Born Effective Charge Tensors Research Articles

Ab initio study of the lattice dynamical and thermodynamic properties of SbXI (X= S, Se, Te) compounds

The Born effective charges, high-frequency dielectrics constants, vibrational and thermodynamics properties of paraelectric SbSeI, SbTeI, SbSI and ferroelectric SbSI phase have been calculated by ab-initio method within the local density approximation. The high-frequency dielectric and Born effective charge tensor components are analyzed within density functional perturbation theory. We find remarkably enhanced/reduced Born effective charges and anomalously high-frequency dielectric constants for SbXI compounds. The phonon dispersion spectra and density of phonon states of these compounds were acquired using the direct method based on calculated Hellmann-Feynman forces. A comparison with the phonon dispersion curves of SbXI compounds displays minor differences that can attributed to the atomic-mass difference among S, Se and Te. The Raman and infrared active phonon mode frequencies of compounds are predicted at the Γ point. The thermodynamic properties of these compounds such as the internal energy, Helmholtz free energy, entropy, heat capacity, and Debye temperature are also investigated and discussed in the framework of the harmonic approximation. The obtained results were compared with available experimental and other theoretical data.

First-principles calculations of zone center phonons and related thermal properties of MgSiN2

MgSiN2 semiconductor is alternative material beyond the family of III-N, which is used widely in optoelectronic devices. This work presents the calculations of the dielectric tensor, Born effective charge tensor, phonon frequencies at Γ point, phonon dispersion, Helmholtz free energy (ΔF), internal energy (ΔE), entropy (ΔS) and specific heat capacity at constant volume (Cv) of MgSiN2 by using the density-functional perturbation theory (DFPT) as implemented in ABINIT. We found that the phonons and thermal properties of MgSiN2 have similar properties to those of ZnSiN2 and Cv of MgSiN2 are in agreement with the available experimental results.

Structure and lattice dynamics of the wide band gap semiconductors MgSiN2 and MgGeN2

We have determined the structural and lattice dynamical properties of the orthorhombic, wide band gap semiconductors MgSiN2 and MgGeN2 using density functional theory. In addition, we present the structural properties and Raman spectra of MgSiN2 powder. The structural properties and lattice dynamics of the orthorhombic systems are compared to those of wurtzite AlN. We find clear differences in the lattice dynamics between MgSiN2, MgGeN2 and AlN, for example, we find that the highest phonon frequency in MgSiN2 is about 100 cm−1 higher than the highest frequency in AlN, and that MgGeN2 is much softer. We also provide the Born effective charge tensors and dielectric tensors of MgSiN2, MgGeN2 and AlN. Phonon related thermodynamic properties, such as the heat capacity and the entropy, have also been evaluated and are found to be in very good agreement with available experimental results.

Universal two-dimensional characteristics in perovskite-type oxyhydrides ATiO2H (A = Li, Na, K, Rb, Cs)

A series of unsynthesized perovskite-type oxyhydrides ATiO2H (A = Li, Na, K, Rb, Cs) are investigated by the density functional calculations. These oxyhydrides are stable in the sense of the formation energies for some possible synthesis reactions. They are crystallized into quite similar crystal structures with the long c-axis, and the corner-sharing TiO4H2 octahedra of the ideal perovskite-type structure are deformed into the 5-fold coordinated titanium atoms with the OH plane and the apical oxygen atoms. All of these oxyhydrides exhibit two-dimensional electronic states at the valence band maximum characterized by the in-plane oxygen 2p and the hydrogen 1s orbitals. While the c-axis becomes short as the ionic radius of the A atom becomes small and the two-dimensional characteristics are weakened, the electronic state at the valence band maximum is still characterized as the O-H in-plane state. Additionally, the Born effective charge tensors, spontaneous electric polarizations, dielectric tensors, and piezoelectric tensors are evaluated. It is found that the spontaneous electric polarizations of these oxyhydrides are much larger than that of tetragonal BaTiO3.

