A comparative study of the electrical properties of reduced and unreduced LiTaO<Subscript>3</Subscript> crystals

We have carried out a comparative study of the electrical properties of lithium tantalate (LiTaO₃) crystals in a wide temperature range (300–900 K) before and after reductive treatment in H₂O vapor and subsequent oxidative annealing. The results demonstrate that, in the temperature range of Li⁺ ion conduction (550–900 K), the activation enthalpy for ionic conduction in the reduced lithium tantalate crystal is H _a = 1.37 eV, which slightly exceeds that in the initial state of the crystal (1.34 eV). In the temperature range 390–450 K, the σ(T) data for the unannealed crystal are well represented by the Arrhenius law in the presence of two carrier types, with activation energies E ₁ = 1.03 eV and E ₂ = 0.29 eV, characteristic of proton and electron hopping conduction, respectively. After reductive annealing, the activation energy for conduction is ~0.65 eV, characteristic of the activation energy for bipolaron conduction. After subsequent oxidative annealing of the reduced crystals in dry air, the activation energy is ~1.2 eV. It seems likely that the presence of oxygen vacancies in the reduced LiTaO₃ crystal stimulates hydrogen release from the crystal during oxidative annealing.     

Thermal Expansion and Isothermal Compressibility of TlGaTe<Subscript>2</Subscript>

The temperature dependences of thermal expansion and isothermal compressibility for TlGaTe₂ indicate that this compound undergoes a second-order phase transition at 98 K. The experimental data are used to evaluate the Debye characteristic temperature, rms dynamic atomic displacements, specific heat difference C _p - C _V , and Gruneisen 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 TlGaTe₂ crystals.     

Semiempirical Model of Equation of State for Condensed Media

An approximation of equation of state for matter is treated, which involves the consistent use of interpolation approach with respect to both density and temperature in the entire nonrelativistic region. The cold component is defined under normal conditions by four experimentally obtained parameters, namely, specific volume, bond energy, bulk modulus, and parameter κ = ?(?lnB _S /?lnV) _S . The thermal ion component describes the transition from lattice vibrations with free Debye energy as a function of characteristic temperature being introduced, which makes possible the extension of the range of its application from zero temperature to ideal gas. The thermal electron component describes the transition of free electrons from ideal degenerate gas to nondegenerate state. A formula is suggested which enables one to calculate the degree of ionization at arbitrary densities and temperatures. Continuous functions are described which approximate ionization potentials and energies. The phase diagram, shock adiabats for continuous and porous matter, and isentropes are calculated. An analytical approximation of the Debye function is suggested. The results are illustrated by dependences on compression ratio in the range ρ/ρ ₀ = 1 to 10⁶. Comparison is made with experimental data.     

Heat Capacity and Mössbauer Study of Self-Flux Grown FeTe Single Crystal

We report mainly the heat capacity and Mössbauer study of self-flux grown FeTe single crystal, which is a ground state compound of the Fe chalcogenides superconducting series i.e. FeTe_1?x(Se/S) _x The as grown FeTe single crystal is large enough to the tune of a few centimetres and the same crystallizes in tetragonal structure having space group of P4/nmm. FeTe shows the structural/magnetic phase transition at 70 K in both magnetic and resistivity measurements. Heat capacity measurement also confirms the coupled structural/magnetic transition at the same temperature. The Debye model fitting of low temperature (below 70 K) heat capacity exhibited Debye temperature (?? _D ) to be 324 K. Mössbauer spectra are performed at 300 and 5 K. The 300-K spectra showed two paramagnetic doublets and the 5-K spectra exhibited hyperfine magnetic sextet with an average hyperfine field of 10.6 Tesla matching with the results of Yoshikazu Mizuguchi et al.     

