Lattice Dynamics Study of Magnesium Chalcogenides

First-principles calculations,which are based on the plane-wave pseudopotential approach to density functional perturbation theory within the local density approximation,have been performed to investigate the structural,lattice dynamical and thermodynamic properties of zinc blende(B3) structure magnesium chalcogenides:MgS,MgSe and MgTe.The results of ground state parameters and phonon dispersion are compared and agree well with the experimental data available and other calculations.We obtain the change of Born effective charge and LO-TO splitting under hydrostatic pressure.Finally,by the calculations of phonon frequencies,some thermodynamic properties such as the entropy,heat capacity,internal energy,and free energy are also successfully obtained.     

Theoretical investigations on the structural, lattice dynamical and thermodynamic properties of XC (X=Si, Ge, Sn)

First-principles calculations, which is based on the plane-wave pseudopotential approach to the density functional perturbation theory within the local density approximation, have been performed to investigate the structural, lattice dynamical, and thermodynamic properties of SiC, GeC, and SnC. The results of ground state parameters, phase transition pressure and phonon dispersion are compared and agree well with the experimental and theoretical data in the previous literature. The obtained phonon frequencies at the zone-center are analyzed. We also used the phonon density of states and quasiharmonic approximation to calculate and predict some thermodynamic properties such as entropy, heat capacity, internal energy, and phonon free energy of SiC, GeC, and SnC in B3 phase.     

First principles lattice dynamical calculation of semiboride Be2B and its ternary alloys XBeB (X=Na, Mg, Al)

We present results of first principles total energy calculations of the structure, electronic and lattice dynamics for beryllium semiboride and its three ternary alloys using generalized gradient and local density approximations under the framework of density functional theory. The generalized gradient approximation is used for all compounds except MgBeB using the Perdew-Burke-Ernzehorf exchange correlation functional while local density approximations use the Perdew-Zunger ultrasoft exchange correlation functional. The calculated ground state structural parameters are in good agreement with those of experimental and previous theoretical studies. The electronic band structure calculations show that Be₂B may transform to a semiconductor after Al substitution. A linear response approach to density functional theory is used to calculate phonon dispersion curves and vibrational density of states. The phonon dispersion curves of Be₂B and AlBeB are positive indicating a dynamical stablility of the structure for these compounds. The phonon dispersion curves of NaBeB and MgBeB show the imaginary phonons throughout the Brillouin zone, which confirms dynamical instability as indicated in band structures for these alloys. We also present the partial phonon density of states for different species of Be₂B and AlBeB to bring out the details of the participation of different atoms in the total phonon density of state, particularly the role played by Al atom. The first time calculated phonon properties are clearly able to bring out the significant effect of isoelectronic substitution in Be₂B.     

Pb-Sn合金及Pb的热力学性质的第一性原理计算

本文基于第一性原理的密度泛函理论(DFT)和密度泛函微扰理论(DFPT)方法,利用虚晶近似的计算方法研究了Pb-Sn合金的晶格结构、电子能带、声子能带及热力学性质,并用晶格能量差可与达到熔化温度时的振动能量相当的固-液相变机理研究了熔化温度,同时与所计算Pb的所有结果进行了对比.     

Density functional study of optical properties of beryllium chalcogenides compounds in nickel arsenide B8 structure

The structural, electronic and optical properties of beryllium chalcogenides BeS, BeSe and BeTe using the full-potential linear augmented plane wave (FP-LAPW) method are investigated. The exchange-correlation energy within the local density approximation (LDA) and the generalized gradient approximation (GGA) are described. The Engel-Vosko (EVGGA) formalism is applied for electronic and optical properties. The structural parameters of our model and the transition pressure from zinc-blende (B3) to the NiAs (B8) phase are confirmed. It is found that these compounds have indirect band gaps except for BeTe in NiAs (B8) phase. The results of reflectivity, refractive index and optical dielectric functions of Be compounds are investigated. An agreement is found between our results and those of other theoretical calculations and the experimental data.     