Mechanical properties, Born effective charge tensors and high frequency dielectric constants of the eight phases of BaTiO3

We have preformed the first-principles calculations for the mechanical properties, Born effective charge tensors and high frequency dielectric constants of the eight phases of BaTiO3. The independent elastic constants, bulk modulus, shear modulus, Young’s modulus and Poisson’s ratio were obtained, which were consistent with the available theoretical and experimental values. The mechanical stability and brittle/ductile behaviors of the eight phases of BaTiO3 have been discussed. The calculated results indicated that the eight phases were all mechanically stable and behaved in a brittle manner. The calculated Born effective charge tensors shown the covalent Ti–O bond and ionic Ba–O bond. Moreover, the high frequency dielectric constants have been given.

First-principle study of the lattice dynamic and thermodynamic properties of Zn-based semiconductors with wurtzite structure

The first-principle calculations for the structural, dynamic and thermodynamic properties of the IIB-VIA Zn-based compound semiconductors ZnX (X=O, S, Se, Te) with wurtzite structure are preformed in this work. The density-functional perturbation theory are applied to obtain the Born effective charge tensors, the phonon dispersion curves, as well as corresponding density of states. The results show that the Born effective charge tensors are anisotropic and exhibit the anisotropy of material itself. The calculated phonon frequencies of ZnO and ZnS at Γ point and along high symmetry directions are in agreement with other calculations and experimental data. The phonon contributions to the internal energy, free energy, entropy and specific heat are determined within the harmonic approximation based on the calculated phonon dispersion relations. The calculated results of entropy are slightly larger than the experimental values by about 4.5%, since the effects of anharmonicity are ignored and the theoretical lattice constants are used in the calculations.

Lattice dynamics properties of chalcopyrite ZnSnP2: Density-functional calculations by using a linear response theory**Project supported by the Open Research Fund of Computational Physics Key Laboratory of Sichuan Province, Yibin University (Grant No. JSWL2014KFZ01), the Scientific Research Fund of Sichuan Provincial Education Department, China (Grant No. 16ZB0209), the Scientific Research Foundation of Chengdu University of Information Technology, China (Grant No. J201611),

We present a first-principles study of the structural, dielectric, and lattice dynamical properties for chalcopyrite semiconductor ZnSnP2. The structural properties are calculated using a plane-wave pseudopotential method of density-functional theory. A linear response theory is used to derive Born effective charge tensors for each atom, dielectric constants in low and high frequency limits, and phonon frequencies. We calculate all zone-center phonon modes, identify Raman and infrared active modes, and report LO–TO splitting of the infrared modes. The results show an excellent agreement with experiment and propose several predictive behaviors.

Topological Insulators: Electronic Band Structure and Spectroscopy

In this study, we present the results of our ab initio calculation of the elastic constants, density of states, charge density, and Born effective charge tensors for ferroelectric (rhombohedral) and paraelectric phases (cubic) of the narrow band ferroelectrics (GeTe, SnTe) pseudopotentials. The related quantities such as bulk modulus and shear modulus using obtained elastic constants have also been estimated in the present work. The total and partial densities of states corresponding to the band structure of Sn(Ge)Te(S,Se) were calculated. We also calculated the Born effective charge tensor of an atom (for instance, Ge, Sn, Te, etc.), which is defined as the induced polarization of the solid along the main direction by a unit displacement in the perpendicular direction of the sublattice of an atom at the vanishing electric field.

Electronic structure, vibrational and thermoelectric properties of AgTaO3: A first-principles study

We investigate electronic structure, vibrational, ferroic and thermoelectric properties of AgTaO3 within the frame of density functional theory. The atomic structure is fully relaxed and the structural parameters are found to differ by less than 1–5% from experimental data. The associated electronic band structure and density of states presented. We obtain the phonon frequencies at the center of Brillouin zone, Born effective charge tensors using density functional perturbation theory and the computed frequencies at the Γ- point agree with the experimental reported values. The born effective charge tensor suggests that the weak ferroelectricity arises because of displacement of Ta and O atoms. The thermoelectric properties such as thermopower, electrical and thermal conductivity and power factor has been discussed with variation of temperature and concentration doping. The highest power factor obtained ∼91 × 1014 W cm−1 K−1 at the optimum carrier concentration 8 × 1021 cm−3 for n-type doping at the 800 K. This results suggests that n-type doping can enhance the thermoelectric properties of AgTaO3 material in the high temperature region.

