Lattice dynamics and inelastic neutron scattering experiments on andalusite,Al2SiO5

We report coherent inelastic neutron scattering measurements of the phonon dispersion relations and lattice dynamics shell model calculations of several microscopic and macroscopic properties of andalusite, Al₂SiO₅. Andalusite has an orthorhombic structure with 32 atoms/unit cell. The inelastic neutron scattering measurements (up to energy transfers of 45 meV) were carried out using the triple axis spectrometer at Dhruva reactor, India using a single crystal of andalusite and the phonon dispersion relations along the [100] direction have been measured. The shell model calculations have been used to compute the crystal structure, elastic constants, phonon frequencies, dispersion relations, density of states and the specific heat. The calculated results are in good agreement with available experimental data. The computed one-phonon neutron scattering structure factors based on the shell model have been very useful in the planning and analysis of the inelastic neutron scattering experiments.     

Inelastic neutron scattering and lattice dynamics of GaPO<Subscript>4</Subscript>

We report here measurements of phonon spectrum and lattice dynamical calculations for GaPO₄. The measurements in low-cristobalite phase of GaPO₄ are carried out using high-resolution medium-energy chopper spectrometer at ANL, USA in the energy transfer range 0–160 meV. Semiempirical interatomic potential in GaPO₄, previously determined using ab-initio calculations have been widely used in studying the phase transitions among various polymorphs. The calculated phonon spectrum using the available potential show fair agreement with the experimental data. However, the agreement between the two is improved by including the polarisability of the oxygen atoms in the framework of the shell model. The lattice dynamical models are also exploited for calculations of various thermodynamic properties of GaPO₄.     

Heusler合金Li2AlX(X=Ga，In)的晶格动力学和电子特性

本文计算了Heusler合金Li₂AlGa和Li₂AlIn的晶格参数、体积模量、体积模量的一阶导数、 电子能带结构、声子色散曲线和声子态密度,并与密度泛函理论中的广义梯度近似计算结果进行比较. 计算的晶格参数与文献有很好的一致性. 两个Heusler合金的电子能带结构表明它们是半金属结构. 并利用声子色散曲线和声子密度图研究Heusler合金晶格动力学. Li₂AlGa和Li₂AlIn Heusler合金在基态呈现动力学稳定.     

Vibrational spectra of ternary ordered vacancy compounds

We have studied the lattice dynamics of A²B ₂ ³ C ₄ ⁵ crystals with the thiogallate structure using a nonpolarized ion model. We computed the interatomic interaction parameters for nine compounds. The calculated phonon spectra, density of vibrational states, and temperature dependence of the specific heats agree well with the available experimental data. V. D. Kuznetsov Siberian Physicotechnical Institute, Tomsk University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 85–89, February, 1998.     

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.     

Ag Raman modes of RBCO (R = Gd, Pr) by density functional theory approach

Ab initio total energy calculations have been performed for superconducting GdBa₂Cu₃O₇ and insulating PrBa₂Cu₃O₇ using the full-potential linear augmented plane-wave method in the local density approximation (LDA) and generalized gradient approximation (GGA). The comparison of the calculated unit cell volume and lattice parameters with the experimental data indicates the improvement of these parameters in the GGA relative to LDA. LDA and GGA give the equilibrium unit cell volume about 6% smaller and 1.25% larger than the experimental data, respectively for both systems. Thus frozen phonon calculations have been performed to determine the eigenvalues and eigenvectors of the k=0 A_g modes of the two systems in equilibrium structure have been obtained in GGA. The calculated frequencies in the GGA are in good agreement with the other LDA calculations for similar systems. Comparison of computational data with experimental data indicates that calculations determine the frequencies about ten percent below the experimental data. Even by improving LDA to GGA in these calculations, the calculated phonon frequencies have remained almost ten percent below the experimental data, even though the calculated unit cell volumes are nearly equal to the experimental data. So, applying GGA has not considerably decreased the difference between the computational and experimental data. The effect of Pr doping on the eigenvalues and eigenvectors have also been investigated.     

