Thermal expansion of the C15 Laves-phase materials TaV2 and TaV2H x

The thermal expansion of the C15 Laves-phase material TaV₂H _x was measured over the temperature range of 130–350?K for x?=?0, 0.34 and 0.53. A primary objective of the study was to see if the anomalous temperature dependence of elastic constants found in an earlier study is due to, or associated with, unusual thermal expansion. The expansion was typical of transition metals, and quite close to theoretical results for related Laves-phase materials. Grüneisen parameters were determined from the measurements, and these were also typical of transition metals, and close to calculated values for related materials. Both the thermal expansion and the Grüneisen parameters of TaV₂ decreased somewhat with the addition of hydrogen. No anomalous behavior of the expansion was found that might account for the highly anomalous temperature dependence of elastic moduli reported earlier. This absence is consistent with the attribution of the elastic constant behavior to an electronic structure effect.     

Elastic properties, thermal expansion coefficients and electronic structures of Ti0.75X0.25C carbides

Elastic properties, thermal expansion coefficients and electronic structures of Ti_0.75X_0.25C carbides (X=W, Mo, Ta, Nb, V, Hf, Zr, Cr and Al) were systematically investigated using ab initio density functional theory (DFT) calculations. The calculated elastic moduli, electronic structures and thermal expansion coefficients α(T) of pure TiC are in good agreement with experimental data and other DFT calculations. Based on a phenomenological formula, the trends of elastic properties and ductile/brittle behavior of Ti_0.75X_0.25C were analyzed. It was found that alloying elements W, Mo, Ta, Nb, V and Hf can increase elastic moduli, while Zr, Cr and Al reduce moduli. The nearly free electron model and Debye approximation were applied in the evaluation of α(T). The anharmonic effect was taken into account by including volume-dependent elastic moduli and Debye temperature. Results show that alloying additions of 3d V, 4d Zr and Mo slightly reduce α(T), while 3d Cr increases α(T), Al, 4d Nb, 5d Hf and W almost keep α(T) unchanged in Ti_0.75X_0.25C at high temperatures. The electronic structures of Ti_0.75X_0.25C were calculated and analyzed, and the electronic density of states was used to interpret variations of elastic properties and ductile/brittle behavior induced by alloying additions.     

Theoretical study of the structural, elastic, electronic and thermal properties of the MAX phase Nb2SiC

Structural, elastic, electronic and thermal properties of the MAX phase Nb₂SiC are studied by means of a pseudo-potential plane-wave method based on the density functional theory. The optimized zero pressure geometrical parameters are in good agreement with the available theoretical data. The effect of high pressure, up to 40 GPa, on the lattice constants shows that the contractions along the c-axis were higher than those along the a-axis. The elastic constants C_ij and elastic wave velocities are calculated for monocrystal Nb₂SiC. Numerical estimations of the bulk modulus, shear modulus, Young’s modulus, Poisson’s ratio, average sound velocity and Debye temperature for ideal polycrystalline Nb₂SiC aggregates are performed in the framework of the Voigt-Reuss-Hill approximation. The band structure shows that Nb₂SiC is an electrical conductor. The analysis of the atomic site projected densities and the charge density distribution shows that the bonding is of covalent-ionic nature with the presence of metallic character. The density of states at Fermi level is dictated by the niobium d states; Si element has a little effect. Thermal effects on some macroscopic properties of Nb₂SiC are predicted using the quasi-harmonic Debye model, in which the lattice vibrations are taken into account. The variations of the primitive cell volume, volume expansion coefficient, bulk modulus, heat capacity and Debye temperature with pressure and temperature in the ranges of 0-40 GPa and 0-2000 K are obtained successfully.     

Thermal conductivity of UC1 − xNx from 100 to 1000 °K

The thermal diffusivities of UC_{1 ? x}N_x of several compositions were measured from 100 to 1000 °K by a laser flash method. The thermal conductivity was separated into electronic and phonon components by assuming the constant Lorenz number. The phonon conductivity showed an anomalous behaviour against composition at low temperatures. The total thermal conductivity of UC_{1 ? x}N_x showed a minimum above 300 °K at an intermediate composition which moved to higher carbon content with increasing temperature. This behaviour was explained by the temperature dependence of the lattice and electronic components.     

