Phase diagram and equations of state of BaSO4

Abstract

The phase diagram and equations of state of BaSO₄, were determined up to 29 GPa and 1000 K in a resistance-heating type diamond anvil cell. At room temperature, barite is the stable form of BaSO₄ which undergoes a reversible phase transition at 10 GPa. The high-pressure form is tentatively determined to be triclinic. At high temperature, a similar phase transition takes place in BaSO₄, but at a pressure higher than that at room temperature. Our results indicate that the phase boundary of the two polymorphs in BasO₄ has a positive slope (dT/dP) of 90 K/GPa. The equations of state for both barite and its high-pressure phase are reported.     

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.     

Influence of quenching on the structural and conduction characteristics of lithium sulfate

Lithium sulfate quenched from the high-temperature melt using conventional quenching as well as rapid quenching techniques has been investigated using X-ray diffraction, scanning electron microscope, differential scanning calorimetry (DSC), vibrational spectroscopy techniques, and electrical conductivity measurements. Crystal structure of the quenched samples studied using X-ray diffraction shows a less ordered β-Li₂SO₄ phase. This is accompanied by a decrease in phase transition temperature in DSC measurements along with a decrease in the molar enthalpy associated with the transition. Raman and FT-IR spectroscopy studies reveal the disorder in the quenched samples by a significant broadening of the bands associated with the fundamental vibrational modes of SO₄ ^2? ion. Temperature variation of conductivity shows an enhancement in ionic conductivity by one order of magnitude with a slight decrease in phase transition point for quenched samples over unquenched Li₂SO₄.     

Pressure-induced structural and electronic transition in KTb(MoO4)2 through Raman and optical studies

Raman and optical absorption studies under pressure have been conducted on KTb(MoO₄)₂ up to 35.5 GPa. A phase transformation occurs at 2.7 GPa when the crystal is pressurized at ambient temperature in a hydrostatic pressure medium. The sample changes to a deep yellow color at the transition and visibly contracts in theα-axis direction. The color shifts to red on further pressure increase. The Raman spectral features and the X-ray powder pattern change abruptly at the transition indicating a structural change. The pressure-induced transition appears to be a property of the layer-type alkali rare earth dimolybdates. However, the color change at the transition in KTb(MoO₄)₂ is rather unusual and is attributed to a valence change in Tb initiated by the structural transition and consequent intervalence charge transfer between Tb and Mo.In situ high pressure X-ray diffraction data suggest that phase II could be orthorhombic with a unit cell having 3 to 4% smaller volume than that of phase I.     

Structural phase transition due to La substitution in Bi4Ti3O12

The structure of Bi_4?x La _x Ti₃O₁₂ (BLaTx) as a function of La content from x = 0.00 to 1.25 was studied by Raman spectroscopy and X-ray diffraction (XRD). It was observed that the Raman modes evolve discontinuously at about x = 1.00, which indicates a structural phase transition. Specifically, the B _2g and B _3g Raman modes shown evidences to coalesce into E _g modes with increasing La content. The evolution of Raman modes corresponds to the weakening of ferroelectric orthorhombic lattice distortion with increasing La substitution when x ≤ 1.00 and the final transition into the paraelectric tetragonal structure at higher La content. XRD and electrical measurement results confirm this phase transition by the observation that diffraction peaks from (200) and (020) planes of BLaT0.75 coalesce into one peak in BLaT1.25 and by the corresponding disappearance of remanent polarisation. The impact of La substitution on electrical properties of BLaTx is discussed briefly.     

High pressure X-ray diffraction study of CdAl2Se4 and Raman study of AAl2Se4 (A=Hg, Zn) and CdAl2X4 (X=Se, S)

We report the results of an X-ray diffraction study of CdAl₂Se₄ and of Raman studies of HgAl₂Se₄ and ZnAl₂Se₄ at room temperature, and of CdAl₂S₄ and CdAl₂Se₄ at 80 K at high pressure. The ambient pressure phase of CdAl₂Se₄ is stable up to a pressure of 9.1 GPa above which a phase transition to a disordered rock salt phase is observed. A fit of the volume pressure data to a Birch-Murnaghan type equation of state yields a bulk modulus of 52.1 GPa. The relative volume change at the phase transition at ∼9 GPa is about 10%. The analysis of the Raman data of HgAl₂Se₄ and ZnAl₂Se₄ reveals a general trend observed for different defect chalcopyrite materials. The line widths of the Raman peaks change at intermediate pressures between 4 and 6 GPa as an indication of the pressure induced two stage order-disorder transition observed in these materials. In addition, we include results of a low temperature Raman study of CdAl₂S₄ and CdAl₂Se₄, which shows a very weak temperature dependence of the Raman-active phonon modes.     

