3.0 GPa and a solid-solid transition around 12.3 GPa. The intensity ratio of the Raman modes related to all-trans conformation (1130 cm-1 and 2850 cm-1) to that of gauche conformation (1090 cm-1 and 2922 cm-1) suggests an increase in the gauche population conformers above 12.3 GPa. This is accompanied with broadening of Raman modes above 12.3 GPa. The high-pressure phase of n-pentane above 12.3 GPa is a disordered phase where the carbon chains are kinked. The pressure-induced order-disorder phase transition is different from the behavior of higher hydrocarbon like n-heptane.     

High-pressure induced conformational and phase transformations of 1,2-dichloroethane probed by Raman spectroscopy

1,2-Dichloroethane (DCE) was loaded into diamond anvil cells and compressed up to 30 GPa at room temperature. Pressure-induced transformations were probed using Raman spectroscopy. At pressures below 0.6 GPa, fluid DCE exists in two conformations, gauche and trans in equilibrium, which is shifted to gauche on compression. DCE transforms to a solid phase with exclusive trans conformation upon further compression. All the characteristic Raman shifts remain constant in fluid phase and move to higher frequencies in the solid phase with increasing pressure. At about 4-5 GPa, DCE transforms from a possible disordered phase into a crystalline phase as evidenced by the observation of several lattice modes and peak narrowing. At 8-9 GPa, dramatic changes in Raman patterns of DCE were observed. The splitting of the C-C-Cl bending mode at 325 cm-1, together with the observation of inactive internal mode at 684 cm-1 as well as new lattice modes indicates another pressure-induced phase transformation. All Raman modes exhibit significant changes in pressure dependence at the transformation pressure. The new phase remains crystalline, but likely with a lower symmetry. The observed transformations are reversible in the entire pressure region upon decompression.     

High-pressure vibrational spectroscopy of sulfur dioxide

Solid sulfur dioxide was investigated by vibrational spectroscopy over a broad pressure and temperature range, extending to 32.5 GPa at 75-300 K in diamond anvil cells. Synchrotron infrared spectra provided the first measurements of the pressure dependence of the lattice modes in the far-IR region. Below 17.5 GPa, two fundamentals exhibit splittings enhanced by pressure. The asymmetric stretching mode of SO(2) exhibits a remarkable pressure-induced softening. The observations are consistent with the ambient pressure Raman measurements indicating that SO(2) crystallizes in an acentric cell, but are inconsistent with a previously proposed interpretation that the structure of the high-pressure phase consists of (SO(2))(3) clusters. Dramatic changes in the Raman spectra are found above 17.5 GPa at room temperature. These indicate major changes in structure and possible formation of SO(2) clustering with an enlarged unit cell. The behavior at low temperature differs from that at room temperature. These findings provide constraints on the phase diagram of sulfur dioxide.     

Structural behaviour of alkaline sulfides under compression: high-pressure experimental study on Cs2S

The high-pressure behaviour of cesium sulphide Cs(2)S has been studied up to 19 GPa at room temperature using angle-dispersive x-ray powder diffraction in a diamond-anvil cell. X-ray results show that the initial anticotunnite-type structure (S.G. Pnma) seems to undertake a continuous transformation to a distorted Ni(2)In-type structure (also with S.G. Pnma), starting below 1 GPa and being almost completed at 5 GPa. The profile of the x-ray diffraction patterns did not change noticeably from this pressure to 17 GPa. The observed structural changes in Cs(2)S are discussed in relation to the high-pressure behaviour of the rest of alkaline sulfides and their systematic trends are pointed out. Finally, we discuss the analogies between the structures of alkaline-metal chalcogenides and those of the cationic arrays of their corresponding oxides (sulfates, selenates, and tellurates) comparing the insertion of oxygen and the application of pressure.     

High-pressure vibrational spectroscopy of energetic materials: hexahydro-1,3,5-trinitro-1,3,5-triazine

Vibrational spectroscopy has been used to investigate the room-temperature high-pressure phases of the energetic material hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). The pressure-induced alterations in the spectral profiles were studied in a compression sequence to 30.2 GPa using Raman spectroscopy and to 26.6 GPa using far-infrared spectroscopy. At pressures near 4.0 GPa, several changes become immediately apparent in the Raman spectrum, such as large frequency shifts, mode splittings, and intensity changes, which are associated with a phase transition from alpha-RDX to gamma-RDX. Our study extends the kinetic stability of gamma-RDX to pressures near 18.0 GPa. Evidence for a new phase was found at pressures between 17.8 and 18.8 GPa and is based on the appearance of new vibrational bands and associated changes in intensity patterns. The new phase has vibrational characteristics that are similar to those of beta-RDX, suggesting the two polymorphs share a related crystal structure.     

Pressure-induced phase transformations in LiAlH4

The pressure-induced phase transformations in pure LiAlH4 have been studied using in situ Raman spectroscopy up to 7 GPa. The analyses of Raman spectra reveal a phase transition at approximately 3 GPa from the ambient pressure monoclinic alpha-LiAlH4 phase (P2(1)/c) to a high pressure phase (beta-LiAlH4, reported recently to be monoclinic with space group I4(1)/b) having a distorted [AlH4]- tetrahedron. The Al-H stretching mode softens and shifts dramatically to lower frequencies beyond the phase transformation pressure. The high pressure beta-LiAlH4 phase was pressure quenchable and can be recovered at lower pressures ( approximately 1.2 GPa). The Al-H stretching mode in the quenched state further shifts to lower frequencies, suggesting a weakening of the Al-H bond.     

