Melting curve of silicon to 15 GPa determined by two-dimensional angle-dispersive diffraction using a Kawai-type apparatus with X-ray transparent sintered diamond anvils

The melting curve of silicon has been determined up to 15 GPa using a miniaturized Kawai-type apparatus with second-stage cubic anvils made of X-ray transparent sintered diamond. Our results are in good agreement with the melting curve determined by electrical resistivity measurements [V.V. Brazhkin, A.G. Lyapin, S.V. Popova, R.N. Voloshin, Nonequilibrium phase transitions and amorphization in Si, Si/GaAs, Ge, and Ge/GaSb at the decompression of high-pressure phases, Phys. Rev. B 51 (1995) 7549] up to the phase I (diamond structure)—phase II (β-tin structure)—liquid triple point. The triple point of phase XI (orthorhombic, Imma)—phase V (simple hexagonal)—liquid has been constrained to be at 14.4(4) GPa and 1010(5) K. These results demonstrate that the combination of X-ray transparent anvils and monochromatic diffraction with area detectors offers a reliable technique to detect melting at high pressures in the multianvil press.     

In situ observations of temperature- and pressure-induced phase transitions in TiH2: Angle-dispersive and synchrotron energy-dispersive X-ray diffraction studies

We investigated the behavior of the structure of titanium hydride (TiH₂), an important compound in hydrogen storage research, at elevated temperatures (0-120 °C) and high pressures (1 bar-34 GPa). Temperature-induced changes of TiH₂ as indicated in the alteration of the ambient X-ray demonstrated a cubic to tetragonal phase transition occurring at about 17 °C. The main focus of this study was to identify any pressure-induced structural transformations, including possible phase transitions, in TiH₂. Synchrotron X-ray diffraction studies were carried out in situ (diamond anvil cell) in a compression sequence up to 34 GPa and in subsequent decompression to ambient pressure. The pressure evolution of the diffraction patterns revealed a cubic (Fm-3m) to tetragonal (I4/mmm) phase transition at 2.2 GPa. The high-pressure phase persisted up to 34 GPa. After decompression to ambient conditions the observed phase transition was completely reversible. A Birch-Murnaghan fit of the unit cell volume as a function of pressure yielded a zero-pressure bulk modulus K₀=146(14) GPa, and its pressure derivative K′₀=6(1) for the high-pressure tetragonal phase of TiH₂.     

Measurement of Seebeck effect (thermoelectric power) at high pressure up to 40 GPa

The paper reports details of a high-pressure thermoelectric power (Seebeck effect) technique up to 40 GPa. Several different types of high-pressure cells with anvil insets are presented. The technique was applied for measurements of pressure dependence of the thermopower of several substances including elemental metals (lead, Pb; indium, In), cerium-nickel alloy, Ce-Ni and sulphur, S. Two peculiarities in the pressure dependences of the thermopower of CeNi were found and attributed to structural transformations, near ∼5 and ∼10 GPa. These transitions were confirmed in direct X-ray diffraction studies. Sulphur compressed to 40 GPa exhibited a hole type conductivity and the thermopower value was about ∼+1 mV/K. Additionally, as an example of pressure calibration, the data on the electrical resistivity of zinc selenide, ZnSe, are given in a range of 0-23 GPa. These data suggest three possible scenarios of phase transitions from a rock salt (RS) high-pressure phase of ZnSe under decompression: RS→zinc blende (ZB), RS→cinnabar→ZB, and RS→wurtzite.     

High pressure behaviour of KHSO4 studied by electrical impedance spectroscopy

Measurements of the electrical conductivity were performed in KHSO₄ at pressures between 0.5 and 2.5 GPa and in the temperature range 120-350 °C by the use of the impedance spectroscopy. The temperatures of the α-β phase transition (T_Tr) and of the melting (T_m), determined from the Arrhenius plots ln(σT) vs. 1/T, increase with pressure up to 1.5 GPa having dT/dP∼+45 K/GPa. Above the pressure 1.5 GPa, the pressure dependencies of T_Tr and T_m are negative dT/dP∼−45 K/GPa. At pressures above 0.5 GPa, the reversible decomposition of KHSO₄ into K₃H(SO₄)₂+H₂SO₄ (and probably into K₅H₃(SO₄)₄+H₂SO₄) affects the electrical conductivity of KHSO₄, with the typical values of the protonic electrical conductivity, c. 10⁻¹ S/cm at 2.5 GPa.     

