Theoretical study of structural, electronic, lattice dynamical and dielectric properties of SrAl2O4

We investigate the structural, electronic, lattice dynamical, and dielectric properties of SrAl₂O₄ within density-function theory. The crystal structure is fully relaxed, and the structural parameters are found to be well consistent with the experimental data. The first pressure derivatives of the bulk modulus are predicted to be 2.5 and 4.3 for local density approximation (LDA) and generalized gradient approximation (GGA), respectively. The electronic band structure shows that the valence band maximum is comprised of O 2p states and a small amount of Al 3s and 3p states, and the conduction band minimum is comprised of Sr 5s and a small amount of O 2p, Al 3s and Al 3p states. The phonon frequencies at the center of the Brillouin zone and the dielectric permittivity tensors are calculated using density-function perturbation theory. The electronic (?_∞) and static (?₀) dielectric permittivity tensors are theoretically predicted by the calculations with both LDA and GGA formalisms. The results show that the electronic dielectric permittivity is isotropic, while the static dielectric permittivity exhibits to be somewhat anisotropic due to the dominant ionic contributions in static dielectric permittivity.     

Lattice dynamics of magnesium fluoride from a semiempirical two-body potential model

A range of lattice dynamical properties of MgF₂ in sellaite phase with rutile structure is calculated using a two-body potential model. The potential energy of the structure is minimized, and the phonon frequencies are calculated for the structural parameters corresponding to the energy minimum. The computed phonon frequencies are in good agreement with the experimental data, and the phonon dispersion curves are reproduced just as well. The frequencies calculated throughout the Brillouin zone are used to construct the one-phonon density of states, and the harmonic contributions to the thermodynamic functions, including the heat capacity, are calculated up to 800 K. The thermal expansion coefficient is calculated in a perturbative approximation. The results calculated for the heat capacity and the thermal expansion coefficient are in good agreement with the experimental results, at least below 200 K.     

Electronic structure of non-centrosymmetric superconductor LaPdSi3 and its reference compound LaPdGe3

Electronic structures of a superconductor without inversion symmetry, LaPdSi₃, and its non-superconducting counterpart, LaPdGe₃, have been calculated employing the full-potential local-orbital method within the density functional theory. The investigations were focused on analyses of densities of states at the Fermi level in comparison with previous experimental heat capacity data and an influence of the antisymmetric spin–orbit coupling on the band structures and Fermi surfaces (FSs) being very similar for both considered here compounds. Their FSs sheets originate from four bands and have a holelike character, but exhibiting pronounced nesting features only for superconducting LaPdSi₃. It may explain a relatively strong electron–phonon coupling in the latter system and its lack in non-superconducting LaPdGe₃.     

First-principles calculations of the elastic,phonon and thermodynamic properties of Al12Mg17

The elastic, phonon and thermodynamic properties of Al₁₂Mg₁₇ have been investigated by first-principles calculations. The obtained structural parameters, phonon dispersion curves and the predicted thermodynamic properties for all the phases studied herein agree well with available experimental data. The temperature-dependent single-crystal elastic constants are also predicted along with the polycrystalline aggregate properties, including bulk modulus, shear modulus, Young’s modulus, and Poisson’s ratio. The brittleness of Al₁₂Mg₁₇ that we predict is consistent with experiments, in contrast to the previous calculation showing ductile behavior. Detailed analysis of density of states further explains the present theoretical findings.     

Thermodynamic properties of Al,Ni, NiAl,and Ni3Al from first-principles calculations

The thermodynamic properties of Al, Ni, NiAl, and Ni₃Al were studied using the first-principles approach. The 0-K total energies are calculated using the ab initio plane wave pseudopotential method within the generalized gradient approximation. The contribution to the free energy from the lattice vibration was calculated using the phonon densities of states derived by means of the ab initio linear-response theory. The thermal electronic contribution to the free energy was obtained from the one-dimensional numerical integration over the electronic density of states. With the deduced Helmholtz free-energy, the thermal expansion and enthalpy as a function of temperature were calculated and compared with the experimental data. Our calculations show that the enthalpies of formation are slightly temperature dependent with a slope of −1.6 J/mol/K for NiAl and −1.2 J/mol/K for Ni₃Al. For Ni, the inclusion of thermal electronic excitation results in a 10% increase in thermal expansion and 15% increase in enthalpy at 1600 K.     