Ab initio calculation of electron–phonon coupling in monoclinic β-Ga2O3 crystal

The interaction between electrons and vibrational modes in monoclinic β-Ga2O3 is theoretically investigated using ab-initio calculations. The large primitive cell of β-Ga2O3 gives rise to 30 phonon modes all of which are taken into account in transport calculation. The electron-phonon interaction is calculated under density functional perturbation theory and then interpolated using Wannier–Fourier interpolation. The long-range interaction elements between electrons and polar optical phonon (POP) modes are calculated separately using the Born effective charge tensor. The direction dependence of the long-range POP coupling in a monoclinic crystal is explored and is included in the transport calculations. Scattering rate calculations are done using the Fermi golden rule followed by solving the Boltzmann transport equation using the Rode's method to estimate low field mobility. A room temperature mobility of 115 cm2/V s is observed. Comparison with recent experimentally reported mobility is done for a wide range of temperatures (30 K–650 K). It is also found that the POP interaction dominates the electron mobility under low electric field conditions. The relative contribution of the different POP modes is analyzed and the mode 21 meV POP is found to have the highest impact on low field electron mobility at room temperature.

Electronic structure and lattice dynamics of rhombohedral BiAlO3 from first-principles

The structural, elastic, electronic, dynamical (zone-center phonon modes and Born effective charge tensors), and ferroelectric properties of the rhombohedral BiAlO3 were calculated within various exchange-correlation functionals. The standard local-density (LDA) and generalized gradient (GGA) approximations, and nonlocal hybrid Heyd-Scuseria-Ernzerhof (HSE) were used. We have also performed the electronic structure calculations with meta-GGA Tran-Blaha functional. BiAlO3 is indirect band gap semiconductor with the value of band gap: 2.87 eV (GGA), 4.14 eV (HSE), and 3.78 eV (TB-mBJ). The calculated spontaneous polarization is 81 μC/cm2 (87 μC/cm2) for GGA (HSE). The vibrational spectrum including LO-TO splitting was calculated within GGA. The zone-center phonon modes with LO-TO splitting for BiAlO3 were compared with those in isostructural BiFeO3.

Vibrational spectra of Ruthenium Carbide structures yielded by the structure search employing evolutionary algorithm

Out of the three dynamically stable structures of Ruthenium Carbides yielded by the exhaustive structure search employing evolutionary algorithm, Born effective charges are computed for the semiconducting RuC in Zinc blende structure using density functional perturbation theory. Using the phonon frequencies and the Born effective charge tensors of Ru and C in this structure, infrared spectrum is generated for this system. Computations of these dynamical quantities and IR spectra from first principles can be helpful in the unambiguous determination of the stoichiometry and structure by comparison of the experimental measurements with the computational predictions. The positive formation energies of the three systems show that high pressure and possibly high temperature may be necessary for their synthesis. Formation energies of these systems at different pressures are computed. One of the structurally stable systems, Ru3C with hexagonal structure (P6¯m2), has negative formation energy at 200GPa. The system reported from the first synthesis of Ruthenium Carbide also has the same symmetry, though it has a different stoichiometry.

Electronic structure and optical properties of orthorhombic and rhombohedral RAlO3 (R = Sm, Nd)

The electronic structure and optical properties of RAlO3 (R = Sm, Nd) have been investigated by full potential linearized augmented plane wave method within the framework of density functional theory in its generalized gradient approximation. The calculations are initiated with the experimental lattice constants obtained by the Rietveld refinement of the XRD data of the synthesized samples. The band gaps of the systems are measured from the UV–visible reflectance spectra using the Kubelka–Munk function and compared with the calculated data. The spin-polarized calculation has been carried out with on-site Coulomb potential U to consider the electron–electron correlation in the system. The X-ray photoemission spectra of the systems are measured. The valence-band density-of-states spectra are generated and compared with the X-ray photoemission spectra. The chemical shifts of these compounds suggest a mixed ionic and covalent character of the bonds. It has been observed that f-state of Nd and Sm hybridizes with O-2p state near the Fermi level in valence band. The Born effective charge tensors of the constituent ions for SmAlO3 and NdAlO3 have also been calculated which shows strong hybridization and charge transfer between Sm and O in SmAlO3 and Nd and O in NdAlO3. The calculated static dielectric tensor is found to be in good agreement with the experimental values.