First principles study and comparison of vibrational and thermodynamic properties of XBi(X= In,Ga, B,Al)

In the present work, vibrational and thermodynamic properties of XBi(X = B, Al, Ga, In) compounds are compared and investigated. The calculation is carried out using density functional theory(DFT) within the generalized gradient approximation(GGA) in a plane wave basis, with ultrasoft pseudopotentials. The lattice dynamical properties are calculated using density functional perturbation theory(DFPT) as implemented in Quantum ESPRESSO(QE) code. Thermodynamic properties involving phonon density of states(DOS) and specific heat at constant volume are investigated using quasiharmonic approximation(QHA) package within QE. The phonon dispersion diagrams for InBi, GaBi, BBi, and AlBi indicate that there is no imaginary phonon frequency in the entire Brillouin zone, which proves the dynamical stability of these materials. BBi has the highest thermal conductivity and InBi has the lowest thermal conductivity. AlBi has the largest and GaBi has the smallest reststrahlen band which somehow suggests the polar property of XBi materials. The phonon gaps for InBi, GaBi, BBi and AlBi are about 160 cm~(-1), 150 cm~(-1), 300 cm~(-1), and 150 cm~(-1), respectively. For all compounds,the three acoustic modes near the gamma point have a linear behavior. C_V is a function of T~3 at low temperatures while for higher temperatures it asymptotically tends to a constant as expected.     

Structural, vibrational and thermodynamic properties of Mg2 FeH6 complex hydride

Mg₂FeH₆, which has one of the highest hydrogen storage capacities among Mg based 3d-transitional metal hydrides, is considered as an attractive material for hydrogen storage. Within density-functional perturbation theory (DFPT), we have investigated the structural, vibrational and thermodynamic properties of Mg₂FeH₆. The band structure calculation shows that this compound is a semiconductor with a direct X-X energy gap of 1.96 eV. The calculated phonon frequencies for the Raman-active and the infrared-active modes are assigned. The phonon dispersion curves together with the corresponding phonon density of states and longitudinal-transverse optical (LO-TO) splitting are also calculated. Findings are also presented for the temperature-dependent behaviors of some thermodynamic properties such as free energy, internal energy, entropy and heat capacity within the quasi-harmonic approximation based on the calculated phonon density of states.     

A first-principles study on the structural, elastic, vibrational, and thermodynamical properties of BaX (X=S, Se, and Te)

Ab-initio calculations based on norm-conserving pseudopotentials and density functional theory (DFT) have been performed to investigate the structural, elastic, thermodynamic, and lattice dynamical (phonon dispersion curves) properties of BaX in rock-salt (B1) and CsCl (B2) structures. The results support the experimental and theoretical data in the existing literature. Findings are also presented for the temperature-dependent behaviors of some thermodynamic properties such as entropy, heat capacity, internal energy, and free energy for the same compounds in the B1 phase.     

Study of DOS and energy band structures in beryllium chalcogenides

The electronic properties of beryllium chalcogenides namely, BeS, BeSe and BeTe have been investigated theoretically using the full potential-linearized augmented plane wave (FP-LAPW) method within density functional theory (DFT). The exchange correlation effects have been treated in the generalized gradient approximation (GGA). The results so obtained were compared with the experimental and the theoretical results. The energy-volume relations for these compounds have been calculated to obtain the equilibrium lattice parameters. The results of density of states (DOS) shows an indirect band gap in BeS and BeSe with no band gap for BeTe.     

Hydrogen storage: Lattice dynamics of orthorhombic NaMgH3

In this work, we investigate the structural, dynamic and thermodynamic properties of NaMgH₃, devoted for hydrogen storage. Density functional theory using pseudopotential methods and generalized gradient approximation has been used. A good agreement between the calculated structural parameters and the experimental data was found. A linear-response approach for the density functional theory is used in order to derive the Born effective charge tensors, the dielectric permittivity tensors, the phonon frequencies at the center of the Brillouin zone, the phonon-dispersion curves and the corresponding density of states for NaMgH₃ material. The obtained phonon frequencies at the zone center (Γ point) for the Raman-active and infrared-active modes are analyzed. Thermodynamic functions using the phonon density of states are also calculated.     