Electronic structure and thermal properties of doped CaMnO3 systems

The electronic and thermal properties of hole (Na) and electron (Ga) doped CaMnO₃ systems are investigated based on the first principle density functional theory calculations using plane wave basis and pseudo-potential method. A semiconductor-to-conductor transition and a distorted band structure are found for the doped systems; enhanced density of states near Fermi level is observed. The phonon transfer speed and the phonon mean free path are lowered; meanwhile, the phonon specific heat is heightened in comparison with that of the undoped CaMnO₃ system, resulting in enhanced phonon thermal conduction. The calculation results indicate that the doped systems should have improved thermoelectric performance.     

CeO2的晶格动力学性质和热输运性质的第一性原理计算

采用基于密度泛函理论的有限位移法和玻尔兹曼方程,计算了CeO2的晶格动力学性质、热力学性质和热输运性质,计算结果和实验结果基本符合。通过分析CeO2所有声子模式的振动频率、Gruneisen系数和散射率,揭示了光学声子对增强晶格振动的非简谐性和声子散射率所起的重要作用。此外,还计算了不同自由程的声子模式对热导率的贡献,发现CeO2的晶格热导率主要由声子自由程在1~10 nm之间的声子所贡献。     

Normal state of metallic hydrogen sulfide

A generalized theory of the normal properties of metals in the case of electron–phonon (EP) systems with a nonconstant density of electron states has been used to study the normal state of the SH₃ and SH₂ phases of hydrogen sulfide at different pressures. The frequency dependence of the real Re Σ (ω) and imaginary ImΣ (ω) parts of the self-energy Σ (ω) part (SEP) of the Green’s function of the electron Σ (ω), real part Re Z (ω), and imaginary part Im Z (ω) of the complex renormalization of the mass of the electron; the real part Re χ (ω) and the imaginary part Imχ (ω) of the complex renormalization of the chemical potential; and the density of electron states N (ε) renormalized by strong electron–phonon interaction have been calculated. Calculations have been carried out for the stable orthorhombic structure (space group Im3?m) of the hydrogen sulfide SH₃ for three values of the pressure P = 170, 180, and 225 GPa; and for an SH₂ structure with a symmetry of I4/mmm (D4h1?7) for three values of pressure P = 150, 180, and 225 GP at temperature T = 200 K.     

First-principles study of lattice dynamics and thermodynamic properties of LiInX2 (X = S, Se, Te)

First-principles calculations of lattice dynamics and thermodynamic properties of orthorhombic LiInS₂ and LiInSe₂ and chalcopyrite LiInTe₂ have been performed within density functional perturbation theory using norm conserving pseudopotentials. Theoretical values of phonon mode frequencies are in good agreement with the experimental data available for these crystals obtained by methods of Raman spectroscopy and infrared one. In the whole frequency range a significant decrease of the vibrational frequencies is observed going from LiInS₂ to LiInSe₂ and LiInTe₂, which is a consequence of the anion radius increase. The lattice vibrations of In-X (X = S, Se, Te) bonds are mainly located in the low-frequency and mid-frequency ranges, and the Li-X bond vibrations are dominated in the higher frequency range. The mixed covalent-ionic nature of the three compounds is manifested by Born effective charge data. The vibration patterns of orthorhombic LiInS₂ and chalcopyrite LiInTe₂ were discussed in detail. The temperature dependences of thermodynamic quantities (including the internal energy, free energy, heat capacity, entropy and the Debye temperature Θ_D) of all the three compounds were also presented in this paper. It is proved that Debye stiffness increases from LiInTe₂ to LiInSe₂ and LiInS₂.     

