Equation of state for solid neon to 20 kbar

Piston-displacement equation-of-state (EOS) data to 20 kbar which are consistent with existingP=0 thermal expansion and bulk modulus data are given for solid neon for three isotherms (4.2, 13.5, and 19.9 K). A Mie-Grüneisen EOS can be used to correlate these results with data from directC _V measurements at several molar volumes if the Grüneisen parameter is assumed to be proportional to the molar volume and to be only slightly (1%) temperature dependent. This EOS predicts reasonably well theP=0 temperature dependence of the Grüneisen parameter which is obtained from other data.     

The high-pressure equation of state of solid molecular tritium

We have calculated the equation of state for solid molecular tritium using a semiempirical approach, closely guided by experimental results for H₂ and D₂. TheT=0 K isotherm is calculated from a self-consistent phonon model of the free energy. Temperature effects are treated by the Mie-Grüneisen model. A tabulation of pressure, bulk modulus, and thermal expansion is given for a dense set of molar volumes, as a function of temperature up toP=22 kbar.     

Thermal Expansion and Isothermal Compressibility of TlGaS2

The thermal expansion and isothermal compressibility of TlGaS₂ were measured between 77 K and room temperature. No anomalies in the temperature dependences of these properties were detected. The experimental data were used to evaluate the Debye temperature, rms dynamic atomic displacements, the difference between the specific heats at constant pressure and constant volume (C _p – C _V), and the Grüneisen parameter. The appreciable discrepancy between the C _p – C _V values calculated using thermodynamic relations and an empirical formula is attributed to the pronounced anisotropy of TlGaS₂.     

Elastic constants of solid hexagonal close-packed hydrogen and deuterium

Zero-pressure elastic constants for hcpp-H₂ ando-D₂ atT=0 K are calculated within the framework of the self-consistent phonon approximation with cubic anharmonicities. A pairwise additive model potential of the Barker-Pompe form, which reproduces the equation of state in the solid over a wide range of pressures, is used for the calculations. Debye temperatures and Grüneisen constants are also presented and the results are compared with recent experiments on the velocity of sound, inelastic neutron scattering, and specific heat.     

Transport in fluid helium-3 under pressure

The thermal conductivity of liquid helium-3 has been measured at temperatures between 1.3° K and 4.2° K and at pressures up to 55 atm. The viscosity data for liquid helium-3, reported briefly in Ref. 1 are here presented in full for temperatures 1.20° K, 2.00° K, and 3.02° K and at pressures up to 20 atm. Thermal conductivity and viscosity data at 2.00° K are combined with estimated values ofC _vto giveMK/ C _v(whereM is molecular weight,K is thermal conductivity, is viscosity andC _vis the molar specific heat), a dimensionless parameter that is 5/2 in gases at limiting low pressures. Finally, the thermal conductivity of gaseous helium-3 at 1-cm Hg pressure has been measured between 1.3° K and 4° K and compared with available theoretical estimates.     

Specific heats of solid natural neon at five molar volumes and of the separated neon isotopes atP=0

Direct measurements of the constant volume specific heatC _v are reported for solid natural neon at several molar volumes (13.39–12.39 cm³/mole) at temperatures from 1 K to the melting line. All samples were solidified in a high-pressure bomb at the melting line (maximum conditions of 53 K and 2.5 kbar) and molar volumes for the melting line are given. The extrapolations of these data to 0 K result in a ₀(V) relation for which ₀(P=0,T=0)=75.1±0.1 K and an average Grüneisen parameter ₀=2.51±0.03. These data can be represented at all temperatures and for all volumes by a single reduced curve asC _V [T/ ₀(V)], with a maximum deviation of ±0.8% nearT/ ₀=0.12. The deviations are systematic with volume and consistent with a temperature-dependent contribution to the Grüneisen parameter of 1±1%. The melting line parameters are used to test the Lindeman melting relation, and systematic deviations are found. Much less complete measurements ofC _V for the separated neon isotopes²⁰Ne and²²Ne are consistent with the reduced curve for natural neon, and with anM ^–0.5dependence for ₀, where M is the isotopic mass.     

