The electronic structure,elastic and optical properties of Cu2ZnGe(SexS1 − x)4 alloys: density functional calculations

The electronic structure, elastic and optical properties of Cu₂ZnGe(Se_xS_{1 ? x})₄ alloys are systematically analysed using first-principles calculations. The lattice parameters agree well with the theoretical and experimental values which are searched as complete as possible indicating our calculations are reliable. The elastic properties are investigated first and are compared with the similar compounds CZTS and CZTSe due to the unavailable experimental data currently. The variation of the optical properties caused by the increase of Se/S ratio is discussed. The static optical constants are calculated and the corrected values are also predicted according to the available experimental data.     

Systematics of electric field gradients in metals

The intensive study of electric field gradients (EFG) at the site of atomic nuclei in noncubic metals has revealed several systematic trends, e.g. in the relation of the total EFG to the so called lattice EFG as well as in its variation with temperature. Numerous investigations have been carried out in order to test these systematics. The results will be reviewed and compared to the known trends. Progress in theoretical calculations of the EFG in pure and impurity — host systems will be discussed and compared to the latest available experimental data. Recent measurements of the EFG at metal surfaces and new calculations of the EFG at host sites in impure cubic metals may contribute to the understanding of the EFG in metals.     

Identification of Defects in Semiconductors via their Electric Field Gradients

Recent theoretical calculations show for defect complexes in semiconductors, characterized by electric field gradients (EFG), that their chemical compositions, electronic charge states and the induced lattice relaxations can be obtained by comparing the experimental and calculated EFG. The experimental data are obtained by perturbed γγ angular correlation experiments and the calculations are performed using ab initio full potential methods in the framework of density functional theory. This revised version was published online in September 2006 with corrections to the Cover Date.     

First-principles calculation of structural stability,lattice dynamic and thermodynamic properties of BeX (X = S,Se and Te) compounds under high pressure

The phase transitions, lattice dynamical and thermodynamic properties of BeS, BsSe and BeTe at high pressure have been investigated with the density functional theory. The calculated equilibrium structural parameters agree well with the available experimental and theoretical values. The phase transition pressures from the zinc-blende (ZB) to the nickel arsenide (NiAs) phase of these compounds are determined. The calculated phonon dispersion curves of these compounds in ZB phase at zero pressure do not show any anomaly or instability. Dynamically, the ZB phase of BeS, BeSe and BeTe is found to be stable near transition pressures P_T. Within the quasiharmonic approximation, the thermodynamic properties including the thermal expansion coefficient, heat capacity at constant volume, heat capacity at constant pressure and entropy are predicted.     

Elastic,electronic and thermodynamic properties of fluoro-perovskite KZnF3 via first-principles calculations

The elastic, electronic and thermodynamic properties of fluoro-perovskite KZnF₃ have been calculated using the full-potential linearized augmented plane wave (FP-LAPW) method. The exchange-correlation potential is treated with the generalized gradient approximation of Perdew-Burke-Ernzerhof (GGA-PBE). Also, we have used the Engel and Vosko GGA formalism (GGA-EV) to improve the electronic band structure calculations. The calculated structural properties are in good agreement with available experimental and theoretical data. The elastic constants C _ij are calculated using the total energy variation with strain technique. The shear modulus, Young’s modulus, Poisson’s ratio and the Lamé coefficients for polycrystalline KZnF₃ aggregates are estimated in the framework of the Voigt-Reuss-Hill approximations. The ductility behavior of this compound is interpreted via the calculated elastic constants C _ij . Electronic and bonding properties are discussed from the calculations of band structure, density of states and electron charge density. The thermodynamic properties are predicted through the quasi-harmonic Debye model, in which the lattice vibrations are taken into account. The variation of bulk modulus, lattice constant, heat capacities and the Debye temperature with pressure and temperature are successfully obtained.     

Mössbauer effect and first principle calculations of the electronic structure and hyperfine interaction parameters of Hf2Fe

A detailed theoretical study of the structure, electronic properties and the electric field gradients of the Hf₂Fe intermetallic compound is presented. Using all-electron full-potential linearized augmented plane wave (FP-LAPW) formalism the equilibrium volume, bulk modulus and electric field gradients are calculated. The obtained results are compared with EFG values inferred from measurements performed using Mössbauer spectroscopy and the earlier reported time differential perturbed angular correlation (TDPAC) measurements. The lattice relaxation and the supercell calculations are found to be essential for the correct interpretation of the experimental results.     