Electronic structure and linear optical property of BaSi2N2O2 crystal

The electronic structure and linear optical property of BaSi₂N₂O₂ (BSNO) have been calculated by density functional method with the local density approximation. A direct band gap of 5.17 eV at G is obtained for BSNO. The calculated total and partial densities of states indicate that the top valence band is mainly constructed from the N 2p and O 2p states, the low conduction band mostly originates from Ba 4d and Si 3p states. The calculated linear optical property of BSNO is in good agreement with the experimental measurement.     

Structural and thermoelastic properties of CaTiO3 perovskite between 7 K and 400 K

The structural and thermoelastic properties of CaTiO₃ perovskite have been studied using high-resolution powder neutron diffractometry at eighty temperatures in the range 7-400 K. The temperature variation of the unit cell volume, the thermodynamic Grüneisen parameter and the isobaric heat capacity are analysed using a two-term Debye model. Structural parameters are presented as the magnitudes of symmetry-adapted basis-vectors of seven normal modes with wavevectors that lie on the surface of the Brillouin zone of the primitive cubic aristotype phase, and a structural basis for the temperature-dependence of the bond lengths is proposed. A consistency between the vibrational Debye temperatures derived from the atomic displacement parameters and the mean values of the vibrational energies of the three atomic species calculated from the partial phonon densities of states has been found.     

Lattice dynamics,thermodynamics and elastic properties of monoclinic Li2CO3 from density functional theory

Monoclinic Li₂CO₃ has been identified as a critical component of the solid electrolyte interphase (SEI), a passivating film that forms on Li-ion battery anode surfaces. Here, lattice dynamics, finite temperature thermodynamics and the elastic properties of monoclinic Li₂CO₃ are examined with density functional theory (DFT) and various exchange–correlation functionals. To account for LO-TO splittings in phonon dispersion relations of Li₂CO₃, which is a polar compound, a mixed-space phonon approach is employed. Bond strengths between atoms are quantitatively explored with phonon force constants. Temperature variations of the entropy, enthalpy, isobaric heat capacity and linear (average) thermal expansion are computed using the quasiharmonic approach. The single-crystal elasticity tensor components along with polycrystalline bulk, shear and Young’s moduli are computed with a least-squares approach based upon the stress tensor computed from DFT. Computed thermodynamic properties as well as structural and elastic properties of the monoclinic Li₂CO₃ are in close accord with available theoretical and experimental data. In contrast to a recent DFT study, however, computed vibrational spectra suggest that neither the monoclinic Li₂CO₃ nor its high-temperature hexagonal phase exhibits either elastic or vibrational instabilities.     

Filler-reduced phonon conductivity of thermoelectric skutterudites: Ab initio calculations and molecular dynamics simulations

The phonon conductivities of CoSb₃ and its Ba-filled structure Ba_x(CoSb₃)₄ are investigated using first-principle calculations and molecular dynamics (MD) simulations, along with the Green–Kubo theory. The effects of fillers on the reduction of the phonon conductivity of filled skutterudites are then explored. It is found that the coupling between filler and host is strong, with minor anharmonicity. The phonon density of states and its dispersion are significantly influenced by filler-induced softening of the host bonds (especially the short Sb–Sb bonds). Lattice dynamics and MD simulations show that, without a change in the host interatomic potentials, the filler–host bonding alone cannot lead to significant alteration of acoustic phonons or lowering of phonon conductivity. The observed smaller phonon conductivity of partially filled skutterudites is explained by treating it as a solid solution of the empty and fully filled structures.     

