Structural,elastic, and lattice dynamical properties of Germanium diiodide (GeI2)

To investigate the structural, elastic, and lattice dynamical properties of the germanium diiodide, we have performed the first-principles calculations by using the local density approximation method based on density-functional theory. Some basic physical parameters such as lattice constant, bulk modulus and its first derivatives, elastic constants, shear modulus, Young’s modulus, and Poisson’s ratio are calculated. The phonon dispersion curves, electronic band-structures, and total and partial density of states have also been calculated for ground state C6 phase of GeI₂. Our results show that this structure has got 1.72 eV direct band gap. Our secondary results on the temperature-dependent behavior of thermodynamical properties such as entropy, heat capacity, internal energy, and free energy are also presented for the same compounds. The obtained results are in good agreement with the available experimental and other theoretical data.     

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.     

Structural, elastic, electronic, chemical bonding and thermodynamic properties of CaMg2N2 and SrMg2N2: First-principles calculations

We report first-principles density functional theory calculations of the structural, elastic, electronic, chemical bonding and thermodynamic properties of the ternary alkaline earth metal nitrides CaMg₂N₂ and SrMg₂N₂. The calculated equilibrium structural parameters agree well with the experimental findings. Single-crystal and polycrystalline elastic constants and some related properties under pressure effect have been predicted. Both compounds exhibit a striking elastic anisotropy and a ductile behavior. Electronic properties and chemical bonding nature have been studied throughout the band structure, density of states and charge distribution analyses. It is found that these two materials have a direct band gap (Γ-Γ) and a transition to an indirect gap (Γ-M) occurs at about 8.63 and 5.16 GPa in CaMg₂N₂ and SrMg₂N₂, respectively. The chemical bonding has a mixture covalent-ionic character. Thermal effects on some macroscopic properties are predicted using the quasi-harmonic Debye model.     

The first-principles study of LaSe and LaTe in B1 and B2 structures

The structural, electronic and dynamical properties of LaSe and LaTe compounds in NaCl(B1) and CsCl(B2) structures are studied by performing ab initio calculations within the generalized gradient approximation (GGA). Bulk properties including lattice constants, static bulk modulus, first-order pressure derivative of the bulk modulus, cohesive energies and first-order phase transitions are reported and compared with available experimental and other theoretical results. The electronic band structure is also presented for these materials. For the first time, the electronic structure results are used, within the implementation of linear-response technique, for calculations of phonon properties. A detailed discussion of atomic displacement pattern for LaTe in B1 structure is also presented. The phonon dispersion curves for two materials are compared and contrasted.     

Structural,elastic, and lattice dynamical properties of YB2 compound

In this work, density functional theory calculations on the structural, mechanical, lattice dynamical, and thermodynamical properties of YB₂ in AlB₂-type and monoclinic (C2/m) structures are reported. The local density approximation has been used for modeling exchange–correlation effects. We have predicted the lattice constants, bulk modulus, elastic constants, shear modulus, Young’s modulus, Poison’s ratio, Debye temperature, and sound velocities. Furthermore, the phonon dispersion curves, corresponding phonon density of states, some thermodynamical quantities such as internal energy, entropy, heat capacity, and their temperature-dependent behaviors are presented. Our structural and some other results are in agreement with the available experimental and theoretical data.     

Vibrational modes and electrical transport in Sr2GdTaO6

The double perovskite oxide strontium gadolinium tantalate, Sr₂GdTaO₆ (SGT) is synthesized by solid-state reaction technique. The Rietveld refinement of the X-ray diffraction pattern of the sample shows monoclinic phase at room temperature. FTIR spectrum shows two primary phonon modes of the sample at around 373 cm⁻¹ and 562 cm⁻¹. The electronic structure of SGT has been investigated by Vienna ab-initio simulation package. The eigen frequencies of different phonon modes have been calculated and compared with the experimental data observed by Raman spectroscopy. Dielectric properties of the sample are investigated in a temperature range from 303 K to 673 K and in a frequency range of 42 Hz–1 MHz. The relaxation peaks are observed in the frequency dependent spectra for imaginary part of the dielectric constant. The modified Cole–Cole equation is used to describe the relaxation mechanism in SGT. The frequency dependent conductivity spectra follow the Jonscher power law.     

