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.     

Effect of high pressure on the crystal and magnetic structure and on the raman spectra in Pr<Subscript>0.7</Subscript>Ba<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> manganite

The crystal and magnetic structure and the Raman spectra in Pr_0.7Ba_0.3MnO₃ manganite have been studied by the neutron diffraction technique at pressures up to 5 GPa as well as by the X-ray diffraction and Raman spectroscopy at pressures up to 30 GPa. The pressure dependence is determined for the lattice parameters, unit cell volume, Mn-O bond lengths in the orthorhombic structure of the Imma symmetry, and bending and stretching vibration modes for oxygen octahedra. In the low-temperature range at pressure P = 1.9 GPa, the magnetic transition from the initial ferromagnetic (FM) ground state (T _C = 197 K) to the A-type antiferromagnetic (AFM) state (T _N = 153 K) has been revealed. The FM and AFM phases coexist at pressures up to 5.1 GPa and exhibit negative and positive values of the pressure coefficient for the Curie and Néel temperature, respectively (dT _C/dP = −2.3 K/GPa and dT _N/dP = 8 K/GPa). The pressure dependence of the Curie temperature in Pr_0.7Ba_0.3MnO₃ differs drastically from that observed in other manganites of nearly the same composition with the orthorhombic Pnma and rhombohedral R[`3]cR\bar 3c structures, where the FM phase is characterized by the positive values of dT _C/dP. The structural mechanisms of these phenomena are discussed.     

Compressibility of Fe1.087Te: a high pressure X-ray diffraction study

Fe_1.087Te exhibits three phases in the pressure range from ambient to 16.6?GPa and becomes amorphous at higher pressures. All three phases have tetragonal symmetry. The low pressure T-phase is stable in the pressure range 0≤P<4.1?GPa and is found to be relatively soft having zero pressure bulk modulus B ₀=36(1)?GPa. The intermediate cT-phase is less compressible with B ₀=88(5)?GPa and stable in the pressure range 4.1≤P<10?GPa while a more compressible phase was observed between 10 and 16.6?GPa.     

High pressure phase transitions and depressurization amorphization of Bi2Fe4O9

High pressure structural behavior of Bi₂Fe₄O₉ has been studied by in situ angular-dispersive X-ray diffraction (ADXD) measurements up to 51.3 GPa. Two phase transitions have been observed at 7.6 and 22.6 GPa, respectively. A second high pressure structure (HP2) involving the tripling of lattice parameter c has been identified. An unusual amorphization occurs after releasing pressure. The high pressure phase transitions can be understood in terms of the increase in the coordination number of Fe³⁺ ion. The depressurization amorphization results from the appearance of the metastable HP2 and its collapse after releasing pressure. The results extend our understanding of pressure-induced amorphization.     

Spectroscopic study ofβ-Ni(OH)2 under pressure

Infrared absorption and Raman study ofβ-Ni(OH)₂ has been carried out up to 25 GPa and 33 GPa, respectively. The frequency ofA _2u internal antisymmetric stretching O-H mode decreases linearly with pressure at a rate of −0.7 cm⁻1/GPa. The FWHM of this mode increases continuously with pressure and reaches a value of ∼ 120 cm⁻¹ around 25 GPa. There was no discernible change observed in the frequency and width of the symmetric stretchingA _1g O-H Raman mode up to 33 GPa. The constancy of the Raman mode is taken as a signature of the repulsion produced by H-H contacts in this material under pressure. Lack of any discontinuity in these modes suggests that there is no phase transition in this material in the measured pressure range.     

Direct measurement of magnetization isotherms of Fe64Ni36 Invar alloy in a diamond anvil cell

Magnetization isotherms of the Fe₆₄Ni₃₆ Invar alloy have been measured under pressure up to 5.3?GPa in magnetic field up to 5?T using a diamond anvil cell and SQUID magnetometer. The unambiguous change of the pressure parameter dlnM_S/dP (from ?9 to ?13×10^?2?GPa^?1) has been observed in a narrow pressure interval from 2.5 to 3.5?GPa at all temperatures in the range from 5 to 300?K. The pressure interval, where the sharp decrease in magnetization was observed, coincides with the critical pressures of the pressure-induced decrease in Fe-moment that were determined by the X-ray Magnetic Circular Dichroism and the X-ray Emission Spectra studies, recently. The pronounced decrease in the Curie temperature of the Fe₆₄Ni₃₆ alloy under pressure, dT_C/dP = ?44 ±2?K/GPa, has been confirmed.     

