Phase diagram of Bi up to 140 kbars

The phase diagram of Bi has been studied by resistometric techniques in the temperature range of 30 to 300°K up to pressures of 140 kbars. Using the original Bridgman phase notation, the phase transitions I–II, II–III, I–III, III–IV and V–VI were observed. Two new phases, designated VIII and IX were observed in this region. The triple points occurring between I–II–III near 29.5 kbars and 160°K, between IV–V–VIII near 55 kbars and 240°K, between V–VI–VIII near 72 kbars and 255°K and between VI–VIII–IX near 135 kbars and 250°K. Earlier measurements were adjusted to the 1970 Drickamer pressure scale and compared to the present results. A phase diagram is proposed for pressures to 140 kbars. Calculations of the volume changes and latent heats of transformation are made near the triple points I–II–III, IV–V–VIII and V–VI–VIII using the measured volume changes of Bridgman for the I–II, IV–V and V–VI transitions. The latent heat associated with the III–IV transition was calculated using the volume data of Bridgman to be less than ? 2 cal/mol.     

Differential thermal analysis at high pressures— VIII: Phase behaviour of solid cyclopentanone,cyclopentanol, and cyclohexanone up to 3 KBAR

The phase behaviour of solid cyclopentanone, cyclopentanol, and cyclohexanone was investigated from 80 K to the melting temperature and up to 3 Kbar, using a low-temperature high-pressure differential thermal analysis apparatus. The melting temperature of cyclopentanone rises from 221.7 K at atmospheric pressure to 278 K at 2900 bar. No solid solid transition was observed. The melting temperature of cyclohexanone rises from 242.4 K at atmospheric pressure to 312 K at 2840 bar. Its well-known solid solid transition at atmospheric pressure (220.3 K) splits into two different solid solid transitions at elevated pressures. The melting temperature of cyclopentanol rises from 256 K at atmospheric pressure to 328 K at 2600 bar. Cyclopentanol exhibits two well-known solid solid transitions (236.4 K and 202.5 K at 1 atm), but an additional metastable form has been observed in the present work. the transition temperature being 195 K at 1 atm. Volume changes accompanying the phase transitions have been calculated using the Clausius Clapeyron equation.     

Structural studies of the P-T phase diagram of sodium niobate

The crystal structure of sodium niobate (NaNbO₃) has been investigated by energy-dispersive X-ray diffraction at high pressures (up to 4.3 GPa) in the temperature range 300–1050 K. At normal conditions, NaNbO₃ has an orthorhombic structure with Pbcm symmetry (antiferroelectric P phase). Upon heating, sodium niobate undergoes a series of consecutive transitions between structural modulated phases P-R-S-T(1)-T(2)-U; these transitions manifest themselves as anomalies in the temperature dependences of the positions and widths of diffraction peaks. Application of high pressure leads to a decrease in the temperatures of the structural transitions to the R, S, T(1), T(2), and U phases with different baric coefficients. A phase diagram for sodium niobate has been build in the pressure range 0–4.3 GPa and the temperature range 300–1050 K. The dependences of the unit-cell parameters and volume on pressure and temperature have been obtained. The bulk modulus and the volume coefficients of thermal expansion have been calculated for different structural modulated phases of sodium niobate. A phase transition (presumably, from the antiferroelectric orthorhombic P phase to the ferroelectric rhombohedral N phase) has been observed at high pressure (P = 1.6 GPa) and room temperature.     

