Collective modes in Ca70Mg30 glass

The self-consistent phonon scheme given by Takeno and Goda, involving multiple scattering and phonon eigen frequencies which are expressed in terms of many-body correlation functions of atoms as well as of interatomic potential in the solids, has been used to generate the collective modes in the Ca₇₀Mg₃₀ glass. A model potential is proposed to describe the effective interaction in the glass. Three different forms of the local field correction functions viz. Hartree, Taylor and Ichimaru and Utsumi are used to examine relative influence of exchange and correlation effects. The phonon frequencies of the longitudinal and transverse modes are computed employing the theoretical formulation of Hubbard and Beeby. The elastic property of the glassy system is then studied using the long wavelength limits of the phonon modes. The theoretical computations reproduce much better dispersion curves (both for the longitudinal and transverse phonons) compared to earlier reports and are found to be in good agreement with the available experimental results due to neutron scattering. Paper presented at the 5th IUMRS-ICA 98, October 1998, Bangalore.     

Computation of phonon dynamics of Mg<Subscript>70</Subscript>Zn<Subscript>30</Subscript> metallic glass

In the present paper, the computation of the phonon dynamics of binary Mg₇₀Zn₃₀ metallic glass is reported using the well-recognized model potential of Gajjar et al. The present study includes the phonon dispersion curves (PDC), elastic and thermodynamic properties such as longitudinal sound velocity υ_L, transverse sound velocity υ_T, Debye temperature θ_D, isothermal bulk modulus B _T, modulus of rigidity G, Poisson’s ratio σ and Young’s modulus Y and specific heat capacity C _V of the glass. Three theoretical models given by Hubbard–Beeby (HB), Takeno–Goda (TG) and Bhatia–Singh (BS) are used to compute the PDC. Five local field correction functions proposed by Hartree (H), Taylor (T), Ichimaru–Utsumi (IU), Farid et al. (F) and Sarkar et al. (S) are employed for the first time to study the effect of exchange and correlation in the aforesaid properties. The pseudo-alloy-atom (PAA) model is applied for the first time instead of Vegard’s Law.     

Phonon dispersion in alkali metals and their equiatomic sodium-based binary alloys

In the present article, the theoretical calculations of the phonon dispersion curves (PDCs) of five alkali metals viz. Li, Na, K, Rb, Cs and their four equiatomic sodium-based binary alloys viz. Na_0.5Li_0.5, Na_0.5K_0.5, Na_0.5Rb_0.5 and Na_0.5Cs_0.5 to second order in a local model potential is discussed in terms of the realspace sum of the Born von Karman central force constants. Instead of the concentration average of the force constants of pure alkali metals, the pseudo-alloy-atom (PAA) is adopted to directly compute the force constants of the four equiatomic sodium based binary alloys and was successfully applied. The exchange and correlation functions due to the Hartree (H) and Ichimaru-Utsumi (IU) are used to investigate the influence of the screening effects. The phonon frequencies of alkali metals and their four equiatomic sodium-based binary alloys in the longitudinal branch are more sensitive to the exchange and correlation effects in comparison with the transverse branches. The PDCs of pure alkali metals are found in qualitative agreement with the available experimental data. The frequencies in the longitudinal branch are suppressed rather due to IU-screening function than those due to static H-screening function.     

Study of Superconducting State Parameters of Binary Metallic Glasses by Pseudopotential Method

The theoretical calculation of the superconducting state properties (SSP) viz electron–phonon coupling strength λ, Coulomb pseudopotential μ ^*, transition temperature T _C , isotope effect exponent α and effective interaction strength N _O V of 50 binary metallic glasses of simple, non-simple, and transition metals have been carried out using a well-recognized model potential. We have employed here five different types of local field correction functions proposed by Hartree (H), Taylor (T), Ichimaru–Utsumi (IU), Farid et al. (F), and Sarkar et al. (S) to study the exchange and correlation effects on the present investigations. Instead of using Vegard’s law, a pseudo-alloy-atom model (PAA) in the present investigation is proposed and found successful. The present results of the SSP are found to be in qualitative agreement with other such earlier reported data wherever they exist, which confirms the superconducting phase in the metallic glasses.     