Inelastic neutron scattering and lattice dynamics of NaNbO<Subscript>3</Subscript> and Sr<Subscript>0.70</Subscript>Ca<Subscript>0.30</Subscript>TiO<Subscript>3</Subscript>

NaNbO₃ and (Sr,Ca)TiO₃ exhibit an unusual complex sequence of temperature- and pressure-driven structural phase transitions. We have carried out lattice dynamical studies to understand the phonon modes responsible for these phase transitions. Inelastic neutron scattering measurements using powder samples were carried out at the Dhruva reactor, which provide the phonon density of states. Lattice dynamical models have been developed for SrTiO₃ and CaTiO₃ which have been fruitfully employed to study the phonon spectra and vibrational properties of the solid solution (Sr,Ca)TiO₃.      

Negative thermal expansion in framework compounds

We have studied negative thermal expansion (NTE) compounds with chemical compositions of NX₂O₈ and NX₂O₇ (N=Zr, Hf and X=W, Mo, V) and M₂O (M=Cu, Ag) using the techniques of inelastic neutron scattering and lattice dynamics. There is a large variation in the negative thermal expansion coefficients of these compounds. The inelastic neutron scattering experiments have been carried out using polycrystalline and single crystal samples at ambient pressure as well as at high pressures. Experimental data are useful to confirm the predictions made from our lattice dynamical calculations as well as to check the quality of the interatomic potentials developed by us. We have been able to successfully model the NTE behaviour of these compounds. Our studies show that unusual phonon softening of low energy modes is able to account for NTE in these compounds.      

Electronic and pressure dependent vibrational properties of silicide Sr(Ni0.5Si0.5)2

We report a detailed ab initio study of the structural, electronic, and volume dependent elastic and lattice dynamical properties of Sr(Ni_0.5Si_0.5)₂. The calculations have been carried out within the local density functional approximation using norm-conserving pseudopotentials and a plane-wave basis. The phonon dispersion curves along the high-symmetry directions and phonon frequencies with their Grüneisen parameters at the Brillouin zone center are computed by using density functional perturbation theory while the elastic constants are calculated in metric-tensor formulation. The band structure, and partial densities of states and Fermi surface topology are also discussed.     

Inelastic neutron scattering study of lattice dynamics in α-ZnCl<Subscript>2</Subscript>

Inelastic neutron scattering experiments have been carried out to measure the phonon density of states in polycrystalline α-ZnCl₂ at Dhruva, Trombay. Lattice dynamical calculations, based on an interatomic potential model, are accomplished to study phonons associated with this otherwise extremely hygroscopic compound. Our calculated data are found to be well-compatible with the available measured ones.     

First principles vibrational dynamics of magnesium telluride

The lattice dynamics of large-gap semiconductor MgTe compound at various crystallographic phases; rocksalt (B1), zincblende (B3), NiAs (B8₁) and wurtzite (B4), has been investigated from first principles calculations based on density functional theory (DFT) within plane-wave pseudopotential method and generalized gradient approximation (GGA) of the exchange-correlation functional. The static equation of states of the compound has been studied with Vinet equation of states. The ground state of the compound is a fourfold coordinated wurtzite structure, which is consistent with experiments and recent theoretical calculations. Full phonon dispersion spectra of all related phases of the MgTe have been calculated using density functional perturbation theory within the linear-response approach. In view of the total energy calculations and the obtained vibrational spectra, it can be emphasized that the MgTe polymorphs with tetrahedral coordination (zincblende and wurtzite structures) are of covalent character rather than ionic. The large TO-LO splitting of phonon branches of rocksalt and NiAs phases reflect the high ionicity of these phases.     

A study of electronic structure and lattice dynamics of CoSb3 skutterudite

The novel filled skutterudite materials have attracted much interest in recent years and experimental studies have revealed that electrical properties (electrical conductivity and Seebeck coefficient) in these materials are dominated by their electronic structure while the effective suppression of thermal conductivity is mainly determined by their lattice dynamics. To clarify the relationship between microstructure and properties in further, we report a systematic study of electronic structures and lattice dynamics of CoSb₃ in this paper using linearized augmented plane waves based on the density functional theory of first principles. By calculating band structure and partial density of states (PDOS), effects of electronic structures of CoSb₃ on electrical properties were investigated. Based on the calculated results of phonon dispersions and phonon density of states of CoSb₃, lattice dynamics of CoSb₃ (heat capacity, Debye temperature, mean free path and lattice thermal conductivity) are discussed in detail. The calculated results are excellently consistent with other work and experimental data.     