Effect of the carbon isotope composition on the properties of diamond

Parameters of the interatomic interaction potential for ¹²C and ¹³C carbon isotopes in the crystal lattice of diamond have been determined. Based on these data, the isotope dependence of the properties of diamond such as the Debye temperature, molar heat capacity, thermal expansion coefficient, vacancy formation energy, self-diffusion activation energy, surface energy, and longitudinal sound velocity is described. This approach is used for estimating a change in the bulk compression modulus of lithium crystals upon the passage from ⁷Li to ⁶Li. As the temperature increases, the isotope dependence of the heat capacity at constant volume vanishes; at a certain temperature, the isotope dependence of the thermal expansion coefficient changes from growth to decay. The expected isotope dependence of the parameters of phase transitions is predicted. It is shown that carbon condensates formed upon deposition from the gas phase must be enriched with the heavy isotope.     

Thermal expansion and isothermal compressibility of TlGaSe<Subscript>2(1 −<Emphasis Type="Italic">x</Emphasis>)</Subscript>S<Subscript>2<Emphasis Type="Italic">x</Emphasis></Subscript> (<Emphasis Type="Italic">x</Emphasis> = 0.1, 0.2)

The temperature dependences of the thermal expansion coefficient and isothermal compressibility for TlGaSe_{2(1 ? x)}S_2x (x = 0.1, 0.2) solid solutions show an anomaly attributable to a second-order phase transition. The thermal expansion data have been used to evaluate the Debye characteristic temperature Θ, rms atomic displacement, and specific heat difference of the solid solutions.     

Heat capacity of bulk boron-doped single-crystal HPHT diamonds in the temperature range from 2 to 400 K

The heat capacity, C _p , of boron-doped single-crystal diamonds grown by the temperature gradient method was studied. The boron contents were < 10¹⁶, ~ 10¹⁸, and ~ 10²⁰ cm^–3. The heat capacity data for all tested crystals match well (within the measurement accuracy 1%) in the temperature range of 150–400 K and obey the Debye law. At low temperatures the heat capacity follows linear law possibly due to metallic inclusions in diamond bulk. Using this data the amount of metal can be calculated for each sample.     

Peculiarities of phase transitions in polytypes of monoclinic TlInS<Subscript>2</Subscript>

The existence of two polytypes at room temperatures, C-TlInS₂ and 2C-TlInS₂, with different monoclinic cell parameters, c and 2c, has been revealed. Significant differences in crystal lattice dynamics of these polytypes have been found. In particular, two phase transitions (PTs) have been detected for the polytype C-TlInS₂ as the temperature varies: a second-order PT from paraphase to incommensurate phase at T _i = 215 K and a first-order ferroelectric PT accompanied by a quadrupling of the parameter c at T _c = 197 K. No PT accompanied by an increase in unit cell parameter c has been found in the polytype 2C-TlInS₂, but a global temperature hysteresis characteristic of crystals with an incommensurately modulated structure has been detected at T = 180–230 K.     

Crystal Growth and Magneto-transport of Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript> Single Crystals

In this letter, we report on the growth and characterization of bulk Bi ₂Se ₃ single crystals. The studied Bi ₂Se ₃ crystals are grown by the self-flux method through the solid-state reaction from high-temperature (950 °C) melt of constituent elements and slow cooling (2 ℃/h). The resultant crystals are shiny and grown in the [00l] direction, as evidenced from surface XRD. Detailed Reitveld analysis of powder X-ray diffraction (PXRD) of the crystals showed that these are crystallized in the rhombohedral crystal structure with a space group of R3m (D5), and the lattice parameters are a = 4.14 (2), b = 4.14 (2), and c = 28.7010 (7) Å. Temperature versus resistivity (ρ?T) plots revealed metallic conduction down to 2 K, with typical room temperature resistivity (ρ _{300 K}) of around 0.53 m Ω-cm and residual resistivity (ρ _{0 K}) of 0.12 m Ω-cm. Resistivity under magnetic field [ ρ(T)H] measurements exhibited large + ve magneto-resistance right from 2 to 200 K. Isothermal magneto-resistance [ ρH] measurements at 2, 100, and 200 K exhibited magneto-resistance (MR) of up to 240 %, 130 %, and 60 %, respectively, at 14 T. Further, the MR plots are nonsaturating and linear with the field at all temperatures. At 2 K, the MR plots showed clear quantum oscillations at above say 10 T applied field. Also, the Kohler plots, i.e., Δρ/ ρ _oversus B/ ρ, were seen consolidating on one plot. Interestingly, the studied Bi ₂Se ₃ single crystal exhibited the Shubnikov-de Haas (SdH) oscillations at 2 K under different applied magnetic fields ranging from 4 to 14 T.     