A high-pressure Raman and X-ray diffraction study of the perovskite SrCeO3

A high-pressure structural study of SrCeO₃ has been performed at room temperature by Raman spectroscopy and X-ray diffraction up to 32 and 45 GPa, respectively. A first-order reversible phase transition is observed at about 12 GPa in both techniques. A second weak structural change, taking place between 18 and 25 GPa, can be suspected from Raman data. The increase in the number of Raman bands and diffraction lines is an indication that the symmetry is lowered and the compound does not evolve towards the ideal cubic perovskite structure. A Rietveld analysis of X-ray data was performed for the low-pressure phase and the atomic positions and the cell lattice parameters variations are reported in this paper. The volume compressibility derived from Raman modes (5.6×10⁻¹² Pa⁻¹), involving mainly bond-stretching for each type of polyhedron, is found to be close to the one obtained from volume cell variations measured by X-ray diffraction (7.9×10⁻¹² Pa⁻¹).     

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.     

Pressure-induced disordering and phase transformations in Eu2Zr2O7 pyrochlore

ABSTRACT

The structural properties of pyrochlore Eu₂Zr₂O₇ under high pressure have been studied by using Raman spectroscopy and in situ angle-dispersive X-ray diffraction (ADXRD). The results of Raman spectra indicate that Eu₂Zr₂O₇ undergoes a reversible structural change around 21.2?GPa. The results of Rietveld refinements from in situ ADXRD data indicate that the ordered pyrochlore structure (Fd-3m) transforms to the defect-cotunnite structure (Pnma) at 26.5?GPa. The phase transition is irreversible and the transformation process is mainly induced by the accumulations of anti-site defects of the cation sublattice and Frenkel defects on the anion sublattice. Besides, the <Zr–O> bonds should play a more important role than the <Eu–O> bonds in the process of the phase transformation.     

High-pressure effect on inverse spinel LiCuVO4:X-ray diffraction and Raman scattering

The effect of external quasi-hydrostatic pressure on the inverse spinel structure of LiCuVO 4 was studied in this paper. High-pressure synchrotron X-ray diffraction and Raman spectroscopy measurements were carried out at room temperature up to 35.7 and 40.3 GPa, respectively. At a pressure of about 20 GPa, both Raman spectra and X-ray diffraction results indicate that LiCuVO4 was transformed into a monoclinic phase, which remained stable up to at least 35.7 GPa. Upon release of pressure, the high-pressure phase returned to the initial phase. The pressure dependence of the volume of low pressure orthorhombic phase and high-pressure monoclinic phase were described by a second-order Birch-Murnaghan equation of state, which yielded bulk modulus values of B 0 = 197(5) and 232(8) GPa, respectively. The results support the empirical suggestion that the oxide spinels have similar bulk modulus around 200 GPa.     

Structural and magnetic phase transitions occurring in Pr0.7Sr0.3MnO3 manganite at high pressures

The crystal and magnetic structures and the vibrational spectra of Pr_0.7Sr_0.3MnO₃ manganite are studied within the pressure range up to 25 GPa by methods of X-ray diffraction and Raman spectroscopy. Neutron diffraction studies have been performed at pressures up to 4.5 GPa. The magnetic phase transition from the ferromagnetic phase (T _C = 273 K) to the A-type antiferromagnetic phase (T _N = 153 K) is found at P ≈ 2 GPa. This transition is characterized by a broad pressure range corresponding to the phase separation. The Raman spectra of Pr_0.7Sr_0.3MnO₃ measured under high pressures significantly differ from the corresponding spectra of the isostructural doped A_{1 ? x} A′ _x MnO₃ manganites, (where A is a rare-earth ion and A′ is an alkaline-earth ion) with the smaller average ionic radius 〈r _A〉 of A and A′ cations. Namely, the former spectra do not include clearly pronounced stretching phonon modes. At P ～ 7 GPa, there appears the structural phase transition from the orthorhombic phase with the Pnma space group to the orthorhombic high-pressure phase with the Imma symmetry. In the vicinity of the phase transition, anomalies in the pressure dependences of the lattice parameters, unit cell volume, and phonon frequencies corresponding to the characteristic lattice vibration modes are observed.     

High pressure structural investigations on hexagonal YInO3

Hexagonal (space group P6₃cm) form of YInO₃ has been investigated under high pressure using synchrotron-based angle-dispersive X-ray diffraction and Raman scattering methods. Our experimental investigations suggest that it undergoes the phase transition to a new phase in the pressure range 12–15?GPa, while the ambient hexagonal phase is found to coexist with the new phase up to 29?GPa. DFT based calculations within the LDA approach on the hexagonal phase of YInO₃ showed that the unit cell volume matches well with the experimentally obtained volume at ambient pressure. As the pressure increases, theoretically obtained values of unit cell volume of the hexagonal phase were found to be significantly lower than that of experimentally obtained values. This discrepancy has been corrected using LDA?+?U_In(4d) (Hubbard interaction parameter between Indium 4d electrons) method. We have proposed the high pressure phase of YInO₃ to be orthorhombic with space group Pnma.     