The influence of hydrostatic pressure on the static and dynamic properties of an InSe crystal: A first-principles study

The pressure dependences of the crystallographic parameters of the γ-InSe structure in the range up to 15 GPa are theoretically investigated using the density-functional method. It is established that, in the range from 7 to 11 GPa, the pressure dependences of the lattice parameters exhibit an anomalous behavior: the rigidity of the lattice increases in the direction of the weak bond and decreases in the plane of the layers. The layer thickness is characterized by a nonmonotonic pressure dependence. The numerical values of the lattice parameters of the crystal structure under compression are determined, and the pressure dependences of the vibrational frequencies at the center of the Brillouin zone are calculated theoretically. The results of the theoretical calculations are in good agreement with the available experimental data.     

Pseudo-potential calculations of structural and elastic properties of spinel oxides ZnX2O4 (X=Al,Ga, In) under pressure effect

First principle calculations of structural and elastic properties of ZnAl₂O₄, ZnGa₂O₄ and ZnIn₂O₄ compounds are presented, using the pseudo-potential plane-waves approach based on density functional theory, within the generalized gradient approximation GGA. The lattice constants and internal parameters are in good agreement with the available experimental results. Young's modulus, Poisson ratio, bulk modulus, elastic constants and their pressure dependence are also calculated. As the experimental elastic constants are not available hence our results were only compared with the available theoretical values obtained at equilibrium volume.     

First-principles study of structural,elastic, lattice dynamical and thermodynamical properties of GdX (X = Bi,Sb)

The results are presented of first-principles calculations of the structural, elastic and lattice dynamical properties of GdX (X = Bi, Sb). In particular, the lattice parameters, bulk modulus, phonon dispersion curves, elastic constants and their related quantities, such as Young's modulus, Shear modulus, Zener anisotropy factor, Poisson's ratio, Kleinman parameter, and longitudinal, transverse and average sound velocities, were calculated and compared with available experimental and other theoretical data. The temperature and pressure variations of the volume, bulk modulus, thermal expansion coefficient, heat capacities, Grüneisen parameter and Debye temperatures were predicted in wide pressure (0?50 GPa) and temperature ranges (0–500 K). The plane-wave pseudopotential approach to the density-functional theory within the GGA approximation implemented in VASP (Vienna ab initio simulation package) was used in all computations.     

Structural and thermodynamic properties of AlB2 compound

We employ a first-principles plane wave method with the relativistic analytic pseudopotential of Hartwigsen, Goedecker and Hutter (HGH) scheme in the frame of DFT to calculate the equilibrium lattice parameters and the thermodynamic properties of AlB₂ compound with hcp structure. The obtained lattice parameters are in good agreement with the available experimental data and those calculated by others. Through the quasi-harmonic Debye model, obtained successfully are the dependences of the normalized lattice parameters a/a₀ and c/c₀ on pressure P, the normalized primitive cell volume V/V₀ on pressure P, the variation of the thermal expansion α with pressure P and temperature T, as well as the Debye temperature \Theta_D and the heat capacity C_V on pressure P and temperature T.     

Angular correlation studies on 172Lu(172Yb) in GaN and measurements at low temperatures

For optoelectronic devices semiconductors with large band gap doped with rare earth are used. Doping is generally performed during growth but for more structured doping the ion implantation technique is preferable. The perturbed γγ-angular correlation technique is an ideal tool to study the behavior of semiconductors after implantation. An adequate rare earth isotope for such investigations of semiconductors is ¹⁷²Yb. The temperature dependence of the hyperfine fields for ¹⁷²Lu(¹⁷²Yb) in GaN has been analysed. The total electric field gradient (EFG) at the site of this probe is a superposition of the lattice EFG due to the GaN wurtzite structure and the EFG due to the 4f shell of the rare earth probe itself. The latter is strongly temperature dependent and opposed to the lattice EFG which in contrast is nearly constant since the lattice parameters change only negligibly with temperature. At elevated temperatures the sublevels of the 4f shell, split by the crystal electric field, are equally populated. But at low temperatures the lowest level is occupied preferentially. Sign and magnitude of the EFG caused by the 4f shell can be calculated (Tomala et al, J Magn Magn Mater 89(1–2):143, 1990) and the result compared to the experimental data.     