Theoretical Study of Structural,Magnetic, Elastic,Phonon, and Thermodynamic Properties of Heusler Alloys Fe<Subscript>2</Subscript>Cr<Emphasis Type="Italic">X</Emphasis> (<Emphasis Type="Italic">X</Emphasis> = Al,Ga)

First-principle calculations based on generalized gradient approximation and quasi-harmonic Debye model were executed to analyze the structural, magnetic, elastic, phonon, and thermodynamic properties of Fe₂CrX (X = Al, Ga) Heusler alloys. The computed lattice parameters concurred well with available experimental and theoretical data. The calculated elastic constants reveal that the Fe₂CrAl is brittle and Fe₂CrGa is ductile. The phonon dispersion relation of Fe₂CrX (X = Al, Ga) are calculated using finite displacement method with a cutoff radius of 5 Å. We likewise explored the thermodynamic properties by utilizing quasi-harmonic Debye model in which bulk modulus, heat capacity, Debye temperature, Grüneisen parameter, and thermal expansion coefficient are resolved at 0–30 Gpa pressure and 0–900 K temperature from the non-equilibrium Gibbs functions.     

The melting of fullerites from small or large fullerenes

The physical limits are studied theoretically for a mass of fullerenes which may be used to produce a stable crystal, i.e., fullerite. The dependence of the parameters of interfullerene interaction on the mass of fullerene C _nc is used to study the evolution of the properties of fullerite as a result of variation of the number of carbon atoms nc in fullerene C _nc . The dependences of the energy of activation process and surface energy are calculated for different values of temperature and pressure in the 15 ≤ nc ≤ 147 range. It is demonstrated that fullerite becomes unstable at nc < 20, because small-sized light fullerenes cannot be localized due to weak Van der Waals forces. At the same time, fullerites with nc ≥ 110 exhibit abnormally low values of surface energy; this must bring about the fragmentation of nanoclusters of hollow spherical C _nc molecules of such a large size. Four empirical equations are used to estimate the dependence of the melting temperature of fullerite on nc. It is demonstrated that the melting temperature in the case of fullerites from small or large fullerenes is lower than that in the case of fullerites at 50 < nc < 90. It is inferred that the 30 < nc < 100 range is optimal for the formation of stable fullerite.     

Insight into Phase Transition,Electronic, Magnetic,Mechanical, and Thermodynamic Properties of TbTe: a DFT Investigation

Theoretical investigation on TbTe for its structural, electronic, magnetic, and thermodynamic stuffs has been carried within density functional theory (DFT) as implemented in WIEN2K code. TbTe was found stable in ferromagnetic phase. The calculated ground-state parameters were found in a good agreement with the experimental data. The compound was found to have a structural stability in cubic B1 (NaCl-type structure) phase, but under the application of high pressure (at 27 GPa), it undergone to B2 (CsCl-type structure) phase of pressure. The second-order elastic constants and mechanical properties like Young’s modulus, Shear modulus, Poisson ratio, Cauchy pressure (C₁₂–C₄₄), and Pugh’s ratio (B/G) were calculated. The present calculations confirmed the ductile nature of TbTe. Further, the thermodynamic investigations have been carried using quasi-harmonic Debye approximation. We have calculated the pressure and temperature dependence of Debye temperature (??_D), bulk modulus (B), thermal expansion (α), heat capacities (C_V), and entropy (S) in the temperature range of 0 to 1000 K and pressure range of 0 to 25 GPa.     

