A high pressure Raman study of TeO2 to 30 GPa and pressure-induced phase changes

The effect of pressure on the Raman modes in TeO₂ (paratellurite) has been investigated to 30GPa, using the diamond cell and argon as pressure medium. The pressure dependence of the Raman modes indicates four pressure-induced phase transitions near 1 GPa, 4.5 GPa, 11 GPa and 22 GPa. Of these the first is the well studied second-order transition fromD ₄ ⁴ symmetry toD ₂ ⁴ symmetry, driven by a soft acoustic shear mode instability. The remarkable similarity in the Raman spectra of phases I to IV suggest that only subtle changes in the structure are involved in these phase transitions. The totally different Raman spectral features of phase V indicate major structural changes at the 22GPa transition. It is suggested that this high pressure-phase is similar to PbCl₂-type, from high pressure crystal chemical considerations. The need for a high pressure X-ray diffraction study on TeO₂ is emphasized, to unravel the structure of the various high pressure phases in the system.     

Phonon Spectrum and Structural Transformations at High Pressures in Vanadyl IV Phthalocyanine Crystals

The Raman scattering spectra and crystalline structure of vanadyl IV phthalocyanine (VOPc) at normal and high pressures has been studied. According to the X-ray diffraction data, the initial microcrystalline powder represented a mixture of the triclinic α phase (79%) and the monoclinic β phase (21%) possessing P$$\bar {1}$$ and P21/c symmetry, respectively. Raman spectra of the two phases are similar, but the phonon modes of the β phase are shifted toward higher frequencies (energies). The pressure dependence of the spectra of the α phase has been determined and it is established that the interval of 2.3–3.4 GPa reveals reversible pressure-dependent variations: above 3 GPa, some phonon modes exhibit splitting and the coefficients of pressure-induced (baric) shift for almost all modes show a decrease. A high-pressure feature observed in the Raman spectra can be related to changes in intermolecular interactions in crystalline structure of the α phase. The pressure dependence of the α phase unit cell volume measured at pressures increasing up to 4 GPa is a smooth monotonic function that can be well described by the Murnaghan equation of state. The obtained data were used to calculate the Grüneisen parameters of VOPc phonon modes.     

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.     

Raman spectroscopic and optical absorption study of the high pressure behaviour and polymorphism in KO2

Abstract

Raman scattering, visible absorption, and optical observation studies have been made on polycrystalline potassium superoxide (KO₂) in a diamond anvil cell as a function of pressure and temperature. Three new phases are observed. With increasing pressure at 298 K, KO₂ transforms from the well known modified CaC₂ structure (Phase II), to two new phases (VII, and VIII). The transformation from III to VII occurs at about 3.2GPa. Phase VII transforms to phase VIII at about 4.4GPa. However, in some samples phase VII does not occur and phase II transforms directly into phase VIII at about 4.2 GPa. These structural transformations are indicated by marked changes in the Raman spectrum. The transitions out of phase II are also marked by a discontinuous red shift in the optical absorption edge. From optical observations we have also determined the pressure and temperature dependence of the transitions from phase II to the high temperature cubic (B1) phase I as well as from the high pressure phases VII and VIII to a new nonbirefringent phase IX. This new phase IX has the cubic B2 (CsCl) structure as is shown by our recent X-ray synchrotron experiments.     

硼酸锂系列晶体的高压拉曼散射研究

本文进行了硼酸锂系列晶体的高压拉曼散射及其压致相变的研究。对于三硼酸锂（ＬｉＢ３Ｏ５），我们发现在５．０ＧＰａ有一可逆的晶态到晶态的相变，在２７．０ＧＰａ有一不可逆的晶态到非晶态的相变。二硼酸锂（Ｌｉ２Ｂ４Ｏ７）不可逆压致非晶相变发生在３２．０ＧＰａ附近。对于一硼酸锂，我们研究了０—５５．８ＧＰａ范围内的高压拉曼光谱，只在２．０ＧＰａ发现了一个晶态到晶态的相变，但未发现不可逆压致非晶化现象。在硼酸锂系列晶体中，不可逆压致非晶化的压力随Ｌｉ２Ｏ的含量的增加而升高。硼酸锂晶体中Ｌｉ２Ｏ的含量越高，压致非晶化越不容易发生，这与熔体急冷法制备硼酸锂玻璃的规律是一致的。     