Vibrational and thermodynamic properties of orthorhombic CaSnO3 from DFT and DFPT calculations

Density functional theory (DFT) and density functional perturbation theory (DFPT) calculations were used to investigate the vibrational and thermodynamic properties of orthorhombic stannate CaSnO₃ compound. Our approach was based on the generalized gradient approximation with dispersion correction (GGA+D), considering the norm-conserved pseudopotentials. The phonon dispersion relation as well as theoretical peaks of the infrared (IR) and Raman spectrum in the frequency range of 100–800 cm⁻¹ was analyzed and assigned. The thermodynamic potentials and the specific heat at constant volume of the CaSnO₃ compound are also calculated, whose dependence with the temperature are discussed.     

Ab initio lattice dynamics and piezoelectric properties of MgS and MgSe alkaline earth chalcogenides

We have studied structural, elastic, dielectric, vibrational, and piezoelectric properties of rock-salt and zinc-blende Mg alkaline earth chalcogenides (MgS, and MgSe) by using the plane-wave pseudopotential method within the local density approximation. The calculated lattice constant, bulk modulus, and elastic constants for these compounds are found to be in good agreement with experiment. The density functional perturbation theory has been employed to derive the Born effective charge and high-frequency dielectric constant and to calculate phonon dispersion curves and density of states. The piezoelectric constant has also been determined. The calculated results are compared with theory and experiment when available and showed reasonable agreement. In other cases, our results are predictions. The pressure dependence of the studied quantities has been examined and discussed. The pressure coefficients and mode Grüneisen parameters are determined.     

First-principles study of lattice dynamics in thallium-V compounds

We have performed first-principles calculations to investigate the structural, lattice dynamics and thermodynamic properties of the zincblende thallium-V compounds: TlAs, TlP and TlN. The ground-state parameters, such as the lattice constant and the bulk modulus, as well as the electronic structure are calculated using the plane wave pseudopotential approach to density functional theory within the local density approximation. Phonon dispersion spectra are derived from the linear response to density functional theory. The present ab initio results for phonon dispersion are compared and contrasted with the common III–V materials. Thermodynamical properties, calculated using quasiharmonic approximations, are also reported.     

Elastic and phonon properties of FeSi and CoSi in the B2 structure

First principles calculations of structural, electronic, elastic, and phonon properties of the intermetallic compounds FeSi and CoSi in the B2 (CsCl) structure are presented, using the pseudopotential plane-wave approach based on density functional theory, within the local density approximation. The optimized lattice constants, independent elastic constants, bulk modulus, and first-order pressure derivative of the bulk modulus are reported for the B2 structure and compared with earlier experimental and theoretical calculations. A linear-response approach to density functional theory is used to derive the phonon dispersion curves, and the vibrational partial and total density of states. Atomic displacement patterns for FeSi at the Γ, X, and R symmetry points are presented. The calculated zone-center optical phonon mode for FeSi is in good agreement with experimental and theoretical data.     

First-principles calculations of elastic,phonon and thermodynamic properties of W

We investigate the elastic properties, lattice dynamical, thermal equation of state and thermodynamic properties of bcc phase W under high pressure using density functional theory. The calculated high-pressure elastic constants of bcc phase W agree well with experimental and theoretical data. Under compression, the phonon dispersion curves of bcc phase W do not show any anomaly or instability. Our calculated zero-pressure phonon dispersion curves are in excellent agreement with experiments. Within the quasiharmonic approximation, we predict the thermal equation of state and other properties including the thermal expansion coefficient, adiabatic bulk modulus, specific heat at constant volume and entropy.     