Plasmon-phonon and plasmon-two different phonon interaction in Pb1−xMnxTe mixed crystals

Far-infrared reflection spectra in wide temperature range was used to investigate the vibrational properties of Pb_1−xMn_xTe (x = 0.0002, 0.002, 0.02 and 0.1) mixed crystals. To analyse the experimental results we use dielectric function that takes into account the existence of plasmon-phonon as well as the plasmon-two different phonon interaction. The best fit method revealed two frequencies of plasmon-phonon coupled modes and three frequencies of plasmon-two different phonon coupled modes. Further, the values for two different LO modes and plasma frequency (ω_P) are calculated. Results obtained from experimental spectra as the best fit, are in very good agreement with the theoretical prediction. The model of phonon behaviour based on Genzel's model was developed. It was found that the long wavelength optical phonon modes of these mixed crystals, exhibit an intermediate and two mode behaviour, coincidentally.     

Study of Screening Dependence Superconducting State Parameters of Cu<Subscript><Emphasis Type="Italic">C</Emphasis></Subscript>Zr<Subscript>100−<Emphasis Type="Italic">C</Emphasis></Subscript> Binary Metallic Glasses by a Pseudopotential

The screening dependence theoretical investigations of the superconducting state parameters (SSP) viz. electron–phonon coupling strength λ, Coulomb pseudopotential μ ^*, transition temperature T _C , isotope effect exponent α and effective interaction strength N _O V of ten binary Cu _C Zr_100−C (C=25, 30, 33, 35, 40, 45, 50, 55, 57 and 60) metallic glasses have been reported using Ashcroft’s empty core (EMC) model potential. Five local field correction functions proposed by Hartree (H), Taylor (T), Ichimaru–Utsumi (IU), Farid et al. (F), and Sarkar et al. (S) are used in the present investigation to study the screening influence on the aforesaid properties. It is observed that the electron–phonon coupling strength λ and the transition temperature T _C are quite sensitive to the selection of the local field correction functions, whereas the Coulomb pseudopotential μ ^*, isotope effect exponent α and effective interaction strength N _O V show weak dependences on the local field correction functions. The values of T _C obtained from the H-local field correction function are in qualitative agreement with available theoretical or experimental data and show almost linear nature with the concentration C of Cu element. A linear T _C equation is proposed by fitting the present outcomes for the H-local field correction function, which is in conformity with other results for the experimental data. Also, the present results are in qualitative agreement with other such earlier reported data, which confirms the superconducting phase in the metallic glasses.     

Thermal stability and elastic properties of Mg2X (X = Si, Ge, Sn, Pb) phases from first-principle calculations

Thermal stabilities, elastic properties and electronic structures of Mg₂Si, Mg₂Ge, Mg₂Sn and Mg₂Pb have been determined from first-principle calculations. The calculated heats of formation and cohesive energies show that Mg₂Ge has the strongest alloying ability and Mg₂Si has the highest structural stability. Gibbs free energy, heat capacity and Debye temperature are calculated and discussed. The elastic parameters are calculated, the bulk moduli, shear moduli, Young’s moduli and poisson ratio value are derived, the brittleness and plasticity of these phases are discussed, and the brittle behavior and structural stability mechanism are also explained through the densities of states (DOS) of these intermetallic compounds.     

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.     

Effect of phonon anharmonicity on the thermal and elastic properties of stabilized δ-plutonium

The Debye temperature, bulk modulus, density, and molar heat capacity of a Pu_0.95Al_0.05 alloy have been calculated as functions of the temperature using a self-consistent thermodynamic model developed according to a quasi-classical approach that takes into account the effect of phonon anharmonicity on the acoustic and thermodynamic properties of metals. The results of self-consistent numerical simulation of the thermal and elastic properties are compared to the available experimental data and theoretical results obtained using alternative approaches. It is established that the phonon anharmonicity plays a significant role in the formation of elastic and thermal properties of the system under consideration.     

Influence of ZnO additions to 0.8(Mg0.95Co0.05)TiO3–0.2Ca0.6La0.8/3TiO3 ceramics on sintering behavior and microwave dielectric properties

The effects of ZnO addition on the microstructures and microwave dielectric properties of 0.8(Mg_0.95Co_0.05)TiO3–0.2Ca_0.6La_0.8/3TiO₃ ceramics were investigated. ZnO was selected as liquid phase sintering aids to lower the sintering temperature of 0.8(Mg_0.95Co_0.05)TiO3–0.2Ca_0.6La_0.8/3TiO₃ ceramics. With ZnO additives, the densification temperature of 0.8(Mg_0.95Co_0.05)TiO₃–0.2Ca_0.6La_0.8/3TiO₃ can be effectively reduced from 1450 to 1200–1325 °C. The crystalline phase exhibited no phase difference at low addition levels (0.25–2 wt.%). It is found that low-level doping of ZnO (0.25–2 wt.%) can significantly improve the density and dielectric properties of 0.8(Mg_0.95Co_0.05)TiO₃–0.2Ca_0.6La_0.8/3TiO₃ ceramics. The quality factors Q × f were strongly dependent upon the amount of additives. Q × f values of 36 000 and 13 000 GHz could be obtained at 1200–1325 °C with 1 and 2 wt.% ZnO additives, respectively. During all additives ranges, the relative dielectric constants were significantly different and ranged from 23.1 to 27.96. The temperature coefficient varies from 14.1–24.3 ppm/°C.     