Ab initio calculations of elastic constants and thermodynamic properties of Li2O for high temperatures and pressures

The elastic constants and thermodynamic properties of Li₂O for high temperatures and pressures are calculated by the ab initio unrestricted Hartree-Fock (HF) linear combination of atomic orbital (LCAO) periodic approach. The lattice constant, elastic constants, Debye temperature, and thermal expansion coefficient obtained are in good agreement with the available experimental data and other theoretical results. It is found that at zero pressure the elastic constants C₁₁, C₁₂ and C₄₄, bulk modulus B and Debye temperature Θ_D decrease monotonically over the wide range of temperatures from 0 to 1100 K. When the temperature , C₁₂ approaches zero, consistently with the transition temperature 1200 K. However, with increasing pressure, they all increase monotonically and the anisotropy will weaken.     

First-principles calculations on structural, elastic, electronic, optical and thermal properties of CsPbCl3 perovskite

The structural, elastic, electronic, optical and thermal properties of the semiconductor perovskite CsPbCl₃ were investigated using the pseudo-potential plane wave (PP-PW) scheme in the frame of generalized gradient approximation (GGA) and local density approximation (LDA). The computed lattice constant agrees reasonably with experimental and theoretical ones. The CsPbCl₃ crystal behaves as ductile material. The valence bands are separated from the conduction bands by a direct band gap R-R. We distinguished hybridization between Pb-p states and Cl-p states in the valence bonding region. Under compression at P=30 GPa, this material will have a metallic character. The thermal effect on the lattice constant, bulk modulus, Debye temperature and heat capacity C_V was predicted using the quasi-harmonic Debye model. To the author's knowledge, most of the studied properties are reported for the first time.     

Heat capacity and thermal expansion of CdCr2Se4 and CdCr2S4

The thermodynamic properties of the spinel ferromagnetic compounds CdCr₂Se₄ and CdCr₂S₄ have been investigated by making heat capacity and thermal expansion measurements on single crystals. For both compounds, the ferromagnetic transition is marked by λ-type thermal anomalies, and the results provide a pressure dependence of the transition temperatures that is in agreement with direct measurements. Below the transition, CdCr₂S₄ shows an anomalous heat-capacity contribution and negative thermal expansion, which are in contrast to the conventional behavior found in CdCr₂Se₄.     

Optical and electronic properties of nano-Cd1−xMnxTe alloy

Cd_1−xMn_xTe thin films were fabricated by thermal interdiffusion of multilayers of sputtered compound semiconductors as well as thermally evaporated elements. Electron microscopy revealed their nanostructures. The alloys have been investigated for evaluation of optical and electronic parameters. Spectrophotometry helped to find out the bandgap and composition; photoluminescence was used for observing relative transition probabilities at room temperature. Photoresponse showed the light dependence of the resistance of the alloy films. Hall measurements and four-probe tests indicated the influence of manganese on the room-temperature electronic properties of the alloy.     

The influence of internal strain on the Néel temperature of rare earth metals

Measurements of the thermal expansion and elastic constant C₃₃ of single crystal 50% TbHo near the Néel temperature (T_N) indicates that T_N increases with the increase in internal strain in the sample. After repeated thermal cycling or direct straining of the sample this effect on T_N is irreversible.     