Stable structure of platinum carbides: A first principles investigation on the structure,elastic, electronic and phonon properties

A comprehensive first principles study of structural, elastic, electronic, phonon and thermodynamical properties of novel metal carbide, platinum carbide (PtC) is reported within the density functional theory scheme. The ground state properties such as lattice constant, elastic constants, bulk modulus, shear modulus and finally the enthalpy of PtC in zinc blende (ZB) and rock-salt (RS) structures are determined. The energy band structure and electron density of states for the two phases of PtC are also presented. Of these phases zinc blende phase of PtC is found stable and phase transition from ZB to RS structure occurs at the pressure of about 37.58 GPa. The phonon dispersion curves and phonon DOS are also presented. All positive phonon modes in phonon dispersion curves of ZB-PtC phase indicate a stable phase for this structure. Within the GGA and harmonic approximation, thermodynamical properties are also investigated. All results reveal that the synthesized PtC would favor ZB phase. The compound is stiffer and ductile in nature.     

Optical, magnetic and electronic properties of Ln2O2Te (Ln = La, Sm and Gd)

The synthesis, the optical and magnetic properties and the electronic structure of the rare-earth oxytellurides of formula Ln₂O₂Te (Ln = La, Sm and Gd) are reported. The magnetic measurements show that La₂O₂Te exhibit a diamagnetic behavior. The value of the magnetization M of Gd₂TeO₂ at our highest available field (50 kOe) is about 5.5μ_B, well below the expected value for two independent Gd ions (2gJ = 14μ_B) confirming the antiferromagnetic character of this compound. The magnetic properties of Sm₂TeO₂ reveal that Sm is in its 3+ oxidation state with a large Van Vleck contribution. The electronic structure calculations, studied by means of a first principles DFT approach, are consistent with the optical measurements and suggest an indirect band gap semiconductor behavior.     

First principles calculations of the electronic, dynamical, and thermodynamic properties of the rocksalt ScX (X = N, P, As, Sb)

We have investigated the electronic, dynamical, and thermodynamic properties of the rocksalt ScX (X = N, P, As, Sb) using a plane-wave pseudopotential method within the generalized gradient approximation in the frame of density functional perturbation theory. The calculated lattice constants are found to differ by less than 0.56% from the available experimental values. These materials have the indirect Γ–X band gaps and a wide and direct band gap at the X-point in band structure, which are closer to experimental results than the previous calculations. A linear-response approach is used to calculate the phonon frequencies, the phonon density of states and LO–TO splitting. The obtained phonon frequencies at the zone-center (Γ-point) for the Raman-active and infrared-active modes are analyzed. We also calculate the thermodynamic functions using the phonon density of states, and the calculated values are in nearly perfect agreement with experimental data.     

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.     

Thermodynamic Properties of MgSc and AlSc from First-Principles Phonon Calculations

The thermodynamic properties of intermetallic compounds MgSc and AlSc with CsCl-type B2 structure have been studied by performing density functional theory and density functional perturbation theory within the quasi-harmonic approximation in comparison with NiAl. The linear thermal expansion of the lattice, coefficients of thermal expansion, isothermal bulk modulus, phonon dispersions, phonon density of states, and specific heat capacities at constant volume and constant pressure have been obtained. Comparisons have been made with available experimental data and the results of previous calculations, and good agreement has been obtained. Our results show that the thermal expansion of MgSc is larger than that of AlSc.     

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.     

Electronic and Phonon Structures of BaFe2As2 Superconductor by Ab-initio Density Functional Theory

Electronic and phonon structures of the BaFe₂As₂ superconductor in the magnetic–orthorhombic phase have been investigated by the ab-initio density functional theory using the pseudopotential Quantum Espresso code. Density of state and electronic band structure for this phase has been studied, but phonon dispersion has been obtained only for the nonmagnetic–orthorhombic phase. Electronic band structure and density of states are in good agreement with other calculations in the literature. The electronic state near the Fermi energy are essentially made from Fe3d and As4p orbital that indicate in-plane conductivity in FeAs layers in this system. Comparing calculated phonon dispersions with experimental data justify magnetic dependence of some phonon modes.     