Investigations of phase transition, elastic and thermodynamic properties of GaP by using the density functional theory

The phase transition of gallium phosphide (GaP) from zinc-blende (ZB) to a rocksalt (RS) structure is investigated by the plane-wave pseudopotential density functional theory (DFT). Lattice constant a₀, elastic constants c_ij, bulk modulus B₀ and the pressure derivative of bulk modulus B₀' are calculated. The results are in good agreement with numerous experimental and theoretical data. From the usual condition of equal enthalpies, the phase transition from the ZB to the RS structure occurs at 21.9 GPa, which is close to the experimental value of 22.0 GPa. The elastic properties of GaP with the ZB structure in a pressure range from 0 GPa to 21.9 GPa and those of the RS structure in a pressure range of pressures from 21.9 GPa to 40 GPa are obtained. According to the quasi-harmonic Debye model, in which the phononic effects are considered, the normalized volume V/V₀, the Debye temperature θ, the heat capacity C_v and the thermal expansion coefficient α are also discussed in a pressure range from 0 GPa to 40 GPa and a temperature range from 0 K to 1500 K.     

z切LiTaO3单晶的冲击相变研究

利用二级轻气炮加载下的冲击Hugoniot线(冲击波速度D-粒子速度u关系)和粒子速度剖面测量，结合基于密度泛函理论的平面波赝势计算研究了z切LiTaO₃单晶的高压相变.实验发现，D-u关系在u=0.95km/s附近出现明显拐折；实测波剖面中25.9 GPa和32.6 GPa时观测到弹-塑性双波结构，而终态压力为42.7 GPa和53.0 GPa时则为三波结构.上述结果都清楚地表明z切LiTaO₃单晶冲击相变的发生，相变起始压力约为37.9 GPa.同时，理论计算的菱形相(R3c对称群)压缩线与低压实验数据符合较好，而正交相(Pbnm对称群)压缩线则与扣除热压贡献的高压实验数据相符，由此推断z-切LiTaO₃的高压相为正交结构.从实验和理论上澄清了z切LiTaO₃的相变起始压力和高压相晶体结构的认识，研究工作亦对类似单晶材料的冲击相变研究有参考价值.     

Synthesis and characterization of the new high pressure phases ACu3 V4O12(A=Gd,Tb, Er)

New ACu₃V₄O₁₂ (A=Gd, Tb, Er) phases have been prepared at high pressure and high-temperature conditions (P～8–9 GPa, T～1000°C) in a toroid-type high pressure cell. These compounds crystallize in the cubic symmetry with a perovskite-like structure. At ambient pressure, they are paramagnetic and have activation-type conductivity. The effect of high pressure (10–50 GPa) on the electrical properties of the materials was analyzed in the temperature range from 78 to 300 K. Pressure ranges of the transition from activation type to metallic conductivity have been determined. The crystal structure of ACu₃V₄O₁₂ (A=Gd, Tb, Er) was found to be stable up to 50 GPa.     

High pressure studies on α-cristobalite form of Al0.5Ga0.5PO4: hydrostatic versus non-hydrostatic conditions

High pressure angle-dispersive X-ray diffraction investigations have been carried out on α-cristobalite form of Al_0.5Ga_0.5PO₄. Our investigations show that the structural stability of this phase under high pressure depends on the nature of pressure conditions in the diamond anvil cell. Under hydrostatic pressure conditions using neon as a pressure transmitting medium, ambient orthorhombic C222₁ phase transforms to orthorhombic Cmcm phase at 4.9?GPa. The high pressure Cmcm phase remains stable up to the highest pressure in the experiment, i.e. 19?GPa. The values of bulk modulus for C222₁ and Cmcm phases are 19(2) and 126(4)?GPa, respectively. In contrast to this, under non-hydrostatic pressure conditions, transformation of ambient C222₁ phase to Cmcm phase has not observed up to 17.4?GPa. Instead, a new monoclinic phase P2₁ is observed which contains layers of six coordinated Al/Ga ions separated by less dense five coordinated Al/Ga ions.     