First-principles calculations of structural and thermodynamic properties of BeB2 compound

The lattice parameter bulk modulus and pressure derivative of BeB2 are calculated by using the Cambridge Serial Total Energy Package （CASTEP） program in the frame of density function theory. The calculated results agree well with the average experimental data and other theoretical results. Through the quasi-harmonic Debye model, the dependences of the normalized lattice parameters a/ao, c/c0 and the normalized primitive cell volume V/Vo on pressure P, the variation of the thermal expansion coefficient ~ with pressure P and temperature T, as well as the dependences of the heat capacity Cv on pressure P and temperature T are obtained systematically.     

Pressure dependence of the electric field gradient in the AgIn2 intermetallic compound

The pressure dependence of the electric field gradient (EFG) in the AgIn₂ intermetallic compound was measured from zero up to 35 kbar using the time differential perturbed angular correlation technique in¹¹¹Cd. The unit cell volume and thec/a ratio variations with pressure were measured up to 80 kbar. The temperature dependence of the cell parameters was also measured, in a range varying from 300 K up to 458 K. The relationship of these results showed that the contribution of the lattice thermal expansion to the EFG variations is about 1/3, a small but not negligible part. The estimated EFG volumetric dependence is at variance with the systematic results found in pure metals. Work supported in part by FINEP and CNPq (Brasil).     

Crystal-field interpretation of the temperature dependence of the57Fe Mössbauer quadrupole splitting in two orthopyroxenes

The temperature dependences of the M2 and the M1 ferrous quadrupole splittings in two orthopyroxenes have been studied. The valence contributions to the EFG were calculated from the Boltzmann averages of the expectation values of the EFG components over the corresponding⁵D levels, while the lattice contributions were obtained from lattice summations. The proper⁵D level schemes were each determined by diagonalization of the full crystal-field Hamiltonian, for which the operator coefficients were calculated so as to take into account the real C₁ symmetry of the FeO ₆ ^10– clusters. The calculations were based on the lattice and the charge data related to ferrosilite.     

Valence fluctuations in CeIn3 under the effect of pressure

The impacts of pressure on the structural and electronic properties of CeIn₃ have been calculated. The calculations are performed in the presence and the absence of spin-orbit interaction as well as GGA+U using density functional theory within the PBE-GGA approximation. It is shown that energy and density of states analyses are considerably influenced by the spin-orbit interaction. The spin and orbital magnetic moments of Ce are calculated under pressure up to 22 GPa. An almost linear relation is observed between the magnetic moment and the density of states of Ce-4f at Fermi level. At ambient pressure, a good agreement between the values of the electric field gradients, EFG, and bulk modulus with experimental results is observed. The strongest anisotropy in charge distribution originates from In-5p orbital, which has the main contribution to EFG.     

金属钒弹性波速与热力学函数的理论计算

在全电子水平上,基于广义梯度近似(GGA)的密度泛函理论和全势能线性缀加平面波方法(FLAPW)计算了钒的晶格参数,弹性波速和格临爱森参数.在德拜模型的基础上,利用弹性波速方法和原子位移方法分别计算了钒的德拜频率,以及在标准条件下(298.15 K,1atm)的热容,熵等热力学函数,并与实验值进行了比较.     

Lattice dynamical calculation of the second Grüneisen constant of CsBr

The second order Grüneisen constant constitutes an additional measure of the anharmonicity in a solid (the Grüneisen constant itself being the first measure). This quantity and the corresponding second mode-Grüneisen parameters are calculated for CsBr using a modified rigid ion model of lattice dynamics. The model parameters are deduced from the three elastic constants and the long wavelength optical mode frequencies. Using the first and second order pressure dependences of these physical observables, the individual second mode-Grüneisen parameters are obtained as a function of wave vector. The temperature variation of the second Grüneisen constant is then deduced in a quasi-harmonic approximation. The second Grüneisen constant is also calculated from available thermodynamic data and is in reasonable agreement with the present theoretical prediction.     

高压下TiC的弹性、电子结构及热力学性质的第一性原理计算

利用基于密度泛函理论的第一性原理平面波赝势方法 并结合准谐徳拜模型研究了NaCl结构的TiC在高压下的弹性性质、电子结构和热力学性质. 计算所得零温零压下的晶格常数、体弹模量及弹性常数与实验值符合得很好. 零温下弹性常数和弹性模量随压强增大而增大. 通过态密度和电荷密度的分析, Ti-C键随压强增大而增强. 运用准谐德拜模型, 成功计算了TiC在高温高压下的体弹模量、熵、热膨胀系数、徳拜温度、 Grüneisen参数和比热容. 结果表明压强对体弹模量、热膨胀系数和徳拜温度的影响大于温度对其的影响. 热容随着压强升高而减小, 在高温高压下, 热容接近Dulong-Petit极限.