Lattice parameter of polycrystalline diamond in the low-temperature range

The lattice parameter for polycrystalline diamond is determined as a function of temperature in the 4–300 K temperature range. In the range studied, the lattice parameter, expressed in angstrom units, of the studied sample increases according to the equation a = 3.566810(12) + 6.37(41) × 10−14T 4 (approximately, from 3.5668 to 3.5673 Å). This increase is larger than that earlier reported for pure single crystals. The observed dependence and the resulting thermal expansion coefficient are discussed on the basis of literature data reported for diamond single crystals and polycrystal

Thermostructural and Elastic Properties of PbTe and Pb 0.884 Cd 0.116 Te: A Combined Low-Temperature and High-Pressure X-ray Diffraction Study of Cd-Substitution Effects

Rocksalt-type (Pb,Cd)Te belongs to IV–VI semiconductors exhibiting thermoelectric properties. With the aim of understanding of the influence of Cd substitution in PbTe on thermostructural and elastic properties, we studied PbTe and Pb0.884Cd0.116Te (i) at low temperatures (15 to 300 K) and (ii) at high pressures within the stability range of NaCl-type PbTe (up to 4.5 GPa). For crystal structure studies, powder and single crystal X-ray diffraction methods were used. Modeling of the data included the second-order Grüneisen approximation of the unit-cell-volume variation, V(T), the Debye expression describing the mean square atomic displacements (MSDs), u2>(T), and Birch–Murnaghan equation of state (BMEOS). The fitting of the temperature-dependent diffraction data provided model variations of lattice parameter, the thermal expansion coefficient, and MSDs with temperature. A comparison of the MSD runs simulated for the PbTe and mixed (Pb,Cd)Te crystal leads to the confirmation of recent findings that the cation displacements are little affected by Cd substitution at the Pb site; whereas the Te displacements are markedly higher for the mixed crystal. Moreover, information about static disorder caused by Cd substitution is obtained. The calculations provided two independent ways to determine the values of the overall Debye temperature, θD. The resulting values differ only marginally, by no more than 1 K for PbTe and 7 K for Pb0.884Cd0.116Te crystals. The θD values for the cationic and anionic sublattices were determined. The Grüneisen parameter is found to be nearly independent of temperature. The variations of unit-cell size with rising pressure (the NaCl structure of Pb0.884Cd0.116Te sample was conserved), modeled with the BMEOS, provided the dependencies of the bulk modulus, K, on pressure for both crystals. The K0 value is 45.6(2.5) GPa for PbTe, whereas that for Pb0.884Cd0.116Te is significantly reduced, 33.5(2.8) GPa, showing that the lattice with fractional Cd substitution is less stiff than that of pure PbTe. The obtained experimental values of θD and K0 for Pb0.884Cd0.116Te are in line with the trends described in recently reported theoretical study for (Pb,Cd)Te mixed crystals

The pressure and temperature evolution of the Ca3V2O8 crystal structure using powder X-ray diffraction

We present a comprehensive experimental study of the crystal structure of calcium vanadate (Ca3V2O8) under systematic temperature and pressure conditions. The temperature evolution (4-1173 K) of the Ca3V2O8 structural properties is investigated at ambient pressure. The pressure evolution (0-13.8 GPa) of the Ca3V2O8 structural properties is investigated at ambient temperature. Across all pressures and temperatures used in the present work, the Ca3V2O8 crystal structure was determined by Rietveld refinement of powder X-ray diffraction data. The experimental high-pressure data are also supported by density-functional theory calculations. According to the high-pressure results, Ca3V2O8 undergoes a pressure-induced structural phase transition at a pressure of 9.8(1) GPa from the ambient pressure trigonal structure (space group R3c) to a monoclinic structure (space group Cc). The experimentally determined bulk moduli of the trigonal and monoclinic phases are, respectively, B0 = 69(2) GPa and 105(12) GPa. The trigonal to monoclinic phase transition appears to be prompted by non-hydrostatic conditions. Whilst the trigonal and monoclincic space groups show a group/subgroup relationship, the discontinuity in the volume per formula unit observed at the transition indicates a first order phase transition. According to the high-temperature results, the trigonal Ca3V2O8 structure persists over the entire range of studied temperatures. The pressurevolume equation of state, axial compressibilities, Debye temperature (264(2) K), and thermal expansion coefficients are all determined for the trigonal Ca3V2O8 structure