High-pressure phases of plutonium monoselenide studied by X-ray diffraction

Abstract

Plutonium monoselenide was studied under high pressure up to 47 GPa, at room temperature, using a diamond anvil cell in an energy dispersive X-ray diffraction facility. At ambient pressure, PuSe has the NaC1-type (B1) structure. The compound has been found to undergo a second-order crystallographic phase transition at around 20 GPa. This phase can be described as a distorted B1 structure, with a rhombohedral symmetry. PuSe transforms to a new phase at around 35 GPa, which can be indexed in the cubic CsCl-type (B2). The volume collapse at this phase transition is 11%. When releasing pressure, we observed a strong hysteresis to the inverse transformation down to 5 GPa. From the pressure-volume relationship, the bulk modulus has been determined to B ₀ = 98 GPa and its pressure derivative as B ₀ = 2.6. These results are compared to those obtained with other actinide monmictides and monochalcogenides.     

High‐pressure Raman spectroscopy study of α and γ polymorphs of AlH3

For the first time, Raman spectroscopy of α and γ polymorphs of AlH₃ has been performed in the pressure range from ambient up to 16.9 and 32.7 GPa, respectively using the diamond anvil cell (DAC) technique. An analysis of pressure response wavenumbers (ν) for α‐AlH₃ showed a change of dν_i/dP at a pressure of about 8 GPa and may indicate a monoclinic distortion from the initially hexagonal α‐AlH₃. The distortion is stable at least up to 16.9 GPa. The γ form exhibited more complex behavior transforming to the α form at a pressure of about 12 GPa. The structural phase transition was shown to be an irreversible and kinetically slow process that required at least 5 h to complete. Copyright © 2008 John Wiley & Sons, Ltd.     

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.     

Effect of high pressure on the crystal and magnetic structures of La<Subscript>0.5</Subscript>Ca<Subscript>0.5</Subscript>CoO<Subscript>3</Subscript> cobaltite

The atomic and magnetic structures of La_0.5Ca_0.5CoO₃ cobaltite have been studied by the neutron diffraction technique at high pressures of up to 4 GPa in the 10- to 300-K temperature range. The pressure dependences of the structural parameters have been obtained. The Curie temperature increases with the pressure with the coefficient dT _C/dP = 1 K/GPa, demonstrating the stability of the ground ferromagnetic (FM) state. The pressure dependence of the ground FM state in La_0.5Ca_0.5CoO₃ is in drastic contrast with that for La_{1 − x} Ca _x CoO₃ at a lower calcium content (x < 0.3). For the latter compound, the pressure suppressed the ground FM state and a large negative pressure coefficient of the Curie temperature (dT _C/dP ∼ −10 K/GPa) was observed. The nature of such a phenomenon is analyzed in the framework of the double exchange model also taking into account the changes in the electron configuration of Co³⁺ ions.     

In situ high-pressure X-ray diffraction experiments and ab initio calculations of Co2P

In situ high-pressure experiments of Co2P are carried out by means of angle dispersive X-ray diffraction with diamond anvil cell technique. No phase transition is observed in the present pressure range up to 15 GPa at room temperature, even at high temperature and 15 GPa. Results of compression for Co2P are well presented by the second-order Birch-Murnaghan equation of state with V0 = 130.99(2)3 (1=0.1 nm) and K0 = 160(3) GPa. Axial compressibilities are described by compressional modulus of the axis: Ka = 123(2) GPa, Kb = 167(8) GPa and Kc = 220(7) GPa. Theoretical calculations further support the experimental results and indicate that C23-type Co2P is stable at high pressure compared with the C22-type phase.     