Molecular-dynamics simulation of the high-pressure properties of rubidium

An embedded-atom potential for rubidium has been calculated with the parameters chosen with the use of the results of the static tests at a temperature of 300 K and pressures up to 45 GPa, as well as the results of the shock tests at pressures up to 39 GPa. The molecular-dynamics simulation has been performed for temperatures of 300–10 000 K and pressures up to ∼94 GPa. The potential determined from the shock-test data does not provide complete agreement with the static data for 300 K. The pressure, energy, and specific heats C _V and C _p have been calculated for the compression up to 20% of the normal pressure and for temperatures up to 10 000 K. The derivative (∂p/∂T) _V is positive for all of the molar-volume and temperature values except for a compression ratio of 30%. Compression up to a factor of 2.5 or more is accompanied by the partial amorphization of the models, which is enhanced with heating. The calculations of the temperature along the Hugoniot curve under the assumption that the Grüneisen parameter and adiabatic compression modulus are independent of the temperature provide an incorrect molar-volume dependence of the pressure at 0 K.     

Lattice dynamics and thermal expansion of lutetium

Dispersion relations, the lattice specific heat, third-order elastic constants, and the temperature variation of the lattice volume Grüneisen function of lutetium are determined on the basis of Keating's method. The lattice heat capacity is calculated using the frequency distribution function g(), and compared with the experimental values of Gerstein et al. The third-order elastic (TOE) constants are calculated using two anharmonic parameters. The low- and high-temperature limits _L and _H of the lattice thermal expansion are evaluated. The variation of lattice volume Grüneisen function with tempera- ture is presented. The pressure dependence of the lattice parameters and the compression ratio V/V ₀ of Lu are calculated using the theoretical TOE constants and Thurston's extrapolation formula.     

Specific heat,pressure, and the Grüneisen relation in solid hydrogen

We report systematic measurements ofC _v and (P/T) _v in solid hydrogen below 1 K and for orthohydrogen molar concentrations 0.58X _ortho0.20. This is the temperature and concentration range associated with a proposed quadrupolar glass phase. The results show no overtly glassy properties such as remanence or hysteresis upon warming and cooling, and there is no evidence of anomalous thermal relaxations. We have calculated the quadrupolar Grüneisen constant from our data and find that is only a slowly varying function of temperature and orthohydrogen concentration. TheC _v results are compared with theoretical models and the behavior of a corresponding classical solid, N₂-argon. There are several similarities between the N₂-argon and H₂ behaviors, but the semiempirical single-particle model does not correspond well with the experimental results.     

The elastic behaviour of Ce3S4 and La3S4

The hydrostatic pressure and temperature dependences of the elastic stiffnesses of the cubic, Th₃P₄ structure compounds Ce₃S₄ and La₃S₄ have been measured using the ultrasonic pulse echo overlap technique. Although Ce₃S₄, unlike La₃S₄ (T _c = 103 K), does not undergo a phase transition when its temperature is lowered to 16 K, its elastic stiffnesses (C₁₁-C₁₂)/2 and C₁₁ soften with decreasing temperature (in a similar manner but less markedly than those of La₃S₄); this lattice instability indicates an incipient phase transition. In both compounds the elastic constants increase under pressure: the long-wavelength acoustic-mode Grüneisen parameters are all positive, and the application of pressure does not induce acoustic phonon-mode softening.     

Determination of the compressibility factorZ(p,T) of three methane-ethane mixtures using the dielectric constant method

The compressibility behavior of three mixtures of the CH₄ C₂ H₆, system has been investigated experimentally by means of the dielectric constant method. Precise ( ± 1 ppm) measurements of the dielectric constant () as a function of the pressure (P) along one isotherm (T) are combined with the first three dielectric virial coefficients (A,B, andC) in order to obtain accurate values of the molar density (p). The compressibility factorZ=P/( _p RT) was obtained from the measured values ofp,P, andT. The coefficientA, is determined from the measurements of as a function ofP, while the higher-order coefficients (B, andC,) are obtained by an expansion technique. We report the measured values ofZ at 295.15 K up to 12 MPa for three mixtures of CH₄-C₂-H₆ containing, respectively. 9.54, 35.3, and 75.4% (molar) of ethane. Their exact composition was determined by weighing during the mixing process. The first three dielectric virial coefficients and the mixed second dielectric virial coefficient for the CH₄,-C₂, H₆ system agree with the calculated or the literature values within the limits of uncertainties. For the mixture containing 90.46% CH₄+C₂H₆, deviations in compressibility are of the order of 0.4% from GERG.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