Crystallographic study of the phase transition in (Me4P)2ZnBr4

The compound (Me₄P)₂ZnBr₄, a member of the β-K₂SO₄ structure class, undergoes a phase transition at 84°C from the room temperature space group P12₁/c1 to the parent Pmcn structure. The room temperature structure corresponds to a ferrodistortive transition of B_1g symmetry at the zone center. At room temperature, the compound has lattice constants a=9.501(1), b=16.055(2), c=13.127(2) Å and β=90.43(1)°. For the high temperature phase, the orthorhombic cell has dimensions a=9.466(2), b=16.351(3) and c=13.284(2) Å. The structures consist of two crystallographically independent Me₄P⁺ cations and the ZnBr₄²⁻ anions. In the room temperature phase, all three ionic species show substantial displacement from the mirror plane perpendicular to the a-axis that exists in the high temperature phase, as well as rotations out of that plane. The thermal parameters of the cations are indicative of substantial librational motion. Measurements of lattice parameters have been made at 2-5°C intervals over the temperature range 40-140°C. The changes in the lattice constants appear continuous at T_c (within experimental limits) indicating that the phase transition is likely second-order. The a lattice constant shows an anomalous shortening as T_c is approached. Thermal expansion coefficients are calculated from this data. An application of Landau theory is used to derive the temperature dependencies of spontaneous shear strain and corresponding elastic stiffness constants associated with the primary order parameter.     

Lattice parameter anomalies in V2O3 at high temperature

The rhombohedral lattice parameters of pure V₂O₃ show anomalous behaviour in the high temperature region. The change in the rate of expansion of the rhombohedral angle α_R occurs at 533 K where the lattice constant a_R reaches a minimum. The sharp dip at 533 K in the a_R curve can be correlated with the peak observed in recent small angle scattering of neutrons and might be due to some dynamic magneto elastic effect.     

An anomalous thermal expansion in the perovskite system,Gd1−xSrxMnO3 (0 ≤ x ≤ 0.3)

The thermal expansion behavior of sintered samples of Gd_1−xSr_xMnO₃ (X = 0.0–0.4) was studied. The sintered bodies in this system showed negative thermal expansion over a wide temperature range. The detailed crystal structure refinements with respect to temperature showed that the volume of the orthorhombic perovskite lattice monotonically increased with temperature, however, in addition to this, the release of distortion from the Jahn-Teller effect of Mn³⁺ ion occurred over a wide temperature range, which brought the negative expansion of the a-axis, although the b- and c-axes increased with temperature. The anomalous thermal expansion is explained by the sum of the effects of the shrinkage of the a-axis and absorption of the b- and c-axes' expansion by the pores in the sintered body.     

Phase stability,elastic, electronic,thermal and optical properties of Ti3Al1−xSixC2 (0≤x≤1): First principle study

The structural parameters with stability upon Si incorporation and elastic, electronic, thermodynamic and optical properties of Ti₃Al_1−xSi_xC₂ (0≤x≤1) are investigated systematically by the plane wave pseudopotential method based on the density functional theory (DFT). The increase of some elastic parameters with increasing Si-content renders the alloys to possess higher compressive and tensile strength. The Vickers hardness value obtained with the help of Mulliken population analysis increases as x is increased from 0 to 1. The solid solutions considered are all metallic with valence and conduction bands, which have a mainly Ti 3d character, crossing the Fermi level. The temperature and pressure dependences of bulk modulus, normalized volume, specific heats, thermal expansion coefficient, and Debye temperature are all obtained through the quasi-harmonic Debye model with phononic effects for T=0−1000 K and P=0−50 GPa. The obtained results are compared with other results available. Further an analysis of optical functions for two polarization vectors reveals that the reflectivity is high in the visible–ultraviolet region up to ∼10.5 eV region showing promise as a good coating material.     

Grüneisen parameter of cerium compounds with unstable 4f shells

We give experimental results of magnetic susceptibility, thermal expansion and elastic constant measurements for CeSn₃ and CePd₃. We interpret these results with a phenomenological scaling function for the free energy and we include a brief discussion of CeAl₃. The coupling between the electronic system and the lattice can be described by an electronic Grüneisen parameter Ω_g associated with the unstable 4f system which turns out to become a system specific constant far below the fluctuation temperatureT ₀. The coupling parameter T₀Ω_g is an order of magnitude larger than the coupling constant for stable valentRE systems extracted from crystal field effects. A generalization to include magnetic field dependent effects is also given.     