Structural, electronic and phonon properties’ investigation of YP and YAs compounds

We have studied the structural, electronic and phonon properties of the YP and YAs compounds in NaCl(B1) and CsCl(B2) structures using the density functional theory within the generalized gradient approximation (GGA). The calculated lattice constants, static bulk modulus, first-order pressure derivative of the bulk modulus and transition pressure are reported and compared with previous calculations. We have carried out the calculations of band structure and density of states (DOS) for YP and YAs. Then, a linear-response approach to the density-functional theory is used to derive the phonon frequencies and DOS in both B1 and B2 structures.     

Effect of doping Zn atom on the structural stability,mechanical and thermodynamic properties of AlLi phase in Mg–Li alloys from first-principles calculations

ABSTRACT

This work uses first-principles total energy calculations on the basis of density functional theory to predict the structural stability, mechanical and thermodynamic properties of Zn atom doped AlLi phase in Mg–Li–Al–Zn alloy. The values of the equilibrium lattice parameters and the formation of enthalpy are highly consistent with the experimental and previous calculations results available. Negative enthalpies of formation ΔH are predicted for all AlLi phase doped concentrations which have positive consequences for its structural stability. The elastic modulus is deduced by Voigt–Reuss–Hill arithmetic approximation. The bulk modulus of the Al–Li–Zn compounds increases as the doping concentrations increase, which are larger than the value of the AlLi phase. In particular, the stability and mechanical anisotropy of the Al–Li–Zn compounds are discussed. The charge density cloud map is drawn to reveal the bonding characteristics of four compounds. The changes in thermodynamic properties are derived by the phonon frequencies within the quasi-harmonic approximation.     

A first principles investigation on the structural,phonon, elastic and thermodynamic properties of the Si0.5Sn0.5 cubic alloy

An investigation on the structural, phonon, elastic and thermodynamic properties of the Si_0.5Sn_0.5 cubic alloy has been conducted using the norm-conserving pseudopotentials within the local density approximation in the frame of the density functional theory (DFT) and the density functional perturbation theory (DFPT) in the ABINIT code. Our results demonstrate that the Si_0.5Sn_0.5 cubic alloy in the zinc-blende phase keeps dynamically stable until 8 GPa. The elastic constants and thermodynamic quantities as a function of pressure and/or temperature are also presented.     

Phonon vibrational frequencies and elastic properties of solid SrFCl. An ab initio study

The phonon vibrational frequencies, electronic and elastic properties of SrFCl, one of the members of the alkaline-earth fluorohalide family crystallizing with the PbFCl-type structure, have been investigated, for the first time, at the ab initio level, by using the periodic CRYSTAL program. Both Hartree-Fock (HF) and density functional theory (DFT) Hamiltonians have been used, with the latter in its local density, gradient-corrected (PW91), and hybrid (B3LYP) versions. The structural and elastic properties are in good agreement with experiment, with the exception of those calculated within the local density approximation, which were found to be systematically under-estimated (distances) or over-estimated (elastic properties). As regards the phonon frequencies, B3LYP and PW91 provide excellent results, the mean absolute difference with respect to the experimental Raman data being 4.1% and 3.6%, respectively.     

Phase Transition and Phonon Spectrum of Zinc-Blende Structure ZnX （X = S,Se, Te）

Calculations have been performed to investigate the pressure-induced solid-solid phase transitions and the mechanical stability for three zinc-blende II-VI semiconductor compounds： ZnS, ZnSe, ZnTe by ab initio plane-wave pseudopotential density functional theory （DFT）. Using the generalized gradient approximation （GGA） for exchange and correlation in the scheme of Perdew-Wang 1991 （P Wgl ）, the ground state properties and equation of state are obtained, which are well consistent with the experimental data available and other calculations. On the basis of the forth-order Birch-Murnaghan equation of states, the transition pressures Pt are determined through the analysis of enthalpy variation with pressure. A linear-response approach is used to calculate the frequencies of the phonon dispersion. Finally, by the calculations of phonon frequencies, some thermodynamic properties such as the vibrational contribution to the Helmholtz free energy （F）, enthedpy （H）, entropy （S）, and the heat capacity （Cv ） are also successfully obtained.