Raman and photoluminescence spectroscopy study of benzoic acid at high pressures

Benzoic acid (C₆H₅COOH, BA) has been studied by high pressure Raman and fluorescence spectroscopy up to about 13.40 GPa using a diamond anvil cell at room temperature. The changes of lattice modes are interpreted as the crystal structure transformation. Three possible phase transitions, with the pressure increasing up to about 0.55, 3.67 and 11.10 GPa, are, respectively, elucidated as crystalline-to-crystalline, crystalline-to-amorphous transitions. A new material formed when the pressure is up to above 11.10 GPa remains stable after the pressure is released.     

Experimental constraints on the phase diagram of elemental zirconium

The phase diagram of zirconium metal has been studied using synchrotron X-ray diffraction and time-of-flight neutron scattering at temperatures and pressures up to 1273 K and 17 GPa. The equilibrium phase boundary of the α-ω transition has a dT/dP slope of 473 K/GPa, and the extrapolated transition pressure at ambient temperature is located at 3.4 GPa. For the ω-β transition, the phase boundary has a negative dT/dP slope of 15.5 K/GPa between 6.4 and 15.3 GPa, which is substantially smaller than a previously reported value of −39±5 K/GPa in the pressure range of 32-35 GPa. This difference indicates a significant curvature of the phase boundary between 15.3 and 35 GPa. The α-ω-β triple point was estimated to be at 4.9 GPa and 953 K, which is comparable to previous results obtained from a differential thermal analysis. Except for the three known crystalline forms, the β phase of zirconium metal was found to possess an extraordinary glass forming ability at pressures between 6.4 and 8.6 GPa. This transformation leads to a limited stability field for the β phase in the pressure range of 6-16 GPa and to complications of high-temperature portion of phase diagram for zirconium metal.     

High-pressure powder diffraction study of TaO2F

TaO₂F, with a ReO₃-type structure, has been studied at up to 12.8 GPa using monochromatic synchrotron powder diffraction and diamond anvil cells. Two-phase transitions at ∼0.7 and 4 GPa were observed on compression. Below ∼0.7 GPa the cubic material was found to have a bulk modulus (K₀) of 36(3) GPa (K_p fixed at 4.0), similar to that reported for NbO₂F but much smaller than that of ReO₃. Immediately above 0.7 GPa on compression, the diffraction data were not fully consistent with a VF₃-type structure as previously proposed for NbO₂F. On decompression, the data between 8 and 4 GPa could be satisfactorily attributed to a single R-3c phase with a VF₃-type structure and an average bulk modulus of 60(2) GPa.     

Deviatoric stress and mean pressure in MgO compressed in a Kawai-type apparatus above 30 GPa: Evidence for reduction of deviatoric stress by annealing

By applying the numerical tensor analysis proposed by Yoneda and Kubo [Simultaneous determination of mean pressure and deviatoric stress based on numerical tensor analysis: a case study for polycrystalline X-ray diffraction of gold enclosed in a methanol-ethanol mixture. J. Phys.: Condens. Matter 18(2006)S979], we have determined deviatoric stress and mean pressure of polycrystalline MgO compressed in the Kawai-type apparatus. After the compression at room temperature, the mean pressure and the deviatoric stress in MgO were determined as 34.5 and 3.2 GPa, respectively. The mean pressure is significantly lower than the nominal pressure of 37.2 GPa determined by the conventional pressure determination method. By heating the sample to 1850 K with the constant press load, the deviatoric stress dramatically decreased to 0.1 GPa with a mean pressure of 34.2 GPa at room temperature after the heating. These results show both the importance of stress analysis to determine pressure more accurately and the effectiveness of annealing to reduce deviatoric stress in the sample.     

Structural stability of tin clathrates under high pressure

High pressure Raman scattering experiments have been performed for Rb₈Sn₄₄□₂ in order to investigate the pressure induced phase transition. At pressures of 6.0 and 7.5 GPa, Raman spectrum was drastically changed, indicating the phase transitions. The irreversibility of the spectral change and the disappearance of Raman peak observed at 7.5 GPa strongly suggest the occurrence of irreversible amorphization.     

Experimental constraints on the phase diagram of titanium metal

The phase transformations of titanium metal have been studied at temperatures and pressures up to 973 K and 8.7 GPa using synchrotron X-ray diffraction. The equilibrium phase boundary of the α-ω transition has a dT/dP slope of 345 K/GPa, and the transition pressure at room temperature is located at 5.7 GPa. The volume change across the α-ω transition is ΔV=0.197 cm³/mol, and the associated entropy change is ΔS=0.57 J/mol K. Except for ΔV, our results differ substantially from those of previous studies based on an equilibrium transition pressure of 2.0 GPa at room temperature. The α-ω-β triple point is estimated to be at 7.5 GPa and 913 K, which is comparable with previous results obtained from differential thermal analysis and resistometric measurements. An update, more accurate phase diagram is established for Ti metal based on the present observations and previous constraints on the α-β and ω-β phase boundaries.     