Local polarization in Lead-Calcium Titanate ceramics

Calcium-modified Lead Titanate ceramics Pb_0.76 Ca_0.24 [(Co_0.5 W_0.5)_0.05 Ti_0.95O₃] are studied by means of dielectric and thermal expansion measurements as a function of temperature. Results show that the phase transition has a diffusive character with an exponent τ that decreases when temperature increases. Above the transition temperature, Tc, the electrical conductivity of samples affects the thermal behaviour of the dielectric constant. From thermal expansion measurements, results show that the thermal elongation and thermal expansion coefficient separate from their normal behaviour at a temperature Td = 460°C, much higher than Tc and close to the phase transition temperature of PbTiO₃. The difference Δα = α − α₀ (anomalous part of the thermal expansion coefficient) decreases to zero when the temperature increases from Tc, following a logarithmic law Δα = F(logΔT). Thermal expansion results may be ascribed to some type of local polarization in the crystalline grains, which disappears near the phase transition temperature of PbTiO₃.     

Heat capacity and lattice dynamics of yttrium diboride in the temperature range 5–300 K

This paper reports an experimental study of the heat capacity and crystal lattice parameters of a polycrystalline sample of yttrium diboride prepared by high-temperature synthesis from elements. The electronic and lattice contributions to the heat capacity are isolated. The temperature dependences of the characteristic temperature, the linear thermal expansion coefficients α_a(T) and α _c (T), the bulk thermal expansion coefficient β(T), and the Grüneisen coefficient are calculated. A region of negative values of α _c (T) and β(T) is revealed. Anharmonicity is found to exert only a minor effect on the YB₂ lattice dynamics over a larger part of the temperature range covered.     

Charge ordering and elastic anomalies in NaxCoO2

We report in-plane resistivity and elastic constant C₃₃ measurements on the Na_0.8CoO₂ and Na_0.5CoO₂ systems. An ordering transition is found at T₀=280 K for Na_0.8CoO₂. The temperature dependence of the elastic constant C₃₃ propagating perpendicular to the CoO₂ layers is interpreted, in a phenomenological approach, as being due to the anharmonicity of atomic vibrations. The effective Grüneisen parameter γ_eff deduced directly from the temperature dependence of C₃₃ exhibits important changes at the charge ordering, magnetic and metal-insulator phase transitions.     

Elasticity of a magnetic lattice near the magnetic critical point

The isothermal elastic constants and the coefficient of anomalous thermal expansion of a magnetic lattice are discussed. The spin system is described by the Ising model with an exchange coupling depending on lattice spacing. A behavior of the elastic constants and the coefficient of thermal expansion is found which is in qualitative agreement with experiments. The isothermal compressibility remains positive nearT _c and no thermo-mechanical instability occurs (which would lead to a first-order phase transitions), in contrast to earlier theories.     

Theoretical investigations of structural, elastic and thermodynamic properties for PtN2 under high pressure

We have investigated structural and elastic properties of PtN₂ under high pressures using norm-conserving pseudopotentials within the local density approximation (LDA) in the frame of density-functional theory. Calculated results of PtN₂ are in agreement with experimental and available theoretical values. The a/a₀, V/V₀, ductility/brittleness, elastic constants C_ij, shear modulus C′, bulk modulus B, shear modulus G, Young's modulus E, Poisson's ratio σ and anisotropy factor A as a function of applied pressure are presented. Through the quasi-harmonic Debye model, we also study thermodynamic properties of PtN₂. The thermal expansion versus temperature and pressure, thermodynamic parameters X (X=Debye temperature or specific heat) with varying pressure P, and heat capacity of PtN₂ at various pressures and temperatures are estimated.     

Theoretical study of thermal properties for SiGe system

Using our presented treatment with the volume effect on the force constants of the pure constituent, we study the thermal properties of Si-Ge solid solution from first principle in the electronic theory of solids. The specific heat at constant volume of Si_1−xGe_x solid solution is monotonous function of the concentration x and is approximately given by the linear interpolation of atomic fraction. The Grüneisen constant and the thermal expansion coefficient of Si_1−xGe_x system have the characteristic concentration dependence. The almost constant value at high temperatures and the negative minimum at low temperature of the Grüneisen constant are not monotonous as function of x, and show a maximum near x = 0.7 and a minimum near x = 0.2, respectively. Then, the linear thermal expansion coefficient at low and high temperatures deviate largely from the linear interpolation of the atomic fraction.