Analysis of charge states in the mixed-valent ionic insulator AgO

The doubly ionized ${d}^{9}$ copper ion provides, originally in ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4}$ and later in many more compounds, the platform for high temperature superconductivity when it is forced toward higher levels of oxidation. The nearest chemical equivalent is ${\mathrm{Ag}}^{2+}$, which is almost entirely avoided in nature. AgO is an illustrative example, being an unusual nonmagnetic insulating compound with an open $4d$ shell on one site. This compound has been interpreted in terms of one ${\mathrm{Ag}}^{3+}$ ion at the fourfold site and one ${\mathrm{Ag}}^{+}$ ion that is twofold coordinated. We analyze more aspects of this compound, finding that indeed the ${\mathrm{Ag}}^{3+}$ ion supports only four occupied $4d\ensuremath{-}\mathrm{based}$ Wannier functions per spin, while ${\mathrm{Ag}}^{+}$ supports five, yet their physical charges (as quantified by their spherical radial charge densities) are nearly equal. The oxygen $2p$ Wannier functions display two distinct types of behavior, one type of which includes conspicuous Ag $4d$ tails. Calculation of the Born effective charge tensor shows that the mean effective charges of the Ag ions differ by about a factor of 2, roughly consistent with the assigned formal charges. We analyze the $4d$ charge density and discuss it in terms of recent insights into charge states of insulating (and usually magnetic) transition-metal oxides. What might be expected in electron- and hole-doped AgO is discussed briefly.

The electronic structures, Born effective charge tensors, and phonon properties of cubic, tetragonal, orthorhombic, and rhombohedral K0.5Na0.5NbO3: A first-principles comparative study

The electronic structures, Born effective charges (BECs), and full phonon dispersions of cubic, tetragonal, orthorhombic, and rhombohedral K0.5Na0.5NbO3 are investigated by the first principles method based on density functional theory. The hybridized states of Nb 4d and O 2p states are observed in the valence band, showing the formation of a strong Nb—O covalent bond which should be responsible for the displacement of Nb and O atoms. The abnormally large BECs of Nb and O indicate the possibility of phase instability induced by the off-center displacement of Nb and O atoms. The phonon dispersions reveal that the ferroelectric instability of K0.5Na0.5NbO3 is dominated by Nb and O displacements with significant Na characteristics. In addition to the ferroelectric instability, there is also rotational instability coming from the oxygen octahedra rotation around one axis. Moreover, the Γ phonon properties of orthorhombic KNbO3, NaNbO3, and K0.5Na0.5NbO3 are also studied in detail.

Dynamical and dielectric properties of MP2O7 (M = Ti, Zr, and Hf): A first-principles investigation

A first-principles investigation on the structural and vibrational properties of MP2O7 (M=Ti, Zr and Hf) has been performed. Using density functional perturbation theory, the Born effective charge tensors, the IR-active phonon frequencies at the center of the Brillouin zone, and the dielectric constants of MP2O7 are obtained. Due to the complex structure of MP2O7, the Born effective charge tensors are quite complex. The anomalously large values of Z* implies the covalent bonding nature of MP2O7. The frequencies of zone center IR-active modes are closely related to the local structural and chemical environments of atoms. The electronic and static dielectric permittivities are analyzed in detail. The theoretical results highlight the vital role of MO6 octahedron underpinning the properties of MP2O7.

