Born Effective Charge Tensors Research Articles

The first principles studies of the MgB7 compound: Hard material

The electronic, mechanical, and vibrational properties of MgB7 are investigated by means of density functional theory as implemented in pseudo-potential plane wave approach. Using the calculated elastic constants, the bulk modulus, shear modulus, Young's modulus, Possion's ratio, Debye temperature, hardness, and anisotropy value were derived. It is found that MgB7 in considered structure is mechanically stable and exhibits brittle character according to the B/G criterion. The band structure of this compound is a semiconductor with an indirect band gap. Furthermore, the phonon dispersion, density of states, Born effective charge tensors, Grüneisen parameter, temperature-dependent variations of the internal energy, free energy, entropy, and heat capacity are also computed and discussed.

Born effective charges of Cu2ZnSnS4 quaternary compound: First principles calculations

Toxicity and scarceness stand as major handicaps for CuInGaSe2 and CuInGaS2 based compounds when applied as photoactive layers in solar cells. As a result, quaternary semiconductors Cu2ZnSnS4 and Cu2ZnSnSe4 are emerging as serious contenders to take their place. We report on the structural and dynamical properties of Cu2ZnSnS4 by using first principles plane-wave pseudopotential calculations. The most stable kesterite and stannite structures were considered and the Born effective charge (transverse charge) tensors were obtained by using self-consistent finite-electric field calculations within the Berry phase approach. These quantities may allow us to understand longitudinal and transverse optical splittings from vibrational spectra of this class of materials and related alloys.

Vibrational properties of BaClF, BaBrF and BaIF under high pressure

We have studied the Raman active optical phonons of layered BaClF, BaBrF and BaIF scintillators at ambient as well as at high pressures. The calculations were carried out using density functional perturbation theory within the local density approximation. We observed the softening of low and high frequency modes Eg(1) and Eg(3) under pressure. The calculated optical dielectric and Born effective charge tensors are found to be isotropic even though the crystal structures are anisotropic.

Density functional theory studies on the electronic, structural, phonon dynamical and thermo-stability properties of bicarbonates MHCO3, M = Li, Na, K

The structural, electronic, phonon dispersion and thermodynamic properties of MHCO3 (M = Li, Na, K) solids were investigated using density functional theory. The calculated bulk properties for both their ambient and the high-pressure phases are in good agreement with available experimental measurements. Solid phase LiHCO3 has not yet been observed experimentally. We have predicted several possible crystal structures for LiHCO3 using crystallographic database searching and prototype electrostatic ground state modeling. Our total energy and phonon free energy (FPH) calculations predict that LiHCO3 will be stable under suitable conditions of temperature and partial pressures of CO2 and H2O. Our calculations indicate that the groups in LiHCO3 and NaHCO3 form an infinite chain structure through O⋯H⋯O hydrogen bonds. In contrast, the anions form dimers, , connected through double hydrogen bonds in all phases of KHCO3. Based on density functional perturbation theory, the Born effective charge tensor of each atom type was obtained for all phases of the bicarbonates. Their phonon dispersions with the longitudinal optical–transverse optical splitting were also investigated. Based on lattice phonon dynamics study, the infrared spectra and the thermodynamic properties of these bicarbonates were obtained. Over the temperature range 0–900 K, the FPH and the entropies (S) of MHCO3 (M =Li, Na, K) systems vary as FPH(LiHCO3) > FPH(NaHCO3) > FPH(KHCO3) and S(KHCO3) > S(NaHCO3) > S(LiHCO3), respectively, in agreement with the available experimental data. Analysis of the predicted thermodynamics of the CO2 capture reactions indicates that the carbonate/bicarbonate transition reactions for Na and K could be used for CO2 capture technology, in agreement with experiments.

The structural, dielectric, elastic, and piezoelectric properties of KNbO3 from first-principles methods

The elastic and dielectric properties of the four experimentally known phases of KNbO3 (KNO) are investigated by first-principles methods. The atomic structure is reported along with the Born effective charge tensor to reveal the relation between Nb–O bonds hybridization and ferroelectric structural distortion. The dielectric, elastic, and piezoelectric properties of each phase are presented and compared to results in the literature. The computed structures are found to match experiment to an accuracy of approximately 2%. Although there have been very few experimental studies of single crystal KNO, it is found that the elastic parameters computed for orthorhombic KNO agree with the measured values to better than 25%, which is within the anticipated exchange-correlation error; however, the computed piezoelectric coefficient differ from the experimental values by as much as 50%, which suggests that the disagreement may not be solely due to the theoretical approximations.