First-principles calculations of phonon and thermodynamic properties of Fe-Si compounds

The phonon approach and the Debye model are combined to predict the vibrational thermodynamic contribution for the following Fe-Si compounds: Fe₃Si, Fe₂Si, Fe₅Si₃, FeSi, β-FeSi₂ and α-FeSi₂. Both the ultrasoft pseudopotential (USPP) and the projector augmented wave (PAW) methods are employed to describe the electron-ion interactions. The generalized gradient approximation including PW91 and PBE is employed to describe the exchange-correlation functional. Lattice parameters, bulk modulus, phonon dispersions, and finite temperature thermodynamic properties are calculated and compared with available experimental data, and good agreement is observed. The thermodynamic data obtained in the present work provide better understanding of the stability of binary Fe-Si compounds and can be used for further thermodynamic modeling of this system.     

Density functional study of the L10-αIrV transition in IrV and RhV

Both IrV and RhV crystallize in the αIrV structure, with a transition to the higher symmetry L1₀ structure at high temperature, or with the addition of excess Ir or Rh. Here we present evidence that this transition is driven by the lowering of the electronic density of states at the Fermi level of the αIrV structure. The transition has long been thought to be second order, with a simple doubling of the L1₀ unit cell due to an unstable phonon at the R point (0 1/2 1/2). We use first-principles calculations to show that all phonons at the R point are, in fact, stable, but do find a region of reciprocal space where the L1₀ structure has unstable (imaginary frequency) phonons. We use the frozen phonon method to examine two of these modes, relaxing the structures associated with the unstable phonon modes to obtain new structures which are lower in energy than L1₀ but still above αIrV. We examine the phonon spectra of these structures as well, looking for instabilities, and find further instabilities, and more relaxed structures, all of which have energies above the αIrV phase. In addition, we find that all of the relaxed structures, stable and unstable, have a density comparable to the L1₀ phase (and less than the αIrV phase), so that any transition from one of these structures to the ground state will have a volume change as well as an energy discontinuity. We conclude that the transition from L1₀ to αIrV is probably weakly first order. We also examine the behavior of similar compounds, and show that the αIrV structures of both IrTi and RhTi are lower in energy than the experimentally observed high-temperature L1₀ structure.     

High pressure Raman studies on the structural conformation of oligophenyls

The goal of this combined experimental and computational study is to investigate the structural conformation of oligo(para-phenylenes) in the crystalline phase, in particular the planarity of this type of molecules. To this end we have performed Raman experiments on para-terphenyl and para-quaterphenyl in a pressure range from 0 to 70 kbar and at temperatures from 10 to 300 K. The positions and the relative intensities of the C–C interring stretch mode at 1280 cm⁻¹ and the C–H in-plane bend mode at 1220 cm⁻¹ have been tracked. We find that upon increasing temperature at ambient pressure the intensity ratio I₁₂₈₀/I₁₂₂₀ drops rapidly at temperatures that coincide with the crystallographic phase transition for the investigated materials. At ambient temperature also, this intensity ratio drops rapidly upon increasing pressure up to about 15 kbar. In the computational part, the Raman frequencies and activities of isolated 3P and 4P molecules were calculated within restricted Hartree–Fock formalism with the interring tilt angles varying from 0 to 90°. These calculations confirm that the I₁₂₈₀/I₁₂₂₀ intensity ratio can be related to the planarity of the molecules. Three-dimensional bandstructure calculations within density functional theory were applied to determine phonon frequencies and estimate Raman activities for the polymer poly(para-phenylene). These simulations show that the same conclusions hold for crystalline environment.     

Optical absorption and structure of energy bands of GdNi<Subscript>5 − x</Subscript>Cu<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> intermetallic compounds

Optical properties of intermetallic compounds GdNi_{5 ? x} Cu _x (x = 0, 0.5, 1, 1.5, and 2) have been studied in a spectral range from 0.22 to 15 μm using the ellipsometry method. The substitution of copper for nickel has been found to lead to local changes in the interband optical conductivity spectra. A new quantum-absorption band, whose intensity increases substantially with increasing copper content, has been found at 3–4 eV. The relaxation and plasma frequencies of conduction electrons, which were calculated using data on the optical parameters, also depend on the concentration. Self-consistent calculations of the electronic structure of the intermetallic binary GdNi₅ compound have been performed using the LSDA + U method. The density of electronic states for two spin projections and the frequency dependence of the interband optical conductivity of the compound have been calculated.     