Photo-and pressure-induced transformations in the linear orthorhombic polymer of C<Subscript>60</Subscript>

Stability of the linear orthorhombic polymer of C₆₀ under pressure and laser irradiation is studied by Raman scattering and X-ray diffraction measurements. The Raman spectrum at ambient pressure remains unchanged, in the time scale of the experiment, up to an intensity of 3200 W/cm² of the 514.5 nm line of an Ar⁺ laser, but irreversible changes are observed at higher intensities. The Raman spectra recorded at increased pressure show similar irreversible changes even at the laser intensity as low as 470 W/cm². The X-ray diffraction and Raman measurements of the pressure-treated samples, performed after pressure release, show that the nonirradiated material does not exhibit any changes in the crystal structure and phonon spectra. This behavior indicates a pressure-enhanced photo-induced transformation to a new polymeric phase characterized by a Raman spectrum that differs from those of the other known polymeric phases of C₆₀. The Raman spectra of the phototransformed linear orthorhombic polymer of C₆₀ were measured at a pressure of up to 29 GPa. The pressure dependence of the Raman mode frequencies show singularities near 4 GPa and 15 GPa, respectively, related to a reversible phase transition and an irreversible transformation to a metastable disordered phase. The diffuse Raman spectrum of the disordered phase does not exhibit substantial changes with an increase in pressure up to 29 GPa. The high-pressure phase transforms to a mixture of pristine and dimerized C₆₀, after pressure release and exposure to ambient conditions for 30 h. The text was submitted by the authors in English.     

Vibrational study of phase transitions and conductivity mechanism in H3OUO2PO4.3 H2O (HUP)

Infrared and Raman spectra of polycrystalline H₃OUO₂PO₄.3 H₂O (HUP) have been envestigated at various temperatures between 50 K and 300 K. The most temperature sensitive bands corresponding to PO₄ and H₂O librations, U-OPO₃ stretching and OH stretching vibrations indicate four different phases of HUP and allow to propose a phase transition mechanism from a quasiliquid state of protonated species in R.T. phase to a fully ordered crystal below 100 K. Protonic conductivity mechanism of room and low temperature phases is discussed.     

White-beam X-ray diffraction and radiography studies on high-boron-containing borosilicate glass at high pressures

ABSTRACT

Multi-angle energy-dispersive X-ray diffraction studies and white-beam X-ray radiography were conducted with a cylindrically shaped (1?mm diameter and 0.7?mm high) high-boron-content borosilicate glass sample (17.6% B₂O₃) to a pressure of 13.7?GPa using a Paris-Edinburgh (PE) press at Beamline 16-BM-B, HPCAT of the Advanced Photon Source. The measured structure factor S(q) to large q?=?19 Å^?1 is used to determine information about the internuclear bond distances between various species of atoms within the glass sample. Sample pressure was determined with gold as a pressure standard. The sample height as measured by radiography showed an overall uniaxial compression of 22.5% at 13.7?GPa with 10.6% permanent compaction after decompression to ambient conditions. The reduced pair distribution function G(r) was extracted and Si–O, O–O and Si–Si bond distances were measured as a function of pressure. Raman spectroscopy of the pressure recovered sample as compared to starting material showed blue-shift and changes in intensity and widths of Raman bands associated with silicate and four-coordinated boron.     

Raman and IR study of high-pressure atomic phase of nitrogen

Atomic phase of nitrogen has been studied up to pressure 250 GPa and temperature 3300 K using a shear diamond anvil cell. This phase was synthesized both from azide NaN₃ and molecular N₂. The atomic phase has been interpreted as a cubic gauche (CG) structure by means of Raman and IR absorption spectroscopy procedures. The phase transition to CG begins at pressure 50 GPa and room temperature for NaN₃ and at 127 GPa for N₂. Observed pressure dependencies and degeneration of phonon modes, the selection rules for IR and Raman spectra, as well equilibrium pressure between molecular N₂ and atomic phase of nitrogen agree well with theoretical predictions for CG.     