Phase Transition and vibration properties of MnCO3 at high pressure and high-temperature by Raman spectroscopy

The high pressure and high-temperature behavior of MnCO₃ was investigated up to 55?GPa at ambient temperature and up to 573?K at ambient pressure by Raman spectroscopy, respectively. Some new modes were detected at ～16 and ～32?GPa, which were assigned to MnCO₃-I below 16?GPa and to MnCO₃-II above 32?GPa, and to a coexisting phase of them in between. The high pressure vibration properties of all Raman modes, especially high frequency modes, were systematically reported. The coexisting phase of MnCO₃-I and MnCO₃-II had much easier compressibility than the MnCO₃-II phase. The thermal stability of MnCO₃ was at least to 573?K and its thermal expansion along the c axis was easier than a and b axes.     

Equation of state and structural characterization of Cu4I4{PPh2(CH2CH = CH2)}4 under pressure

Combined high pressure single crystal X-ray diffraction experiments and ab initio simulations based on the density functional theory have been performed on a copper(I) iodide cluster formulated [Cu₄I₄{PPh₂(CH₂CH?=?CH₂)}₄] under high pressure up to 5?GPa. An exhaustive study of compressibility has been done by means of determination of isothermal equations of state and structural changes with pressure at 298?K taking advantage of the single crystal is more precise than powder X-ray diffraction for this type of experiments. It allows us to report the evidence of the existence of an isostructural phase transition of second order at 2.3?GPa not detected so far.     

Pressure evolution of two polymorphs of Tb 2(MoO 4)3

We have studied the high pressure behavior of the α and β^′-phases of Tb ₂(MoO ₄)₃ using a combination of powder X-ray diffraction and ab initio calculations. The α-Tb ₂(MoO ₄)₃ phase did not undergo any structural phase transition in the pressure range from 0 up to the maximum experimental pressure of 21 GPa. We observed line broadening of the diffraction patterns at pressures above 7 GPa, which may be due to non-hydrostatic conditions. The complete amorphization of the sample was not reached in the pressure range studied, as expected from previous Raman studies. The behavior under pressure of the β^′-Tb ₂(MoO ₄)₃ phase is similar to that of other rare-earths trimolybdates with the same structure at room temperature. A phase transition was observed at 2 GPa. The new phase, which can be identified as the δ-phase, has never been completely characterized by diffraction studies. A tentative indexation has been performed and good refined cell parameters were obtained. We detect indications of amorphization of the δ-Tb ₂(MoO ₄)₃ phase at 5 GPa.     

First-order character of the displacive structural transition in BaWO4

Nearly all displacive transitions have been considered to be continuous or second order, and the rigid unit mode （RUM） provides a natural candidate for the soft mode. However, in-situ X-ray diffraction and Raman measurements show clearly the first-order evidences for the scheelite-to-fergusonite displacive transition in SaWO4： a 1.6% volume collapse, coexistence of phases, and hysteresis on release of pressure. Such first-order signatures are found to be the same as the soft modes in BaWO4, which indicates the scheelite-to-fergusonite displacive phase transition hides a deeper physical mechanism. By the refinement of atomic displacement parameters, we further show that the first-order character of this phase transition stems from a coupling of large compression of soft BaOs polyhedrons to the small displacive distortion of rigid WO4 tetrahedrons. Such a coupling will lead to a deeper physical insight in the phase transition of the common scheelite-structured compounds.     

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.     

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.     

X-ray diffraction and Raman spectra of merrillite at high pressures

ABSTRACT

The compressibility and effect of pressure on the vibrations of merrillite, Ca₉NaMg(PO₄)₇, were studied by using diamond anvil cell at room temperature combined with in-situ synchrotron X-ray diffraction and Raman spectroscopy up to about 18 and 15?GPa, respectively. The pressure-volume data was fitted by a third-order Birch–Murnaghan equation of state to determine the isothermal bulk modulus as K₀ ?=?87.2(32) GPa with pressure derivative K₀′?=?3.2(4). If K₀′?=?4, the isothermal bulk modulus was obtained as 81.6(10) GPa. The axial compressibility was estimated and an axial elastic anisotropy exists since a-axis is less compressible than the c-axis. The Raman frequencies of all observed modes for merrillite continuously increase with pressure, and the pressure dependences of stretching modes (v ₃ and v ₁) are larger than those of the bending modes (v ₄ and v ₂) and external modes. The isothermal mode Grüneisen parameters and intrinsic anharmonicity of merrillite were also calculated.