Pressure–volume–temperature equations of state of Au and Pt up to 300 GPa and 3000 K: internally consistent pressure scales

By using a Mie–Grüneisen-type analysis method, the pressure–volume–temperature equations of state (P–V–T EOSs) of Au and Pt have been determined up to 300?GPa and 3000?K based on the experimental shock Hugoniot and thermodynamic data. The calculated results of Au and Pt show an excellent agreement with available experimental volume compression data over a wide range of pressures and temperatures. A comparison of our results with previous theoretical investigations has also been done. In addition, we have further examined the consistency of our results and the P–V–T EOS of MgO [K. Jin, X.Z. Li, Q. Wu, H.Y. Geng, L.C. Cai, X.M. Zhou, and F.Q. Jing, The pressure–volume–temperature equation of state of MgO derived from shock Hugoniot data and its application as a pressure scale, J. Appl. Phys. 107 (2010), pp. 113518] using simultaneous volume measurements of Au, Pt, and MgO at various temperatures. The good agreement among the P–V–T EOSs of Au, Pt, and MgO implies that these EOSs can be used as the reliable pressure scales in high pressure–temperature diamond anvil cell experiments.     

In situ observation of pressure-induced electrical resistance changes in zirconium: pressure calibration points for the large volume press at 8 and 35 GPa

Simultaneous in situ pressure–resistance measurements were carried out up to 40 GPa using a multianvil apparatus with synchrotron-based X-ray diffraction (XRD) measurements. Pressure-induced electrical resistance changes in zirconium were measured at ambient temperatures and two discontinuities were observed around the α–ω and ω–β structural phase transitions. The transition pressures were strictly determined from simultaneous measurements of the electrical resistance and in situ XRD as 7.96±0.16 and 34.5±0.3 GPa, respectively, using an equation of state for gold as the pressure scale. The precisely determined transition pressures are available for room temperature pressure calibration points for large volume presses installed at offline laboratories.     

Characteristics of the thermodynamic and kinetic properties of NbH0.83 in the phase transition region

Measurements were made of the specific heat (in the temperature range 80–400 K), electrical resistivity (4.2–300 K), thermo-emf (4.2–300 K), thermal conductivity (7–300 K), magnetic susceptibility (4.2–400 K), and lattice parameters (30–300 K) of the alloy NbH_0.83. The λ′→λβ phase transitions were studied. It was established that the structure of the niobium matrix of the hydride remains unchanged as a result of these transitions. It is shown that the β→λ phase transition (one-dimensional hydrogen ordering) is a three-stage one and is accompanied by a substantial change in the shear modulus of the hydride. The λ′ phase existing at T<110 K was determined. The electron thermal conductivity κ_el calculated for the λ′ phase in the range T<23 K is 25% higher than the measured thermal conductivity. In order to explain this fact and also the discontinuity in the concentration dependence of the coefficient of thermal expansion of NbHx for x?0.83–0.84 and the approximately 1.5% compression of the NbH_0.84 volume it is assumed that in the region of x _c in the λ′ phase the topology of the Fermi surface of the NbH_x interstitial alloy changes substantially.     

High–pressure electrical conductivity and Raman spectroscopy of chalcanthite

Abstract

The phase transitions and dehydration of chalcanthite were investigated by electrical conductivity and Raman spectroscopy at 1.0–24.0?GPa and 293–673?K in a diamond anvil cell. At ambient temperature, two secondary phase transitions were observed according to discontinuous changes in the slope of Raman shifts, full width at half maximum and electrical conductivities at ～7.3 and ～10.3?GPa. The dehydration temperatures were determined by the splitting of Raman peaks and changes in electrical conductivity as ～350 and ～500?K at respective ～3.0 and ～6.0?GPa. A positive relationship for chalcanthite between dehydration temperature and pressure is established.     

Isothermal compressibility and bulk modulus for methanol under the effect of pressure and temperatures

It has been demonstrated that the spinodal model produces very well the isothermal compressibility of liquid methanol for a wide range of pressures and temperatures. We have used the pseudospinodal model further to determine pressure derivatives, first-order as well as second-order of isothermal compressibility and bulk modulus for liquid methanol in the range of pressures (0–100 MPa) and temperatures (208.17 K–298.16 K). The results have been found to present close agreement with the available experimental data. We have also calculated the values of densities as a function of pressure and temperature for methanol using the Stacey equation of state.     