First principles prediction of structural stability,elastic, lattice dynamical and thermal properties of osmium carbides

Abstract

First principles calculations are performed to investigate the structural stability, elastic, lattice dynamical and thermal properties of osmium carbides with various crystal structures. Our calculation indicates that the I₄Te type structure is energetically the most favourable for Os₄C. Based on stress–strain relationships, elastic constants are obtained, and the relevant mechanical properties are also discussed. The phonon dispersion relation and the dynamical stability are also predicted. We have found that the predicted structures are mechanically stable as well as dynamically stable except for cubic-Os₄C. Through the quasi-harmonic Debye model, the temperature and pressure effects on the bulk modulus, thermal expansion coefficient, heat capacity, Grüneisen parameter and Debye temperature are presented.     

Electronic properties of semiconducting silicides: fundamentals and recent predictions

This review emphasizes progress in theoretical simulation and experiments that have been performed in the past years for semiconducting silicides. New fundamental electronic and optical properties of Ca₂Si and BaSi₂, recently found RuSi₂ phase, ternaries in Fe-Os-Si and Ru-Os-Si systems, β-FeSi₂, Mg₂Si and CrSi₂ with stretched and compressed lattices as well as transport properties of β-FeSi₂, ReSi_1.75, Ru₂Si₃ are presented. Prospects for practical applications of semiconducting silicides are discussed.     

Ca3Co4O9 ceramics consolidated by SPS process: Optimisation of mechanical and thermoelectric properties

Ca₃Co₄O₉ (349) thermoelectric (TE) oxide ceramics were successfully prepared by Spark Plasma Sintering process. The effects of the uniaxial pressure (30-100 MPa), the dwell temperature (700-900 °C) and the cooling rate were investigated. Microstructure analyses have revealed strong enhancements of the bulk density as the pressure level and the applied temperature during the SPS process are increased. Mechanical properties were investigated by using instrumented nanoindentation and three point bending tests. Hardness, elastic modulus, strength and fracture toughness were shown to improve drastically and depend on the processing parameters. Thermal expansion measurements reveal a noticeable anisotropy induced by unidirectional hot pressing. The mechanical, thermal and thermoelectric properties were correlated to the microstructure and crystallographic texture of the resulting ceramics.     

Effects of Zn on the microstructure,mechanical property and bio-corrosion property of Mg–3Ca alloys for biomedical application

The effects of the addition of Zn element on the microstructures, mechanical properties and bio-corrosion properties of Mg–3Ca alloys are investigated. The microstructure and X-ray diffraction topography indicate that as-cast Mg–3Ca alloys are composed of primary Mg and eutectic (α-Mg + Mg₂Ca) phases, while Mg–3Ca–2Zn alloys are constituted of primary Mg and eutectic (α-Mg + Mg₂Ca + Ca₂Mg₆Zn₃) phases. Mechanical properties results show that the element Zn could improve both tensile strength and elongation of Mg–3Ca alloys. The ultimate tensile strength is enhanced by 22%. Meanwhile, the corrosion resistance is increased by the addition of Zn element. It is thought that the presence of Ca₂Mg₆Zn₃ phase mainly contributes to these improvements. Mg–3Ca–2Zn alloy provides moderate strength and excellent corrosion resistance for biomedical application.     

Structural and lattice dynamical properties of Zintl NaIn and NaTl compounds

The first-principles calculations based on the density-functional theory have been performed using both the generalized–gradient approximation (GGA) and the local-density approximation (LDA) to investigate many physical properties of NaIn and NaTl compounds. Specifically, the structural (lattice constant, bulk modulus, pressure derivative of bulk modulus, phase transition pressure (P_t)), mechanical (second-order elastic constants (C_ij), Young’s modulus, isotropic shear modulus, Zener anisotropy factor, Poisson’s ratio, sound velocities), thermo dynamical (cohesive energy, formation enthalpy, Debye temperature), and the vibrational properties (phonon dispersion curves and one-phonon density of states) are calculated and compared with the available experimental and other theoretical data. Also, we have presented the temperature variations of various thermo dynamical properties such as free energy, internal energy, entropy and heat capacity for the same compounds.     