Heat Capacity and thermodynamic functions of DyMe<Superscript>II</Superscript>Cr<Subscript>2</Subscript>O<Subscript>5.5</Subscript>(Me<Superscript>II</Superscript>-Mg,Ca) in the range from 298.15 to 673 K

The calorimetric method is used to investigate the heat capacity of DyMe^IICr₂O_5.5(Me^II-Mg, Ca) chromites in the range from 298.15 to 673 K. The C _p ⁰ ～ f(T) curves exhibit λ-like effects at 348 and 548 K for DyMgCr₂O_5.5 and at 473 K for DyCaCr₂O_5.5, which apparently relate to second-order phase transitions. The temperature dependences are calculated for thermodynamic functions C _p ⁰ (T), H ⁰(T)-H ⁰(298.15), S ⁰(T), and Φ**(T).     

The interaction of ultrasound with electrons in hybridized states of iron impurity in a mercury selenide crystal

We have studied the propagation of transverse ultrasonic waves with a frequency of 52 MHz in an iron-doped mercury selenide (HgSe:Fe) crystal with an impurity concentration of 10¹⁹ cm^?3, for which the effects of resonant electron scattering on the impurity are most pronounced in the conduction behavior. The temperature dependences of the absorption coefficient α and the phase velocity v of the transverse acoustic waves, which have been studied in the temperature interval T = 1.48-80 K, exhibit anomalies at a temperature of about 5 K: a peak in the α(T) curve for a slow transverse wave and a minimum in the v(T) curve. These anomalies are explained using the theory of interaction between ultrasound and electrons in hybridized states on iron impurity atoms.     

Novel Solid-State Growth of <Emphasis Type="Italic">p</Emphasis>-Terphenyl: the Parent High-<Emphasis Type="Italic">T</Emphasis><Subscript>c</Subscript> Organic Superconductor (HTOS)

We report an easy and versatile route for the synthesis of the parent phase of the newest superconducting wonder material, i.e., p-terphenyl. Doped p-terphenyl has recently shown superconductivity with transition temperature as high as 120 K. For crystal growth, the commercially available p-terphenyl powder is pelletized, encapsulated in an evacuated (10^?4 Torr) quartz tube and subjected to high-temperature (260 ^°C) melt followed by slow cooling at 5 ^°C/h. A simple temperature-controlled heating furnace is used during the process. The obtained crystal is one piece, shiny, and plate like. Single crystal surface XRD (X-ray diffraction) showed unidirectional (00l) lines, indicating that the crystal is grown along the c-direction. Powder XRD of the specimen showed that as grown p-terphenyl is crystallized in monoclinic structure with space group P2 ₁/a space group, having lattice parameters a = 7.672 (2) Å, b = 5.772 (5) Å, and c = 13.526(3) Å and β = 91.484 (3)^°. Scanning electron microscopy (SEM) pictures of the crystal showed clear layered slab-like growth without any visible contamination from oxygen. Characteristic reported Raman active modes related to C–C–C bending, C–H bending, C–C stretching, and C–H stretching vibrations are seen clearly for the studied p-terphenyl crystal. The physical properties of the crystal are yet underway. The short letter reports an easy and versatile crystal growth method for obtaining quality p-terphenyl. The same growth method may probably be applied to doped p-terphenyl and to subsequently achieve superconductivity to the tune of as high 120 K for the newest superconductivity wonder, i.e., high- T _c organic superconductor (HTOS).     

Ab Initio Study of the Electronic Structure,Elastic Properties,Magnetic Feature and Thermodynamic Properties of the Ba<Subscript>2</Subscript>NiMoO<Subscript>6</Subscript> Material

We report first-principles calculations of the elastic properties, electronic structure and magnetic behavior performed over the Ba₂NiMoO₆ double perovskite. Calculations are carried out through the full-potential linear augmented plane-wave method within the framework of the Density Functional Theory (DFT) with exchange and correlation effects in the Generalized Gradient and Local Density Approximations, including spin polarization. The elastic properties calculated are bulk modulus (B), the elastic constants (C₁₁, C₁₂ and C₄₄), the Zener anisotropy factor (A), the isotropic shear modulus (G), the Young modulus (Y) and the Poisson ratio (υ). Structural parameters, total energies and cohesive properties of the perovskite are studied by means of minimization of internal parameters with the Murnaghan equation, where the structural parameters are in good agreement with experimental data. Furthermore, we have explored different antiferromagnetic configurations in order to describe the magnetic ground state of this compound. The pressure and temperature dependence of specific heat, thermal expansion coefficient, Debye temperature and Grüneisen parameter were calculated by DFT from the state equation using the quasi-harmonic model of Debye. A specific heat behavior C_V?≈?C_P was found at temperatures below T = 400 K, with Dulong–Petit limit values, which is higher than those, reported for simple perovskites.     