Equations of state of MgO at high pressure and temperature

Abstract

A synchrotron X-ray diffraction study on MgO has been done at simultaneous high pressure and temperature. The lattice parameter of MgO has been measured up to a static pressure of 6 GPa and a temperature of 1273 K, using a large volume pressure cell and energy-dispersive synchrotron X-ray powder diffraction. The compression was made following six high-temperature isotherms. A Vinet equation of state was used to fit the experimental P-V-T data. The Vinet's model compares very well with the experimental data above the Debye temperature (760 K) and allows the use of MgO as an alternative internal pressure calibrant for experiments at high temperature.     

Study of monazite under high pressure

Monazite was studied under high pressures of up to 20 GPa and 12 GPa by using synchrotron X-ray diffraction and Raman spectroscopy, respectively. X-ray diffraction data suggested that there was a structural distortion at ～11.5 GPa. The pressure–volume data of the monazite was fitted to a third-order Birch–Murnaghan equation of state and yielded a bulk modulus of 109(1) GPa and a pressure derivative of 6.7(1), in agreement with an empirical formula.     

HFC143a的热力学性质研究

1引言鉴于HFC143a在CFCs替代中的意义以及早期PVT实验数据的缺乏与误差，近期文献［l－7］报导了对该物系所做PVT实验研究，但尚缺乏整理与关联。本文在补充测量部分数据的基础上对已有数据进行关联综合，并进行热力学性质计算。2实验结果实验用定容式PVTx装置的一般测量精度为IAT【528inK、IAPD51．4kPa和卜对yiS0．1％，采用直接观察泡点法测量饱和液体密度精度为卜TIS38inK和卜p／pl三0．05％【‘－‘」。HFC143a样品为美国环保局提供，纯度99．95mass％。实验结果见表1－2。表IHFC143a的饱和蒸汽压实验结果表2HFC143a的…     

Evidence of a fcc-hcp transition in aluminum at multimegabar pressure

The structure phase transition and the equation of state (EOS) of the third-period simple metal Al were investigated at pressure up to 333 GPa by powder x-ray diffraction experiments. The theoretically predicted fcc-hcp transition was observed at the reduced volume V/V0 of 0.509(1), corresponding to the pressure of 217+/-10 GPa. From the obtained pressure-volume data, the pressure derivative of the bulk modulus K0' for the EOS of fcc-Al was determined to be 4.83(3) by fitting to the Vinet formulation with a fixed value 72.7 GPa of K0 obtained from previous ultrasonic experiments.     

Phase transformations of Fe3N-Fe4N iron nitrides at pressures up to 30 GPa studied by in situ X-ray diffractometry

The parameters of the equation of state of the stable ?-Fe₃N _x (where x = 0.8) nitride in the Fe-N system have been determined at pressures up to 30 GPa and temperatures up to 1273 K. The parameters V ₀ = 81.48(2) Å^?3, K _T = 162(3) GPa, K _T = 4.0 = 1.66(2), γ₀ = 555 K, and q = 1 have been determined for ?-Fe₃N_0.8 by the approximation of the P-V-T data with the Vinet equation of state and the thermal parameters within the Mie-Grüneisen-Debye formalism. No anomalous change in the volume of the cell owing to possible magnetic transitions has been revealed. The instability of Fe₄N at high pressures and Fe₃N in the presence of a deficit of nitrogen in the system has been established. The stable phase in the temperature range of 300–673 K and the pressure range of 20–30 GPa is ?-Fe₃N rather than ?-Fe₃N_0.8.     

Phase transition and thermodynamic properties of TiO2 from first-principles calculations

The pressure induced phase transitions of TiO2 from anatase to columbite structure and from rutile to columbite structure and the temperature induced phase transition from anatase to rutile structure and from columbite to rutile structure are investigated by ab initio plane-wave pseudopotential density functional theory method(DFT),together with quasi-harmonic Debye model.It is found that the zero-temperature transition pressures from anatase to columbite and from rutile to columbite are 4.55 GPa and 19.92 GPa,respectively.The zero-pressure transition temperatures from anatase to rutile and from columbite to rutile are 950 K and 1500 K,respectively.Our results are consistent with the available experimental data and other theoretical results.Moreover,the dependence of the normalized primitive cell volume V/V0 on pressure and the dependences of thermal expansion coefficient α on temperature and pressure are also obtained successfully.     