Measurements and ab initio molecular dynamics simulations of the high temperature ferroelectric transition in hexagonal RMnO3

Measurements of the structure of hexagonal RMnO3 [R = rare earths (Ho) and Y] for temperatures significantly above the ferroelectric transition temperature (TFE) were conducted to determine the nature of the transition. The local and long range structural measurements were complemented by ab initio molecular dynamics simulations. With respect to the Mn sites in YMnO3 and HoMnO3, we find no large atomic (bond distances or thermal factors), electronic structure changes, or rehybridization on crossing TFE from local structural methods. The local symmetry about the Mn sites is preserved. With respect to the local structure about the Ho sites, a reduction of the average Ho–O bond with increased temperature is found. Ab initio molecular dynamics calculations on HoMnO3 reveal the detailed motions of all ions. Above ∼900 K there are large displacements of the Ho, O3, and O4 ions along the z axis which reduce the buckling of the MnO3/O4 planes. The changes result in O3/O4 ions moving to toward central points between pairs of Ho ions on the z axis. These structural changes make the coordination of Ho sites more symmetric thus extinguishing the electric polarization. At significantly higher temperatures, rotation of the MnO5 polyhedra occurs without a significant change in electric polarization. The Born effective charge tensor is found to be highly anisotropic at the O sites but does not change appreciably at high temperatures.

Thermodynamic functions from lattice dynamic of KMgH 3 for hydrogen storage applications

The dynamic and the thermodynamic properties of KMgH 3 have been investigated by density functional theory (DFT). We have found that the calculated lattice parameters differ from the experimental data by less than 0.6% and the electronic density of states (DOS) reveals that the KMgH 3 is an insulator. The formation energy of KMgH 3 from binary hydrides (MgH 2 and KH) has been calculated. Using density-functional perturbation theory, we have calculated the phonon dispersion curves, the phonon density of states, the Born effective charge tensors, the dielectric permittivity tensors and the phonon frequencies at the center of the Brillouin zone of KMgH 3. Also we have assigned the calculated phonon frequencies at the gamma point for Infrared-active and Raman-active modes. For the first time, the thermodynamic functions are computed using the phonon density of states.

Vibrational Properties of Body-Centered Tetragonal C4

Body-centered tetragonal C4 (bct C4) is a new form of crystalline sp3 carbon, which is found to be transparent, dynamically stable at zero pressure and more stable than graphite beyond 18.6 GPa. Symmetry analysis of the vibrational modes of bct C4 at Brillouin zone center is performed, Raman and infrared active modes are identified. The analysis results show that, different from cubic diamond and hexagonal diamond, there is an infrared active mode in bct C4. Based on first-principle method within the local density approximation, vibrational frequencies, Born effective charge tensors, and infrared absorption intensity of bct C4 are obtained. The vibrational modes of bct C4 are presented and compared with those of cubic diamond and hexagonal diamond in detail.

Origin of the multiferroicity in BiMn 2O 5 from first-principles calculations

Electronic structure, Born effective charges, and spontaneous polarization of multiferroic single crystal BiMn 2O 5 have been investigated in the framework of density functional theory. The relative stability of the ground state and the origin of multiferroicity for magnetism and ferroelectricity are addressed. The results reveal that the stability of antiferromagnetic (AFM) state is better than the ferromagnetic (FM) and ferrimagnetic configurations. The Born effective charge tensors ( Z ⁎) have been calculated for this compound using a Berry-phase approach, compared to their nominal ionic values, the Z * of Mn atoms show anomalous difference. By investigating the electric structure of BiMn 2O 5, there exists obviously hybridization between Bi 6s and O 2p states, our calculations indicate that the 6s 2 lone pair on the formally trivalent Bi ion plays an important role in inducing the ferroelectric distortion.

Lattice dynamical, dielectric and thermodynamic properties of LiNH2 from first principles

The lattice dynamical, dielectric properties and thermodynamic properties of LiNH2 are investigated by first principles calculations. Based on the density functional perturbation theory within the framework of linear response theory, the phonon dispersion curves and the phonon density of phonon states throughout the Brillouin zone are obtained. The calculated frequencies of the Raman active and infrared active modes are compared with previous experimental and theoretical results, and Born effective charge tensor as well as electronic dielectric permittivity tensor is presented. We find that the Born effective charge tensor of LiNH2 has quite small anisotropy. These calculated results are in good agreement with available experimental and theoretical values. Furthermore, the thermodynamic functions are predicted using the phonon density of states.