The study of the vibrational spectra of NH4Cl and NH4Br at high pressures

The vibrational spectra of NH₄Cl at pressures of up to 2.6 GPa and of NH₄Br at pressures of up to 7 GPa are investigated by the method of inelastic incoherent scattering of neutrons. It is found that a linear baric dependence of a librational mode changes its slope above the pressure of transition from a disordered cubic phase into an ordered cubic phase with a CsCl-type structure. The slope of the baric dependence of the transverse optical translational mode remains invariant. Estimates for the Grüneisen parameters are presented and the shape of the potential function is calculated in the one-dimensional approximation for librational vibrations in disordered and ordered cubic phases with a CsCl-type structure. It is shown that the phenomena observed are attributed to the high anharmonicity in the disordered phase.     

Vibrational spectra of the ammonia halides NH4I and NH4F at high pressures

The vibrational spectra of ammonium iodide NH₄I at pressures up to 4.1 GPa and ammonium fluoride NH₄F at pressures up to 4.7 GPa were investigated by inelastic incoherent neutron scattering. The pressure dependences of the transverse optical translational and librational modes were obtained. The behavior of the rotational potential barrier for the ammonium ion as a function of the lattice parameter for disordered and ordered cubic phases of ammonium halides with CsCl type structure were calculated. The results obtained confirm that the transition from an orientationally disordered cubic phase into an ordered cubic phase in ammonium halides occurs at close critical values of the positional parameter of hydrogen (deuterium).     

High pressure raman spectra of crystalline sulfur

Abstract

Well—resolved Raman spectra of crystalline sulfur have been recorded in a diamond anvil cell (DAC) in the pressure range from atmospheric pressure to 50 GPa at room temperature, using an 0.6 m triple spectrograph and a CCD multichannel detector. The spectra indicate two phase transitions in the pressure region between 10 and 15 GPa.     

Evolution of Raman and Mössbauer spectra at high pressures up to 75 GPa and a phase transition in CaSnO<Subscript>3</Subscript> perovskite

Raman and Mössbauer spectra from ¹¹⁹Sn nuclei in CaSnO₃ perovskite have been studied at high pressures up to 75 GPa. A linear increase in the frequency of the main Raman modes and a monotonic decrease in the isomer shift in Mössbauer spectra in the pressure range of 0–40 GPa are established. It is shown that the pressure-induced increase in Raman frequencies can be associated with the variation of the angle between the Sn–O–Sn bonds in chains of oxygen octahedra SnO₆ along the c axis. The sharp variation of the parameters of the Raman and Mössbauer spectra is observed in the pressure region of 40–55 GPa, indicating the structural phase transformations, which can be associated with the transition into the post-perovskite state. Raman spectra of CaSnO₃ samples with the ilmenite structure have been obtained for the first time.     

Raman and infrared investigations at room temperature of the internal modes behaviour in solid nitromethane‐h3 and ‐d3 up to 45 GPa

Raman and infrared spectra of internal phonons in solid nitromethane‐h₃ and ‐d₃ were measured as a function of pressure in the range 0–40 GPa at room temperature. Experiments were performed in diamond anvil cells. The evolution of the splitting of the various modes in condition of nearly hydrostatic compression supports the maintenance of the P2₁2₁2₁ crystal structure until the material chemically transforms into an amorphous phase. The observed pressure‐induced shifts of vibrational wavenumbers are consistent with computations recently reported in the literature. Infrared and Raman spectroscopies deliver complementary information on the internal modes behaviour. The continuous evolution of the infrared band shapes suggests a weak molecular distortion during the compression process. The strong modifications that are observed in the Raman bands of the nitro group are attributed to polarization effects arising from a rearrangement of the molecules inside the unit cell in the pressure range 10–12 GPa, a consequence of a close intermolecular O…H approach. Copyright © 2007 John Wiley & Sons, Ltd.     

Triple point on the melting curve and polymorphism of nitrogen at high pressure

Raman spectra of solid and fluid nitrogen to pressures up to 120 GPa and temperatures up to 2500 K reveal that the melting line exhibits a maximum near 70 GPa, followed by a triple point near 87 GPa, after which the melting temperature rises again. Fluid nitrogen remains molecular over the entire pressure range studied, and there is no sign of a fluid-fluid transition. Solid phases obtained on quenching from the melt above 48 GPa are identical to the recently discovered iota and zeta' phases. We find that kinetics plays a major role in the experimentally observed phase changes and account for the metastability of various crystalline molecular phases and the existence of an amorphous single bonded eta-N.     