FP-LAPW calculations of structural, electronic, and optical properties of alkali metal tellurides: M2Te [M: Li, Na, K and Rb]

Structural, electronic, and optical properties of alkali metal tellurides M₂Te [M: Li, Na, K, and Rb] are investigated in the framework of density functional theory within generalized gradient approximation. The calculated structural parameters are in excellent agreement with the experimental data. The electronic band structure calculations show that tellurides of Li, K, and Rb have an indirect fundamental energy band gap, whereas Na₂Te has a direct fundamental energy band gap. To explicate the contribution of anion and cation states to the electronic band structure, the electronic density of states for these compounds has been analyzed. Optical properties such as complex dielectric function, absorption coefficient, refractive index, extinction coefficient, and reflectivity are reported for a wide range of photon energy and are discussed on the basis of corresponding electronic band structure. Furthermore, the electron energy-loss functions for M₂Te compounds are also predicted. In order to validate the performance of the ab initio calculation reported herein, we systematically study the electronic and optical properties of wide band gap M₂Te compounds and compare them with available theoretical and experimental data of M₂O, M₂S, and M₂Se compounds.     

Ab initio, crystal field and experimental spectroscopic studies of pure and Ni-doped KZnF3 crystals

Pure and Ni²⁺ doped KZnF₃ single crystals were studied using the combination of the DFT-based ab initio methods, crystal field theory and experimental spectroscopic techniques. The electronic, optical and elastic properties have been calculated and compared with available experimental data and good agreement was achieved. Elastic anisotropy of pure KZnF₃ was modeled; calculations of the sound velocity, Debye temperature, Grüneisen parameter and specific heat capacity were performed. Comparison of the calculated results for the pure and doped material, which is reported for the first time for the considered material, enabled to identify the changes in the optical and electronic properties, which are due to the introduced nickel impurity ions. In particular, it was shown that the lowest Ni 3d states appear in the host's band gap at about 1.0 eV above the valence band. The changes of the electron density distribution after doping were also shown. Microscopic analysis of the crystal field effects based on the performed ab initio calculations of the Ni²⁺ density of states at different external pressures enabled to estimate the constants of the electron–vibrational interaction, Huang-Rhys factor, Stokes shift and local bulk modulus around impurity ions. The crystal field calculations of the Ni²⁺ energy levels were performed to analyze and assign the experimental absorption spectrum. Such a combination of the ab initio and semi-empirical calculating techniques leads to a complementary picture of the physical properties of KZnF₃:Ni²⁺ and can be applied to other doped crystals.     

Structural,thermodynamic, electronic,and optical properties of NaH from first-principles calculations

The structural stability, thermodynamic, electronic, and optical properties of NaH with rock salt (B1) structure and cesium chloride (B2) structure under high pressure are investigated by first-principles calculations using norm-conserving pseudopotential applying a generalized gradient approximation (GGA) for exchange and correlation. Through the analysis of energy–volume variation, we find the phase transition of NaH from B1 to B2 structure occurs at 32.3 GPa, which in good agreement with the diamond-anvil-cell high-pressure experimental value of 29.3 ± 0.9 GPa [Phys. Rev. B 36 (1987) 7664]. By using the quasi-harmonic Debye model, the thermodynamic properties including the Debye temperature Θ_D, heat capacity C_V, thermal expansion coefficient α, and Grüneisen parameter γ are successfully obtained in the temperature range from 0 to 700 K and pressure ranges from 0 to 32 GPa and 33 to 100 GPa for NaH B1 and B2 phases, respectively. Analysis of band structures suggests that the NaH has an indirect band gap that the valence band maximum is at the W point and the conduction minimum locates at L point. The calculated energy gaps is very close to that value obtained in recent full potential augmented plane wave calculations. The optical properties including dielectric function ?(ω), absorption coefficient α(ω), reflectivity coefficient R(ω), and refractive index n(ω) are also calculated and analyzed.     