A first principles study of the lattice dynamics property of LiFePO4

Lattice dynamical properties of LiFePO4 were studied using first principles density functional theory taking into account the on-site Coulomb interaction within the GGA+U scheme. The Born effective charge tensors, phonon frequencies at the Brillouin zone center and phonon dispersion curves were calculated and analyzed. The Born effective charge tensors exhibit anisotropy, which gives an indirect evidence for the one-dimensional Li migration tunnel along the [010]direction in LiFePO4, which has been proposed by other theoretical calculations and experimental observations. The calculated phonon frequencies at the Г point of the Brillouin zone agree well with the available experimental results.

Hydrogen storage: Lattice dynamics of orthorhombic NaMgH 3

In this work, we investigate the structural, dynamic and thermodynamic properties of NaMgH 3, 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 NaMgH 3 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.

Ab initio vibrational and dielectric properties of chalcopyrite CdGeAs 2

We have performed a first-principles study of structural, vibrational and dielectric properties of CdGeAs 2. The calculations have been carried out within the local density functional approximation using norm-conserving pseudopotentials and a plane-wave basis. Calculations of the zone-center optical-mode frequencies (including longitudinal-optical/transverse-optical splitting), Born effective charge tensors for each atom, and dielectric constants, using density functional perturbation theory, are reported. The fully relaxed structural parameters are found to be in good agreement with experimental and other calculation data. All optical modes are identified and agree with infra-red and Raman measurements.

Phonons, Instabilities and Origin of Polarization in KDP Crystals

The Γ-phonons of KH2PO4 (KDP) and its deuterated analog DKDP are studied via first-principles linear response calculations. The paraelectric phase shows two instabilities. One for a z-polarized mode, which leads to the spontaneous polarization Ps of the ferroelectric phase. The other corresponds to a two-fold degenerate xy-polarized mode. Other phonons are analyzed, which couple to the ferroelectric one at large amplitudes and are relevant for the ferroelectric transition. We show that Ps is mainly of electronic nature, since it arises mostly from an off-diagonal component of the Born effective charge tensor of H, with minor contribution from P atoms displacements.

Ab initiothermodynamic and elastic properties ofAGaH4hydrides (A=Li, Na, K, Rb, and Cs)

Systematic properties of the $A{\text{GaH}}_{4}$ alkali gallium hydrides ($A=\text{Li}$, Na, K, Rb, and Cs) are investigated within density functional theory. Seven ground-state crystal structures are identified, with two energetically indistinguishable structures found for both ${\text{LiGaH}}_{4}$, whose structure is as yet undetermined experimentally, and ${\text{CsGaH}}_{4}$. Born effective charge tensors, static and high-frequency dielectric tensors, and phonon dispersion relations incorporating longitudinal-optical/transverse-optical mode splittings are computed. Our results indicate that ${\text{LiGaH}}_{4}$ and ${\text{NaGaH}}_{4}$ have technologically interesting standard enthalpies of formation near $\ensuremath{-}30\text{ }\text{kJ}/\text{mole}$ ${\text{H}}_{2}$. We find, however, that ${\text{LiGaH}}_{4}$ is thermodynamically unstable with respect to both LiGa and LiH, providing a possible explanation for its challenging synthesis. The Born stability criteria are evaluated with the computed elasticity tensor components, ${C}_{ij}$. All seven structures are found to be both elastically and vibrationally stable.

First-principles calculations of Born effective charges and spontaneous polarization offerroelectric bismuth titanate

In this study, we present the results of our first-principles calculations of the band structure,density of states and the Born effective charge tensors for the ferroelectric (ground stateB 1a 1) andparaelectric (I4/mmm) phases of bismuth titanate. The calculations are done using the generalized gradientapproximation (GGA) as well as the local density approximation (LDA) of thedensity functional theory. In contrast to the literature, our calculations on theB 1a 1 structure using GGA and LDA yield smaller indirect bandgaps as compared tothe direct bandgaps, in agreement with the experimental data. The density ofstates shows considerable hybridization among Ti 3d, Bi 6p and O 2p statesindicating the covalent nature of the bonds leading to the ferroelectric instability.The Born effective charge tensors of the constituent ions for the ground state (B 1a 1) andparaelectric (I4/mmm) structures were calculated using the Berry phase method. This is followedby the calculation of the spontaneous polarization for the ferroelectricB 1a 1 phase using the Born effective charge tensors of the individual ions. Thecalculated value for the spontaneous polarization of ferroelectric bismuthtitanate using different Born effective charges was found to be in the range of55 ± 13 µC cm − 2 in comparison to the reported experimental value of (50 ± 10 µC cm − 2) for single crystals. The origin of ferroelectricity is attributedto the relatively large displacements of those oxygen ions in theTiO6 octahedra thatlie along the a axis of the bismuth titanate crystal.