Specific heat of quench-condensed hydrogen films

The specific heatC(X, T) of quench-condensed films of H₂ has been measured as a function of ortho concentration X with 0.28–2 at temperatures between 0.4 and 3.0 K. The films were condensed on evaporated gold substrates held at several temperaturesT. _cond between 1.0 and 3.5 K. The observed specific heat is attributed to orientational ordering of the ortho-H₂ molecules. For the films withX = 0.74 condensed atT _cond>2.5 K, there is a peak which indicates a bulk-like ordering transition. At temperatures below the peak, there is a large contribution toC, which is not present in bulk H₂, and which we attribute to short-range ordering size effects. AsT _cond is decreased below 2.5 K, the shape of the specific heat curve changes, and the peak at 1.5 K is replaced by a gradual rise with a sharp drop above 2.6 K. Despite this strong dependence ofC onT _cond, the entropy per molecule at 3 K is only weakly dependent onT _cond and comparable to that for bulk H₂. Film annealing at 3.4 K produces a change in the specific heat curve, and a study of this effect is presented. The ortho-para conversion rate of the films condensed at the various temperatures is found to be same as in bulk, well-annealed H₂. As in bulk H₂, the transition temperature inferred from the location of the specific heat peak or anomaly decreases withX. Unlike in bulk H₂, there is no temperature hysteresis inC for any of the quench-condensed films. This implies that the ordering transitions are not accompanied by a martensitic transformation.     

The Excess Enthalpy of (Steam+Ethane) in the Supercritical Region up to T=699.4 K and p=25.3 MPa

Flow calorimetric measurements of the excess molar enthalpy H ^E _m of (0.5H₂O+0.5C₂H₆) in the supercritical region carried out at temperatures from 573.7 to 699.4 K and at pressures in the range 5.05 to 25.3 MPa are reported. The measurements are fitted by a two-reference-fluid corresponding-states model which gives a good representation of the excess enthalpies up to the highest pressure.     

PVT relationships in a carbon dioxide-rich mixture with ethane

Comprehensive isochoric PVT measurements have been obtained for the system (0.99 CO₂ + 0.01 C₂H₆). The range of state points studied includes those with densities from 2 to 24 mol·dm^–3, temperatures from 245 to 400 K, and pressures to 35 MPa. Extensive comparisons have been made with two predictive conformai solution models, one which uses the 32-term BWR-type equation of Stewart and Jacobsen as a reference and the other using the newer Schmidt-Wagner functional form. Results obtained with the Schmidt-Wagner equation are better in the near-critical region owing to the flatter critical isotherm associated with this functional form.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

Magnetic torque of κ-(BEDT-TTF)2Cu(NCS)2 single crystals

The magnetic torque of-(BEDT-TFF)₂Cu(NCS)₂ was measured as a function of field direction with respect to thea ^*-axis under constant magnetic fields,H, up to 8 kOe in the temperature range from 1.3 to 8 K. A sharp cusp, C₁, in the irreversible region was found at _c1 near theHbc-plane between 1.3 and 7 K. In addition, extra cusps, C₂ and C₃, were observed at _c2 and _c3, respectively, between 2.5 and 6 K. At each temperature, the perpendicular component ofH giving each cusp is kept constant as i.e., cusps C₁, C₂, and C₃ are ruled by the characteristic field perpendicular to thebc-planeH _cp1,H _cp2, andH _cp3, respectively. These behaviors are almost the same as those we found in the oxide superconductor Bi₂Sr₂CaCu₂O₈. These results suggest that the cusps are intrinsic for irreversible vortex states of these layered superconductors.     

Third-order elastic constants and the low-temperature limit of the Grüneisen parameter of Mg2Pb on axe's shell model

The third-order elastic constants and the pressure derivatives of the second-order elastic constants of Mg₂Pb are calculated. The Grüneisen parameters of the acoustic modes in six symmetry directions are then computed. It is significant that most of the transverse elastic modes have negative Grüneisen parameters. The low-temperature limit of the Grüneisen parameter is then evaluated. Its relevance to the low-temperature thermal expansion of Mg₂Pb is discussed.     

Magnetic properties of cobalt films at the initial stage of ion-beam deposition

Variations in the magnetic properties of ion-beam-deposited cobalt (Co) films from the onset of nucleation until the passage to a bulk-like state have been studied using ferromagnetic resonance (FMR) measurements at 9.55 GHz and SQUID magnetometry. Depending on the Co film thickness, the FMR line width ΔH exhibits a sharp transition from large values (0.24 kOe < ΔH < 0.33 kOe) at the initial deposition stage to slightly varying values of ΔH < 0.16 kOe for film thicknesses above 1 nm. Similarly, Co films with thicknesses below 1 nm exhibit a significant coercivity (H _C > 0.54 kOe at 10 K), while thicker films are characterized by H _C < 0.16 kOe in the entire range of temperatures up to 300 K. Large values of ΔH and H _C at the nucleation and initial growth stages are related to the contribution from a transition Co/Si layer formed under the action of self-irradiation with a high-energy component of the deposited flux, which is inherent in the ion-beam sputtering in high vacuum. This fraction of high-energy Co atoms does not exceed 10% of their total flux and is characterized by a mean projected range of 0.8 nm in the growing Co layer and 1.2 nm in the Si substrate. Conditions of using Co films with intermediate thicknesses within 0.8 nm < t ≤2 nm for the injection of a spin-polarized current into silicon at room temperature are discussed.     