Melting of B12P2 boron subphosphide under pressure

Melting of boron subphosphide (B₁₂P₂) to 26?GPa has been studied by in situ synchrotron X-ray powder diffraction in a laser-heated diamond anvil cell, and by quenching and electrical resistance measurements in a toroid-type high pressure apparatus. B₁₂P₂ melts congruently, and the melting curve has a positive slope of 23(6)?K/GPa. No solid-state phase transition was observed up to the melting in the whole pressure range under study.     

Strength and equation of state of molybdenum triboride under 80 GPa pressure,using radial X-ray diffraction

The strength and equation of state of molybdenum triboride have been determined under nonhydrostatic compression up to 80?GPa, using an angle-dispersive radial X-ray diffraction technique in a diamond anvil cell (DAC). The RXD data yield a bulk modulus and its pressure derivative as K_0?=?342(6)?GPa with K₀′?=?2.11(17) at ψ?=?54.7°. Analysis of diffraction data using the strain theory indicates that the ratio of differential stress to shear modulus (t/G) ranges from 0.002 to 0.050 at pressures of 4–80?GPa. Together with theoretical results on the high pressure shear modulus, our results here show that molybdenum triboride sample under uniaxial compression can support a differential stress of ～10?GPa when it started to yield with plastic deformation at ～30?GPa. In addition, we draw a conclusion that MoB₃ is not a superhard material but a hard material.     

Energy spectrum and phase transitions in C70 fullerite crystals at high pressure

Measurements have been made of the Raman, optical absorption, and luminescence spectra of single crystals and pellets of the fullerite C₇₀ at T=300 K and at pressures up to 12 GPa. The baric shift dω/dP and the Grüneisen parameters of the Raman-active intramolecular phonon modes have been determined. It has been established that the d ω/dP value for certain phonon modes abruptly changes at pressures of P ₁≈2 GPa and P ₂≈5.5 GPa, as do the half-widths of the Raman lines. These features in the Raman spectrum are associated with phase transitions at high pressure. The baric shifts of the absorption and luminescence edges of C₇₀ crystals have been determined and are −0.12 eV/GPa and −0.11 eV/GPa, respectively, for absorption and luminescence. The baric shift of the absorption edge decreases significantly with increasing pressure and is −0.03 eV/GPa at 10 GPa. These data have been used to determine the deformation potential of the fullerite C₇₀, which is about 2.1±0.1 eV. Zh. éksp. Teor. Fiz. 111, 262–273 (January 1997)     

High pressure X-ray diffraction study on BaWO4-II

BaWO₄-II has been synthesized at 5 GPa and 610°C. Its high pressure behavior was studied by in situ synchrotron X-ray diffraction measurements at room temperature up to 17 GPa. BaWO₄-II retains its monoclinic structure. Bulk and axial moduli determined by fitting a third-order Birch–Murnaghan equation of state to lattice parameters are: K ₀=86.2±1.9 GPa, K ₀(a)=56.0±0.9 GPa, K ₀(b)=85.3±2.4 GPa, and K ₀(c)=146.1±3.2 GPa with a fixed K′=4. Analysis of axial compressible modulus shows that the a-axis is 2.61 times more compressible than the c-axis and 1.71 times more compressible than the b-axis. The beta angle decreases smoothly between room pressure and 17 GPa from 93.78° to 90.90°.     

Phase transition and high pressure behavior of Zirconium and Niobium carbides

We have predicted the phase transition pressure (P _T )and high pressure behavior of Zirconium and Niobium carbide (ZrC, NbC). The high pressure structural phase transitions in ZrC and NbC has been studied by using a two body inter-ionic potential model, which includes the Coulomb screening effect, due to the semi-metallic nature of these compounds. These transition metal carbides have been found to undergo NaCl (B1) to CsCl (B2)-type structural phase transition, at high pressure like other binary systems. We predict such structural transformation in ZrC and NbC at a pressure of 98GPa and 85GPa respectively. We have also predicted second order elastic constant and bulk modulus. The present theoretical work has been compared with the corresponding experimental data and prediction of LAPW and GGA and LDA theories.