Ab initio lattice dynamics of Ni2MnX (X = Sn, Sb) magnetic shape memory alloys

In this study, we present the results of first principles calculations of elastic constants and phonon properties of nickel-manganese based magnetic shape memory compounds Ni₂MnSn and Ni₂MnSb in stoichiometric composition. The plane wave basis sets and pseudopotential method within spin-polarized generalized gradient approximation (σ-GGA) scheme of the density functional theory is applied. In investigation of the phonon dispersion spectra, linear response technique of the Density Functional Perturbation Theory is used. Phonon softening is observed in dispersion spectra at the transverse acoustic mode (TA₂) in [ζ ζ 0] direction as an indication of the structural instability of these systems to shear deformation. The vibrational instability of Ni₂MnSb system is larger than that of Ni₂MnSn yielding negative phonon frequencies. This vibrational anomaly is also verified by the low shear modulus and large elastic anisotropy ratio. The minority spin Fermi surfaces of both systems exhibit strong nesting features.     

Mechanical and phonon properties of the superhard LuB2, LuB4, and LuB12 compounds

We have studied structural, elastic, and lattice dynamical properties of the LuB_2, LuB_4, and LuB₁₂ compounds by using the plane-wave pseudopotential approach to the density-functional theory within the generalized gradient approximation. We have considered three different crystal structures of LuB_x: LuB₂ (P6/mmm), LuB₄ (P4/mbm), and LuB₁₂ (Fm-3m). The most stable structure is found to be tetragonal (P4/mbm) structure. The comparative results on the basic physical parameters such as lattice constants, bulk modulus, bond distances, elastic constants, shear modulus, Young's modulus, and Poison's ratio are reported. Also, we have predicted that LuB₄ and LuB₁₂ compounds are potential superhard materials. Furthermore, the phonon dispersion curves and corresponding phonon density of states (DOS) are computed for considered phases. Our structural and some other results are in agreement with the available experimental and other theoretical data.     

The Effect of Platinum on β-NiAl/α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Interfacial Tensile Stress: A Combined Ab Initio DFT and Mechanics-Based Model Study

The beneficial effect of adding Pt in diffusion β-NiAl coating on reducing the tensile stress normal to the coating/α-Al₂O₃ scale interface has been investigated using a combination of ab initio density functional theory (DFT), phonon dispersion theory and mechanics-based interfacial stress modeling. The coefficient of thermal expansion (CTE) for Pt, β-NiAl and β-NiAl-6.25 at% Pt was calculated using the total energy DFT combined with the phonon dispersion theory. The calculated CTE of β-NiAl and β-NiAl-6.25 at% Pt and experimentally measured CTE of α-Al₂O₃ were used to evaluate the tensile stress of the undulated β-NiAl/α-Al₂O₃ and β-NiAl-6.25 at% Pt/α-Al₂O₃ interfaces resulting from the CTE mismatch between the coating and the oxide scale during cooling from elevated temperatures. It was found that the addition of Pt to β-NiAl is capable of lowering the interfacial tensile stress as a result of the reduced CTE of the Pt-modified β-NiAl coating, thus beneficial for improving thermal cyclic durability of the coating. The calculated results showed that the interfacial tensile stress is a function of oxide scale thickness and the interfacial wave amplitude and wavelength of an undulated interface. A thicker oxide scale and a rougher interface with a larger ratio of wave amplitude versus wavelength yield higher interfacial tensile stresses during thermal cycling. The addition of 6.25 at% Pt to β-NiAl coating reduces the coating/oxide scale interfacial tensile stress by about 27% over a wide range of scale thickness and interfacial wave amplitude and wavelength.