高压诱致的反式联苯乙烯酮结构相变与化学反应

 利用金刚石对顶砧（DAC）高压装置在室温下对反式联苯乙烯酮（Trans, Trans-Dibenzylideneacetone）分子晶体进行了高压拉曼谱、荧光光谱和能量色散X射线衍射（EDXRD）研究。结果表明,在压力为1.0~1.3 GPa时,反式联苯乙烯酮发生了晶体—晶体的结构相变,同时开始伴随有压力诱导的化学反应,反应完成的压力为6.5 GPa。高压X射线研究表明,这次相变有新的共价键产生,可能的化学变化过程是,C＝C双键打开再与相邻的分子结合生成新的共价键。在压力大约为11 GPa时,反式联苯乙烯酮分子晶体再次发生了晶体的结构相变。新产生的物质在卸压后依然保持稳定。     

Pressure and photo-induced phase transitions in sulphur investigated by Raman spectroscopy

Abstract

The phase transition of orthorhombic sulphur α-S₈ to a high pressure amorphous sulphur allotrope (a-S) has been investigated by Raman spectroscopy. The conversion is found to be induced by the absorption of laser light and can be discussed in terms of ring opening followed by cis-trans conversion of the dihedral angle of S₈ molecules. Laser energy and transition pressure are correlated due to the pressure tuned red shift of the absorption edge of α-S₈. The amorphous (a-S) phase is observed up to 15 GPa at laser intensities below 30μW/μm² at 514.5 and 488.0 nm. Above this threshold power a-S transforms into a second photo-induced phase (p-S), whose discrete Raman spectrum implies an ordered molecular and crystalline structure. By further increasing pressure crystalline S₆ can be created which is found to be the dominant molecular species at pressures above 10 GPa and low temperatures. A phase diagram in the range T < 300 K and p < 15 GPa is also presented.     

Infrared Spectra of Crystalline,Glassy and Liquid Methanol at High Pressures

Abstract

Mid-infrared spectra in the range 400–1800 cm^?1 of methanol samples in diamond anvil cells at ambient temperature and pressures up to 11 GPa are reported. The freezing pressure is confirmed to be 3.6 GPa, and the spectra of the resulting metastable glass are very similar to those of the liquid. When maintained at high pressure, the glass spontaneously transforms to an ordered crystalline phase which is stable over the range 3.6 to 11 GPa. Small changes in peak wavenumbers for 14 internal modes as a function of pressure are observed, indicating that distortion of the molecules is minimal. A slight decrease for the C-O-H bending mode is attributed to charge transfer from the molecular 0-H bond to the strengthening intermolecular hydrogen bond.     

High pressure study of phase transitions inα-FePO4

High pressure behaviour of FePO₄ in berlinite form has been investigated up to 10 GPa using vibrational Raman spectroscopy and energy dispersive x-ray diffraction. Combination of these techniques along with studies on pressure quenched samples reveal structural transitions in this material from its room pressure trigonal phase to a disordered and a crystalline phase near 3±0.5 GPa. The latter is the Cmcm phase which is the equilibrium structure at high pressures. These high pressure phases do not revert back to its initial structure after release of pressure. Irreversibility of these transformations indicates that FeO₄ tetrahedra do not regain their initial coordination. These high pressure transitions can be rationalized in terms of the three level free energy diagram for such systems.     

Raman scattering and phase changes of CulnSe2 and LiInSe2 at high pressure

Abstract

We have investigated the effect of pressure on the Raman spectra of the ternary chalcogenides CulnSe₂ (chalcopyrite structure) and LiInSe₂ (β-NaFeO₂ structure) for pressures extending well above their first pressure-induced phase transitions. Sign and magnitude of Griineisen parameters are discussed by comparing to related tetrahedrally coordinated chalcogenides. Discontinuous changes of the Raman frequencies indicate pressure-induced phase transitions at 8.0±0.3 and 4.2±0.2 GPa in CuInSe₂ and in LiInSe₂, respectively. The Raman spectra of the low-pressure phases are not recovered after pressure release but a highly disordered structure is induced. In the case of LiInSe₂, the effect of laser heating on the Raman spectra of the high-pressure phase is investigated and discussed in light of recent high-pressure x-ray investigations.