p, @rho, T, x-Relations for Water–Hydrocarbon Gaseous Mixtures in a Wide Range of the Parameters of State

Experimental data on p, @rho, T, x-Relations (gas phase) of the water–hydrocarbon binary systems (CH₄–C₈H₁₈, C₆H₆, C₇H₈) are given in the subcritical and supercritical regions, including states close to the critical region, at temperatures from 523.15 to 673.15 K and pressures up to 63 MPa, obtained by the piezometer method at constant volume. The special features of the thermodynamic behavior of mixtures of this class are determined.     

A Robust Molecular Porous Material for C2H2/CO2 Separation

A molecular porous material, MPM-2, comprised of cationic [Ni₂(AlF₆)(pzH)₈(H₂O)₂] and anionic [Ni₂Al₂F₁₁(pzH)₈(H₂O)₂] complexes that generate a charge-assisted hydrogen-bonded network with pcu topology is reported. The packing in MPM-2 is sustained by multiple interionic hydrogen bonding interactions that afford ultramicroporous channels between dense layers of anionic units. MPM-2 is found to exhibit excellent stability in water (>1 year). Unlike most hydrogen-bonded organic frameworks which typically show poor stability in organic solvents, MPM-2 exhibited excellent stability with respect to various organic solvents for at least two days. MPM-2 is found to be permanently porous with gas sorption isotherms at 298 K revealing a strong affinity for C₂H₂ over CO₂ thanks to a high (ΔQ_st)_AC [Q_st (C₂H₂) − Q_st (CO₂)] of 13.7 kJ mol⁻¹ at low coverage. Dynamic column breakthrough experiments on MPM-2 demonstrated the separation of C₂H₂ from a 1:1 C₂H₂/CO₂ mixture at 298 K with effluent CO₂ purity of 99.995% and C₂H₂ purity of >95% after temperature-programmed desorption. C-H···F interactions between C₂H₂ molecules and F atoms of AlF₆³⁻ are found to enable high selectivity toward C₂H₂, as determined by density functional theory simulations.     

Phase Composition of Nanocrystalline Titania Synthesized under Hydrothermal Conditions from Different Titanyl Compounds

Data are presented on the phase composition and physicochemical properties of nanocrystalline TiO₂ powders prepared via hydrothermal treatment of aqueous titanyl sulfate (TiOSO₄), titanyl nitrate (TiO(NO₃)₂), and aqua complex titanyl oxalate acid (H₂TiO(C₂O₄)₂) solutions and amorphous titanyl hydroxide (TiO₂ · nH₂O) gel (synthesized by precipitating H₂TiCl₆ with an excess of aqueous ammonia) at 423 and 523 K for 10 min to 6 h at solution concentrations from 0.07 to 0.5 M. The synthesized samples were characterized by x-ray diffraction, thermogravimetry, transmission electron microscopy, and nitrogen BET surface area measurements. It is shown that, independent of the precursor, short-term (10 min) hydrothermal treatment leads to the formation of nanocrystalline anatase (crystallite size d = 10–30 nm), a metastable form of titania. Upon an increase in hydrothermal-treatment time to 6 h, the systems studied exhibit different behaviors. Nanocrystalline anatase may persist (titanyl sulfate solutions and amorphous titanyl hydroxide gel) or transform into rutile (d = 50–100 nm), the thermodynamically stable form of TiO₂, through recrystallization processes (titanyl nitrate solutions; 0.07 M H₂TiO(C₂O₄)₂ solutions, partial conversion at 423 K and full conversion at 523 K; 0.28 M H₂TiO(C₂O₄)₂ solutions, full conversion at 423 K). Also possible is the formation of mesoporous anatase (0.28 M H₂TiO(C₂O₄)₂ solution, 523 K, 70- to 90-nm aggregates of crystallites, 10- to 20-nm closed pores containing the solution). A model is proposed according to which the formation of mesoporous oxides is possible at comparable rates of anatase nucleation and growth.