The band gap and electrical resistivity of FeS2-pyrite at high pressures

The indirect energy gap and electrical resistivity of FeS₂-pyrite have been measured at high pressures and 300 K using optical absorption spectroscopy and electrical conductivity measurements. Absorption spectra extend to ∼28 GPa, while resistivity is determined to ∼34 GPa. The band gap of FeS₂ is indirect throughout this pressure range and decreases linearly with pressure at a rate of −1.13(9)×10⁻² eV/GPa. If this linear trend continues, FeS₂ is expected to metallize at a pressure of 80(±8) GPa. The logarithm of resistivity also linearly decreases with pressure to 14 GPa with a slope of −0.101(±0.001)/GPa. However, between 14 and 34 GPa, the logarithm of resistivity is nearly constant, with a slope of −0.011(±0.003)/GPa. The measured resistivity of pyrite may be generated predominantly by extrinsic effects.     

Stiffening of nanoscale anatase Ti0.9Zr0.1O2 upon multiple compression cycles

The compression behavior of nanoscale Zr-doped anatase was studied by means of a diamond anvil cell experiment with alternating cycles of compression and decompression in the stability field of anatase (up to 13 GPa). We found that multiple cycles of compression lead to stiffening of the material: Precompressed samples of nanoanatase Ti_0.9Zr_0.1O₂ have a higher bulk modulus (K₀=249(9) and 266(6) GPa) compared with the sample when compressed for the first time (K₀=211(7) GPa). Upon compression, the crystallite size remains the same and the crystalline areas are free of defects. After the experiment, the crystallites are surrounded by amorphous rims, confirming the theoretical prediction by Pischedda et al. [Ultrastability and enhanced stiffness of similar to 6 nm TiO₂ nanoanatase and eventual pressure-induced disorder on the nanometer scale, Phys. Rev. Let. 96 (2006) 035509] for nanoscale anatase, but yielding much lower pressures (12 GPa) for the onset of partial amorphization.     

Structural stability,electronic structure and mechanical properties of 4d transition metal nitrides TMN (TM=Ru,Rh, Pd)

Ab initio calculations are performed to investigate the structural stability, electronic, structural and mechanical properties of 4d transition metal nitrides TMN (TM=Ru, Rh, Pd) for five different crystal structures, namely NaCl, CsCl, zinc blende, NiAs and wurtzite. Among the considered structures, zinc blende structure is found to be the most stable one among all three nitrides at normal pressure. A structural phase transition from ZB to NiAs phase is predicted at a pressure of 104 GPa, 50.5 GPa and 56 GPa for RuN, RhN and PdN respectively. The electronic structure reveals that these nitrides are metallic. The calculated elastic constants indicate that these nitrides are mechanically stable at ambient condition.     

Electrical and thermal studies in the commensurate incommensurate phase region of (NH4)2ZnCl4 single crystal

DC electrical conductivity for a virgin and poled annealed (NH₄)₂ZnCl₄b-axis single crystal shows a defect controlled property. A Schottky mechanism is a probable mechanism of conduction in regions of strong structural transitions. The rise of conductivity in the incommensurate and paraelectric phases is linked to an increase in discommensurations density. The activation energies (ΔE) in the three phases region were calculated. DTA measurements shows that the crystal is stable up to 200 °C and the phase transition temperatures were observed at 42, 94.8 and 137 °C. The effective activation energy (E_e) was obtained using Kissinger and Mahadevan equations. It was found to be equal to 0.49 eV. This correlates with the value obtained through DC conductivity.     

Ab initio study of ultra-incompressible ternary BeCN2 polymorph

The structural, mechanical, thermodynamic, and electronic properties calculated by projector-augmented wave method are presented for BeCN₂ in chalcopyrite and wurtzite-like structures. The calculated high bulk modulus (321 and 309 GPa) and large shear modulus (302 and 298 GPa) suggest that they are ultra-incompressible and hard materials. The ultra-incompressibility is attributed to a stacking of strongly three-dimensional covalent bonded CN₄ and BeN₄ tetrahedrons connected by corners. Thermodynamic study demonstrates that these two structures can be synthesized at ambient condition. Furthermore, the structural transformation from the wurtzite-like to the chalcopyrite phase was predicted at about 17 GPa according to the enthalpy difference calculations.     