First-principles investigation of electronic, structural, and vibrational properties ofa-Si3N4

Using a density-functional scheme, we investigate the electronic, structural, and vibrational properties of amorphous silicon nitride. Through a Car-Parrinello molecular-dynamics simulation, we generate a model structure formed mainly by a network of ${\text{SiN}}_{4}$ tetrahedra a large fraction of which are edge sharing. Only a small fraction of atoms are overcoordinated and undercoordinated. First, the structural properties such as angular distributions, atomic arrangements in first-neighbor shells, the neutron total structure factor, the radial distribution function, and pair-correlation functions are examined. Next, the electronic properties are analyzed by considering the quasiparticle density of states which is calculated through the $GW$ method. Good agreement is found with experimental data when available. Successively, we focus on a range of vibrational spectra. First, the vibrational density of states is analyzed in terms of its decomposition into N and Si contributions. Then, we investigate the Born effective charge tensors, the high-frequency, and static dielectric constants and calculate the real and imaginary parts of the dielectric function in the infrared. Therefrom we obtain the infrared-absorption spectrum and the refractive index that are found to be in accord with experimental measurements. Moreover, we address the Raman spectrum which is compared with available experimental data. Electronic structure and vibrational properties of the point defects present in our model are also discussed. Density-functional and $GW$ schemes appear to be appropriate for modeling materials based on silicon nitride. In particular, our modeling of silicon nitride achieved a successful level of comparison with experiments. This allows us to infer that $a{\text{-Si}}_{3}{\text{N}}_{4}$ features a high content of edge-sharing tetrahedra, which are absent in the crystalline phases of silicon nitride at ambient conditions.

The structure, dynamical and thermodynamic properties of [formula omitted]: A first-principles study

We have investigated the structure, dynamical properties and thermodynamic properties of α-Li 3N from ab initio density functional theory within local-density approximation (LDA). The results for the lattice constant agree well with available experimental values. A liner response approach to the density functional theory is used 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 of Li 3N. The Born effective charge and the dielectric permittivity tensors are diagonal, which are analyzed in detail. The calculated phonon frequencies at the Γ point of the Brillouin zone show good agreement with the experimental values for most vibrational modes. The thermodynamic properties are investigated within the harmonic approximation based on the calculated phonon dispersion relations. The extensive and successful comparisons with experiments demonstrate once more that the method can provide reliable predictions for the temperature dependence of these quantities, such as the Helmholtz free energy F , the entropy S , Gibbs free energy G , and the enthalpy changes of H − H 298.15 , and the heat capacity in detail.

First-principles study of lattice dynamics of LiFePO 4

Lattice dynamics of lithium iron orthophosphate (LiFePO 4) isostructural with olivine have been investigated using the first-principles calculations taking into account the on-site Coulomb interaction within the GGA + U scheme. Born effective charge tensors, phonon frequencies at the Brillouin zone center and phonon dispersion curves are calculated and analyzed. The Born effective charge tensors exhibit anisotropy, which gives a convincing evidence for the one-dimensional Li migration tunnel along the [010] direction in LiFePO 4, which has been proposed by other theoretical calculations and experimental observation. The calculated phonon frequencies at the Γ point of the Brillouin zone show good agreement with the available experimental observations.

First-principles investigation of the bonding, optical and lattice dynamical properties of CeO2

Abstract Bonding, optical and lattice dynamical properties of CeO 2 are investigated using first-principles density-functional theory taking into account the on-site Coulomb interaction within the LDA + U scheme. Results of the charge-density and electron-localization function distributions indicate that the bonding nature in CeO 2 is covalent bonding between Ce and O atoms. Calculated dielectric constants, Born effective charge tensors, phonon dispersion curves are in agreement with available experimental data.