Thermal properties of BeX (X = S, Se and Te) compounds from ab initio quasi-harmonic method

We report ab initio calculations of the structure, elastic constants, lattice dynamics and thermodynamic properties of BeS, BeSe and BeTe compounds. The fully minimized structure parameters and elastic constants of BeS, BeSe and BeTe compounds are in good agreement with previous theoretical and experimental data. The density functional perturbations theory with quasi-harmonic approximation QHA methods are applied to determine the phonon dispersion relations, phonon density of states, phonon decomposition density of states, and thermal quantities. The computed thermodynamic properties such as Debye temperature is in agreement with the previous work. The vibrational entropy and constant-volume specific heat are shown for the first time.     

Composition-dependent elastic modulus,vibration frequency and polaron properties of ZnSexTe1−x system

Using first-principles pseudopotential approach based on the density functional perturbation theory under the virtual crystal approximation, the compositional dependence of the elastic modulus, zone-center optical phonon modes, and polaron properties of the alloy system ZnSe_xTe_1−x over the whole compositional range from pure ZnTe to pure ZnSe is presented and discussed. Generally, a reasonably good agreement is obtained between our results and the available experimental data for ZnTe and ZnSe parent compounds. Other case, our results are predictions and may serve for a reference for future work.     

FP-APW + lo study of the elastic, electronic and optical properties of the filled skutterudites CeFe4As12 and CeFe4Sb12

Using a full-relativistic version of the full-potential augmented plane wave plus local orbitals (FP-APW + lo) method within the local density approximation (LDA), we have studied the elastic, electronic and optical properties of the filled skutterudites CeFe₄As₁₂ and CeFe₄Sb₁₂. Structural parameters, including lattice constant, internal free parameters and, bulk modulus and its pressure derivative were calculated. We have determined the full set of first-order elastic constants, Young’s modulus, Poisson’s ratio and the Debye temperature of these compounds. Band structures, density of states, pressure coefficients of energy band gaps are also given. It is found that both CeFe₄As₁₂ and CeFe₄Sb₁₂ are indirect band gap semiconductors. The valence band maximum (VBM) is located at Γ point, whereas the conduction band minimum (CBM) is located at N point. Optical constants, including the dielectric function, optical reflectivity, refractive index and electron energy loss were calculated for radiation up to 30 eV. This is the first quantitative theoretical prediction of the elastic and optical properties for these compounds, and it still awaits experimental confirmation.     

Enhancing the microwave dielectric properties of Nd (Mg0.5Sn0.5)O3 ceramics by substituting Nd3+ with Ca2+

The microwave dielectric properties of Nd_(1?2x/3)Ca_x(Mg_0.5Sn_0.5)O₃ ceramics were examined to evaluate their exploitation for mobile communication. Nd_(1?2x/3)Ca_x(Mg_0.5Sn_0.5)O₃ ceramics were prepared by the conventional solid-state method with various sintering temperatures. The X-ray diffraction patterns of the Nd_2.9/3Ca_0.05(Mg_0.5Sn_0.5)O₃ ceramics revealed no significant variation of phase with the sintering temperature. Nd_2.9/3Ca_0.05(Mg_0.5Sn_0.5)O₃ ceramics that were sintered at 1,550 °C for 4 h had the following properties: a density of 6.86 g/cm³, a dielectric constant (ε_r) of 19.3, a quality factor (Q × f) of 99,000 GHz, and a temperature coefficient of resonant frequency (τ _f ) of ?65 ppm/°C.     

First-principles study of structural and mechanical properties of AgB2 and AuB2 compounds under pressure

In this work, density functional theory calculations on the structural, mechanical, and lattice dynamical properties of AgB₂ and AuB₂ compounds in AlB₂, OsB₂, and ReB₂ structures are reported. Generalized gradient approximation has been used for modeling exchange-correlation effects. The detailed information is given for the energetically most stable structure for AgB₂ and AuB₂ compounds. Specifically, the lattice parameters, bulk modulus, cohesive energies, elastic constants, shear modulus, Young’s modulus, Poison’s ratio, Debye temperature, sound velocities, and anisotropic factors are studied. The elastic properties are also studied under pressure. The phonon dispersion curves and corresponding phonon density of states are calculated and discussed. Our structural and some other results are in agreement with the available experimental and theoretical data.