Ionic to electronic dominant conductivity in Al2(WO4)3 at high pressure and high temperature

Electrical conduction and crystal structure of Al₂(WO₄)₃ at 400 °C have been studied as a function of pressure up to 5.5 GPa using impedance methods and synchrotron radiation X-ray diffraction, respectively. AC impedance spectroscopy and DC polarization measurements reveal an ionic to electronic dominant transition in electrical conductivity at a pressure as low as 0.9 GPa. Conductivity increases with pressure and reaches a maximum at 4.0 GPa, where the conductivity value is 5 orders of magnitude greater than the 1 atm value. Upon decompression, the conductivity retains the maximum value until the sample is cooled at 0.5 GPa. The high pressure-temperature X-ray diffraction results show that the lattice parameters decrease as pressure increases and the crystal structure undergoes an orthorhombic to tetragonal-like transformation at a pressure ∼3.0 GPa. The change of conduction mechanism from ionic to electronic may be explained by means of pressure-induced valence change of W⁶⁺→W⁵⁺, which results in electron transfer between W⁵⁺-W⁶⁺ sites at high pressure.     

Shock-induced phase transitions in the MgO-FeO system to 200 GPa

We report new shock-compression data for single-crystal MgO at 114 and 192 GPa. Our data together with the existing shock-wave data revealed a volume discontinuity at 170±10 GPa along with the MgO Hugoniot. The discontinuity gives a volume increase of 1.9%, indicating a possible phase transition from a rock-salt structure (B1) to a high-temperature phase along with the MgO Hugoniot. We re-examined the Hugoniot data on polycrystalline sample (Mg_0.6, Fe_0.4)O up to 200 GPa [M.S. Vassiliou, T.J. Ahrens, The equation of state of Mg_0.6Fe_0.4O to 200 GPa, Geophys. Res. Lett. 9 (1982) 127-130], which showed similar discontinuity with a 2.2% volume increase at 135±10 GPa. Our results add to fundamental understandings of the behavior of MgO and the lower mantle mineral magnesiowüstite (Mg, Fe)O at ultrahigh pressure and temperature.     

Structural,electronic and optical properties of LiBeP in its normal and high pressure phases

An investigation on the structural stabilities, electronic and optical properties of LiBeP under high pressure was conducted using the all-electron density functional theory within the local density approximation. Our results show that the sequence of the pressure induced phase transition of LiBeP is the Cu₂Sb-type structure (P4/nmm), the MgSrSi-type structure (Pnma) and the LiGaGe-type structure (P6₃mc). The first transition (P4/nmm to Pnma) takes place at 2.95 GPa and the second (Pnma to P6₃mc) at 6.65 GPa. In the three phases, the bandgap is indirect and the valence band maximum is at the zone center. With increasing pressure LiBeP in the LiGaGe structure becomes a direct gap semiconductor at 19.75 GPa. The assignments of the structures in the optical spectra and the band structure transitions are discussed. The mean value of the optical dielectric constant for the Cu₂Sb phase is smaller than that for the MgSrSi and the LiGaGe ones. This compound has a positive uniaxial anisotropy in the LiGaGe structure. The absorption coefficient along the z   direction, _αzz${\alpha }_{\mathit{zz}}$, for the MgSrSi structure is higher than that in the other two structures in the visible regime.     

Resistance anomaly and structural disorder in NiSi2 under high pressure

Results of X-ray diffraction, electrical resistance, thermoelectric power measurements and electronic band structure calculations on NiSi₂ under high pressure are reported. The thermoelectric power (TEP) changes sign near 0.5 GPa (from +30 to −20 μV/K). As the pressure is increased, the value of TEP increases further in magnitude and near 7 GPa it becomes −50 μV/K. The pressure vs. resistance curve measured up to 30 GPa using diamond anvil (DAC)-based technique exhibits a broad hump near 12 GPa and exhibits hysteresis on pressure release. The ADXRD patterns up to 42 GPa show a gradual irreversible loss of long-range order in NiSi₂ with the diffraction lines progressively broadening under pressure. The FWHM of the diffraction lines show a rapid increase in the half-widths close to 0.5 GPa and also near 12 GPa. The computed band structure at a compression (without any disorder) corresponding to 12 GPa, exhibits an electronic topological transition (ETT). The rapid increase in disorder above 12 GPa implies that the ETT may be facilitating the structural disorder. It is suggested that the pressure drives the material through a region of entropic and energetic barriers and induces disorder in the material.     

Thermodynamics of high-pressure ice polymorphs: ices III and V

Thermodynamic properties of high-pressure ice polymorphs, ices III and V, are studied theoretically. The results of TIP4P molecular dynamics simulations in the NPT ensemble are used to calculate the temperature dependence of the specific volume of ices III and V at pressures 0.25 and 0.5 GPa, respectively. New P-V-T equations of state of ices III and V are derived using a method generalizing the one proposed by Fei et al. [J. Chem. Phys. 99 (1993) 5369], and new results concerning the equilibrium phase transitions ice III-water and ice V-water are presented.