The surface energy of cryocrystals

An expression for the surface energy σ₀₀ of a crystal at T = 0 and P = 0 is obtained with allowance for the zero-point vibrations of atoms in the crystal. Particular calculations are performed for the cryocrystals of inert gases and hydrogen isotopes, in which the energy of zero-point vibrations is comparable with the energy of interatomic interactions. It is established that σ₀₀ exhibits highly correlated dependences of the same kind on the atomic (molecular) mass m, the melting temperature T_m, and the interatomic interaction potential D/k_B (k_B is the Boltzmann constant), whereby the function σ₀₀(m, T_m, D/k_B) exhibits nonlinear growth with each argument. The ratio σ₀₀/σ_liquid, where σ_liquid is the surface tension of the liquid phase at T = T_m, also exhibits highly correlated dependences on m, T_m, and D/k_B, which can be divided into two parts of linear growth corresponding to the quantum and classical domains. The σ₀₀/σ_liquid ratio, being smaller in the quantum case than in the classical one, grows with m, T_m, and D/k_B much faster in the quantum than in the classical domain.     

Elastic and Piezoelectric Parameters of the Crystals of Histidine Phosphite <Emphasis Type="Italic">L</Emphasis>-Hist · H<Subscript>3</Subscript>РО<Subscript>3</Subscript> Measured by the Method of Electromechanical Resonance

Single crystals of L-histidine phosphite (L-Hist · H₃РО₃) have been grown from aqueous solution by slow cooling method. The results of elemental analysis, of X-ray-diffraction studies of the crystal structure, and the habitus of the obtained crystals are given. The elastic and piezoelectric coefficients have been measured on the plates with the (010) natural faces by the method of electromechanical resonance for stretching vibrations in the temperature range 295–340 K. Elastic compliance values s₃₃ and s₂₂ and the corresponding elastic-moduli values, piezoelectric coefficients d₂₃ and d₂₂, coefficients of electromechanical coupling, and temperature coefficient of the resonance frequency are obtained. A comparison is given with other crystals, which are compounds of amino acids with phosphorous or phosphoric acid.     

Heat Capacity and Thermodynamic Functions of Ga<Subscript>2</Subscript>Se<Subscript>3</Subscript> from 14 to 320 K

The heat capacity of Ga₂Se₃ is measured from 14 to 320 K by adiabatic calorimetry. The smoothed heat capacity data are used to evaluate temperature-dependent thermodynamic functions (entropy, enthalpy increment, and reduced Gibbs energy) of gallium selenide. Under standard conditions, the thermodynamic properties of Ga₂Se₃ are C _p ⁰ (298.15 K) = 120.8 ± 0.2 J/(K mol), S⁰(298.15 K) = 180.4 ± 0.4 J/(K mol), H⁰(298.15 K) - H⁰(0) = 25.32 ± 0.05 kJ/mol, and Φ⁰(298.15 K) = 95.52 ± 0.19 J/(K mol). The Debye characteristic temperature of Ga₂Se₃ evaluated from heat capacity data is 340 ± 10 K.     

Linear relationship between crystal lattice parameters within one phase of ferroelastic lead orthophosphate

An analysis of experimental data on the temperature dependence of the crystal lattice parameters of lead orthophosphate Pb₃(PO₄)₂ shows that, within one (ferro-and paraelastic) phase, the unit cell parameter c is linearly related to the other lattice parameters. The bending of this linear dependence corresponds to a c value in the vicinity of a phase transition. An additional bending on the b versus c plots for the ferroelastic phase of lead orthophosphate is revealed, which is probably indicative of a second phase transition in the region of 90–120°C. A linear relationship between crystal lattice parameters has been also found in some other ferroelastic and ferroelectric crystals.