High‐pressure Seebeck coefficients and thermoelectric behaviors of Bi and PbTe measured using a Paris‐Edinburgh cell

A new sample cell assembly design for the Paris‐Edinburgh type large‐volume press for simultaneous measurements of X‐ray diffraction, electrical resistance, Seebeck coefficient and relative changes in the thermal conductance at high pressures has been developed. The feasibility of performing in situ measurements of the Seebeck coefficient and thermal measurements is demonstrated by observing well known solid–solid phase transitions of bismuth (Bi) up to 3 GPa and 450 K. A reversible polarity flip has been observed in the Seebeck coefficient across the Bi‐I to Bi‐II phase boundary. Also, successful Seebeck coefficient measurements have been performed for the classical high‐temperature thermoelectric material PbTe under high pressure and temperature conditions. In addition, the relative change in the thermal conductivity was measured and a relative change in ZT, the dimensionless figure of merit, is described. This new capability enables pressure‐induced structural changes to be directly correlated to electrical and thermal properties.     

Capacitive discharge mode transition in moderate and atmospheric pressure

Current–voltage characteristics of α- and γ-modes were investigated in moderate and atmospheric pressure capacitive discharges. The α- and γ-modes, the co-existence of both modes, and the α- to γ- and γ- to α-mode transitions were observed with the changes in voltage, current, and plasma volume. Changing of gas pressure, (100–760) Torr, the α to γ and the γ to α transition occurred with respect to input power increasing and decreasing, respectively. The hysteresis in current and voltage curve was observed and became more evident at higher pressure. Using a simple electrical circuit model, the relation between the gas pressure and the α-sheath thickness before mode transition was described.     

Calculation of B/A for n-alkane liquids using the Tait equation

The B/A parameter of acoustic nonlinearity was calculated for a series of n-alkane liquids using the Tait PVT equation of state supplemented with specific heat data. The calculations of sound speed, sound speed derivatives, the two components of B/A, and the value of B/A itself were compared with experimental data taken from the literature and with earlier calculations using a different equation of state. In addition, a comparison of the results with Ballou's rule (linear relation of B/A and reciprocal sound speed) was made. It is concluded that B/A can be calculated from the Tait equation of state with about the same accuracy as direct measurements of sound speed versus pressure and temperature, though the the temperature derivatives of the sound speed are calculated with much lower accuracy than pressure derivatives. The calculations made using the Tait equation are about the same accuracy as calculations made using our equation of state. Also, Ballou's rule does not hold for these liquids.     

高压下钙钛矿结构MgSiO3的分子动力学研究

利用分子动力学方法,研究了高温高压下钙钛矿结构MgSiO3的状态方程.研究表明,分子动力学模拟结果很好地再现了广泛温度和压强范围内钙钛矿结构MgSiO3的摩尔体积.温度300 K压强上升到120 GPa模拟的钙钛矿结构MgSiO3状态方程和有效的实验结果基本一致.在更高温度和更高压强下模拟的钙钛矿结构MgSiO3状态方程和他人的计算值吻合的很好.另外,还分别计算了温度300 K,900 K,1500 K和2500 K压强上升到120 GPa时MgSiO3的体积压缩率.     

Elastic properties of nanocrystalline forsterite under high pressure and high temperature

A simple theoretical model is developed to study the pressure–volume–temperature relationship and applied for nanocrystalline forsterite in the temperature range 300–1573 K and pressure range 0–9.6 GPa. The results obtained with the present model are in quite close agreement to the experimental values. The model is therefore extended to study the variation of bulk modulus and the coefficient of volume thermal expansion under high pressure and high temperature. The present study also reveals that the quasi-harmonic approximation, i.e., the product of bulk modulus and the coefficient of volume thermal expansion as constant, is valid at least up to the temperature 1573 K and pressure 9.6 GPa in case of nanocrystalline forsterite.