Raman spectroscopic investigations of pressure-induced phase transitions in n-hexane

We report high-pressure Raman studies on n-hexane up to 16 GPa. n-Hexane undergoes solid-solid transition around 9.1 GPa along with an already reported liquid-solid transition around 1.4 GPa. The intensity ratio of the Raman modes relating the all-trans conformation (1147 and 2872 cm-1) to that of the gauche conformation (1074 and 2923 cm-1) shows a sudden change across 9.1 GPa, suggesting an increase in the all-trans population conformers above 9.1 GPa. The disappearance of the torsional modes suggests a steric hindrance to the methyl end group, similar to the n-heptane case, suggesting that the high-pressure phase (above 9.1 GPa) is an orientationally disordered phase. In general, the transition pressure for the solid-solid transition is inversely proportional to the length of the carbon backbone in the medium chain length n-alkanes.     

高压下铌酸锌钶铁矿结构相变的原位拉曼光谱和X射线衍射研究

通过原位高压拉曼光谱和X射线衍射对ZnNb₂O₆晶体在29 GPa以下的结构转变进行了研究.拉曼光谱显示, 多数拉曼峰强度减弱, 且随着压力增加向高波数方向移动.压力频移曲线分别在10, 16 和20 GPa处形成了拐点.原位X射线衍射谱在10.6 GPa以上有旧峰消失和新峰出现.结果分析表明, ZnNb₂O₆钶铁矿结构压缩过程中发生了一个可逆压致相变, 此相变从10 GPa左右开始, 到16 GPa左右完成, 继续增加压力到20 GPa以上则形成无序状态.     

Compressional, temporal, and compositional behavior of H2-O2 compound formed by high pressure x-ray irradiation

X-ray irradiation was found to convert H(2)O at pressures above 2 GPa into a novel molecular H(2)-O(2) compound. We used optical Raman spectroscopy to explore the behavior of x-ray irradiated H(2)O samples as a function of pressure, time, and composition. The compound was found to be stable over a period of two years, as long as high pressure conditions (>2 GPa) were maintained. The Raman shifts for the H(2) and O(2) vibrons behaved differently from pure H(2) and O(2) as pressure was increased on the compound up to 70 GPa, indicating that it remains a distinct, molecular compound. Based on spectra taken from different locations in a single sample, it appears that multiple forms of the H(2)-O(2) compound exist. The structure and composition of the starting material plays an important role in compound formation, as we found that hydrogen-filled ice clathrate C(2) (H(2))H(2)O did not undergo the same dissociation as observed in ice VII upon x-ray irradiation until pressure was increased to above 10 GPa.     

Structural and dynamical properties of solid ammonia borane under high pressure

The structural and dynamical properties of solid ammonia borane were investigated by means of extensive density functional theory calculation up to 60 GPa. Molecular dynamics simulations suggest that the Cmc2(1) phase found by recent room-temperature x-ray diffraction experiments can be obtained from the Pmn2(1) structure at high pressure and low temperature. Two new high-pressure phases were found on further compression at room temperature. We also found that all three high-pressure phases have proton-ordered structures, and the separation of the NH(3) and BH(3) rotation observed in the simulations can be explained by their distinct rotational energy barriers. The role of dihydrogen bonds in the high-pressure phases is discussed.     

High pressure Raman spectroscopy of single crystals of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX)

To gain insight into the high-pressure polymorphism of RDX, an energetic crystal, Raman spectroscopy results were obtained for hydrostatic (up to 15 GPa) and non-hydrostatic (up to 22 GPa) compressions. Several distinct changes in the spectra were found at 4.0 +/- 0.3 GPa, confirming the alpha-gamma phase transition previously observed in polycrystalline samples. Detailed analyses of pressure-induced changes in the internal and external (lattice) modes revealed several features above 4 GPa: (i) splitting of both the A' and A' ' internal modes, (ii) a significant increase in the pressure dependence of the Raman shift for NO2 modes, and (iii) no apparent change in the number of external modes. It is proposed that the alpha-gamma phase transition leads to a rearrangement between the RDX molecules, which in turn significantly changes the intermolecular interaction experienced by the N-O bonds. Symmetry correlation analyses indicate that the gamma-polymorph may assume one of the three orthorhombic structures: D2h, C2v, or D2. On the basis of the available X-ray data, the D2h factor group is favored over the other structures, and it is proposed that gamma-phase RDX has a space group isomorphous with a point group D2h with eight molecules occupying the C1 symmetry sites, similar to the alpha-phase. It is believed that the factor group splitting can account for the observed increase in the number of modes in the gamma-phase. Spatial mapping of Raman modes in a non-hydrostatically compressed crystal up to 22 GPa revealed a large difference in mode position indicating a pressure gradient across the crystal. No apparent irreversible changes in the Raman spectra were observed under non-hydrostatic compression.     

Vibrational dynamics, intermolecular interactions, and compound formation in GeH4-H2 under pressure

Optical microscopy, spectroscopic and x-ray diffraction studies at high-pressure are used to investigate intermolecular interactions in binary mixtures of germane (GeH(4)) + hydrogen (H(2)). The measurements reveal the formation of a new molecular compound, with the approximate stoichiometry GeH(4)(H(2))(2), when the constituents are compressed above 7.5 GPa. Raman and infrared spectroscopic measurements show multiple H(2) vibrons substantially softened from bulk solid hydrogen. With increasing pressure, the frequencies of several Raman and infrared H(2) vibrons decrease, indicating anomalous attractive interaction for closed-shell, nonpolar molecules. Synchrotron powder x-ray diffraction measurements show that the compound has a structure based on face-centered cubic (fcc) with GeH(4) molecules occupying fcc sites and H(2) molecules likely distributed between O(h) and T(d) sites. Above ca. 17 GPa, GeH(4) molecules in the compound become unstable with respect to decomposition products (Ge + H(2)), however, the compound can be preserved metastably to ca. 27 GPa for time-scales of the order of several hours.