Infrared Absorption Spectrum of Brushite from First Principles

Brushite, CaHPO4·2H2O, is a crystalline hydrated acidic form of calcium phosphate that occurs in both physiological and pathological biomineralization processes. Additionally, it is biocompatible in humans. Several groups have investigated the experimental Fourier transform infrared vibrational spectrum of brushite. These investigations have led to a long-standing debate concerning the correct assignment for a few of the observed frequencies, particularly, the water stretching and libration modes. Here, we perform a comprehensive first principles theoretical investigation of the vibrational spectrum of brushite with calculations based on dispersion-corrected density functional theory. We obtain both the vibrational frequencies, using the frozen phonon approach, and their corresponding peak intensities, based on the Born effective charge tensor. This allows for an unambiguous assignment of all vibrations, including water vibration and libration modes.

Structural and ferroelectric transitions in magnetic nickelate PbNiO3

Density functional calculations have been tremendously useful in understanding the microscopic origin of multiferroicity and in quantifying relevant properties in many multiferroics and magnetoelectrics. Here, we focus on a relatively new and promising compound, PbNiO3. The structural, electronic and magnetic properties of its two polymorphs, i.e. the orthorhombic structure with space group Pnma and the rhombohedral LiNbO3-type structure with space group R3c have been studied by using density functional calculations within DFT + U and hybrid functional schemes. Our data convey an accurate description of the pressure-induced phase transition from the rhombohedral to orthorhombic phase at a predicted critical pressure of 5 GPa in agreement with the measured value of 3 GPa. Both phases show the G-type antiferromagnetic configuration as a magnetic ground state, but differ in the spatial anisotropy associated with nearest-neighbor exchange couplings, which is strongly weakened in the rhombohedral LiNbO3-type phase. The predicted large ferroelectric polarization of the rhombohedral phase (Hao et al 2012 Phys. Rev. B 014116) has been re-explored and analyzed in detail using partial density of states, Born effective charge tensors, charge density difference, electron localization function analysis and distortion mode analysis. The asymmetric bonding between the Pb 6s and O 2p orbitals along the [111]-direction is responsible for the polar cationic displacement, giving rise to a predicted large ferroelectric polarization as high as ∼ 100 μC cm−2.

Physical properties of zircon and scheelite lutetium orthovanadate: Experiment and first-principles calculation

Pure zircon and scheelite LuVO4 were prepared by solid state reaction and high-pressure route, respectively. Structure, elastic constants, lattice dynamics and thermodynamics of LuVO4 polymorphs were studied by experiments and first principles calculation. Calculations here are in good agreement with the experimental results. The phonon dispersions of LuVO4 polymorphs were studied by the linear response method. The calculated phonon dispersions show that zircon and scheelite LuVO4 phases are dynamically stable. Raman-active frequencies were measured and assigned to different modes according to the calculations. The internal frequencies shift downward after phase transition from zircon to scheelite. Born effective charge tensors elements for both phases are analyzed. The finite temperature thermodynamic properties of LuVO4 polymorphs were calculated from the obtained phonon density of states by quasi-harmonic approach.

Structural, electronic, elastic and vibrational properties of BiAlO3: A first principles study

We present a study of the structural, electronic, elastic and vibrational properties of the rhombohedral BiAlO3 structure within the local density approximation of density functional theory using norm-conserving pseudopotentials. The calculated equilibrium lattice constant, angle and atomic position are in reasonable agreement with the available experimental and theoretical dates. Based on the elastic constants and their related parameters, the crystal mechanical stability have been discussed. The elastic constants for BiAlO3 are also needed to completely determine its elastic properties including polycrystalline bulk, shear and Young’s moduli, Poisson’s ratio and the elastic anisotropy. Energy band structure shows that the rhombohedral BiAlO3 has an indirect band gap between D and Г-D symmetry points. We compute Born effective charge tensor, which is found to be quite anisotropic of Bi and O atoms. BiAlO3 have been studied by applying the direct method and deriving the phonon dispersion relations which include the longitudinal/transverse optical phonon mode splitting. In the rhombohedral phase the phonon dispersion curves show a soft mode between X and Г-point. This soft mode leads to the observed rhombohedral-cubic phase transition. The results are compared with the previous calculations and available experimental data.