Elastic properties and lattice dynamics of alkali chalcogenide compounds Na2S,Na2Se and Na2Te

We report on first principles study of the elastic, vibrational and dielectric properties of the alkali chalcogenide compounds Na₂S, Na₂Se and Na₂Te using the pseudopotential method within the local density approximation and linear response theory. The calculated lattice constants for the studied compounds are in good agreement with available experimental data as well as with other theoretical results. The phonon dispersion curves and phonon density of states are calculated by using density functional perturbation theory. For Na₂S the experimental features of lattice dynamics data are well reproduced by our calculations. The width of the acoustic frequency range decreases with increasing chalcogen atomic number. The phonon densities of states show that Na⁺ ions are involved in the lower frequency modes in Na₂S. However, the inverse occurs in the case of Na₂Se and Na₂Te. The mean-square ionic displacements show that Na⁺ ions perform large thermal vibrations even at temperatures well below the melting point. The Born effective charge increases, while the IR oscillator strength decreases with anion atomic number.     

First-principles calculations of structural, electronic, and elastic properties of MgZrSi2O7

For the first time, the recently synthesized pyrochlore MgZrSi₂O₇ [J. Xu et al., Mater. Chem. Phys. 128 (2011) 410] has been analyzed using the first principles calculations. The electronic and elastic properties were predicted; in particular, the band gap is indirect and has the value of 6.75 eV. The bulk modulus equals to 186.51 ± 1.95 GPa. Anisotropy of elastic properties was analyzed by comparing the upper and lower estimates of the shear moduli. In addition, directional dependence of the Young's modulus was calculated and visualized; its value varies in the range from 249.7 GPa (along the a, b, c crystallographic axes) to 136.84 GPa (along the bisector direction in any of the ab, bc, ac planes).     

First principles investigations of structural, electronic, elastic, and dielectric properties of KMgF3

An ab initio study of structural, electronic, elastic, and dielectric properties of KMgF₃ in cubic perovskite structure is presented in the framework of density functional theory. The calculations presented here employ generalized gradient approximation with projector augmented wave method. The fully relaxed structural parameters are found to be in reasonable agreement with available experimental data and with previous theoretical work. The independent elastic constants of cubic KMgF₃ are derived from the derivative of total energy as a function of lattice strain in full detail. The bulk modulus and its first pressure derivative are obtained by fitting total energy versus volume data to a Murnaghan equation of state. The electronic band structure, total density of states, and projected density of states on each of the K, Mg, and F atoms are calculated and found to be in good agreement with previous theoretical results. First principles computed phonon dispersions for the cubic KMgF₃ are reported for the first time. The imaginary part of frequency-dependent dielectric function is determined by summing over all possible transitions from occupied to unoccupied states and taking the appropriate transition matrix element into account. The Born effective charges computed by linear response within density functional perturbation theory are used together with the mode eigenvectors to decompose the lattice dielectric susceptibility tensor into contributions arising from individual IR-active phonon modes. Our results for the static and optical dielectric constant are in good agreement with previously reported experimental results.     

Lattice dynamics of layered ferroelectric semiconductor compound TlGaSe2

We present the first-principles calculation of the lattice dynamics of the TlGaSe₂ ternary semiconductor having highly anisotropic crystal structure. Calculations have been performed using open-source code ABINIT on the basis of the density functional perturbation theory within the plane-wave pseudopotential approach. Results on the frequencies of phonon modes in the centre of Brilloin zone and the dispersion of transverse shear acoustic branch of the phonon spectra agree well with the experimental data on Raman scattering, infrared reflectivity and ultrasound wave propagation in TlGaSe₂. The calculated and experimental temperature dependencies of heat capacity are in a good agreement up to the room temperature. Along the layer, the low-frequency acoustic branch displays the bending wave behavior which is characteristic of the layer crystal structures.