Analysis of the crystal lattice instability for cage–cluster systems using the superatom model

We have investigated the lattice dynamics for a number of rare-earth hexaborides based on the superatom model within which the boron octahedron is substituted by one superatom with a mass equal to the mass of six boron atoms. Phenomenological models have been constructed for the acoustic and lowenergy optical phonon modes in RB₆ (R = La, Gd, Tb, Dy) compounds. Using DyB₆ as an example, we have studied the anomalous softening of longitudinal acoustic phonons in several crystallographic directions, an effect that is also typical of GdB₆ and TbB₆. The softening of the acoustic branches is shown to be achieved through the introduction of negative interatomic force constants between rare-earth ions. We discuss the structural instability of hexaborides based on 4f elements, the role of valence instability in the lattice dynamics, and the influence of the number of f electrons on the degree of softening of phonon modes.     

Ground state and lattice dynamical study of ionic conductors CaF2, SrF2 and BaF2 using density functional theory

The present paper reports a comprehensive and complementary study on structural, electronic and phonon properties of face centered cubic fluorites, namely CaF₂, BaF₂ and SrF₂, using first principles density functional calculations within the generalized gradient approximation. The calculated lattice constants and bulk modulus are in good agreement with available experimental data. The analysis of band structure and density of states confirms the ionic character for all the three fluorides. The phonon dispersion curves and corresponding phonon density of states obtained in the present work are consistent with the available experimental and other theoretical data. The LO-TO splitting is maximum for CaF₂, which confirms that the ionicity is maximum in the case of CaF₂. The phonon properties for SrF₂ have been calculated for the first time.     

Inelastic neutron scattering and lattice dynamics of minerals

We review current research on minerals using inelastic neutron scattering and lattice dynamics calculations. Inelastic neutron scattering studies in combination with first principles and atomistic calculations provide a detailed understanding of the phonon dispersion relations, density of states and their manifestations in various thermodynamic properties. The role of theoretical lattice dynamics calculations in the planning, interpretation and analysis of neutron experiments are discussed. These studies provide important insights in understanding various anomalous behaviour including pressure-induced amorphization, phonon and elastic instabilities, prediction of novel high pressure phase transitions, high pressure-temperature melting, etc.      

Ground state and phonon spectrum of NiSi2

We have studied structural, electronic, elastic and dynamical properties of NiSi₂ by employing the plane wave pseudopotential method based on density functional theory within the local density approximation. The calculated lattice constant, bulk modulus and first-order pressure derivative of the bulk modulus are reported and compared with earlier available experimental and theoretical calculations. Numerical first-principles calculations of the elastic constants were used to calculate C₁₁, C₁₂ and C₄₄ for NiSi₂. The calculated electronic band structure has been compared with angle-resolved photoemission spectroscopy experimental data along the [100] and [111] symmetry directions. A linear response approach to density functional theory is used to derive the phonon dispersion curves and phonon partial density of states. Atomic displacement patterns for NiSi₂ at the Γ, X and L symmetry points are also presented.     

Investigation of structural,electronic, elastic and phonon properties of cubic spinel ZnM2O4(M = Co,Rh and Ir) compounds

The results obtained from ab initio calculations on ZnM₂O₄ (M = Co, Rh and Ir) compounds have been reported. The elastic constants, Bulk, Shear and Young modulus, and Poisson's ratios of the compounds are presented. In addition, full phonon dispersion curves and projected density of states of the compounds have been computed using the direct method. The lattice parameters (a) and internal parameters (u) are found to be in a good agreement with experimental results. According to both the B/G values and the Poisson's ratio, these compounds have covalent bondings. The analysis of the band structure of these compounds have indicated indirect band gaps of 1.25 eV for ZnCo₂O₄ and 1.14 eV for ZnRh₂O₄ and 0.86 eV for ZnIr₂O₄. The full phonon spectra of these compounds show that they are dynamically stable in the cubic spinel structure.     

Ab initio calculation of Ni50−xFexTi50

The structure, lattice dynamics and electronic band structure of Ni_43.75Fe_6.25Ti₅₀ were obtained using ab initio calculations. The phonon dispersion relations and phonon density of states were calculated using the direct method. The stability of Ni_50−xFe_xTi₅₀ structure for x=0.0, 6.25, 12.5, 25 has been investigated and shown that the orthorhombic structure is the most stable phase for x=25.     

Lattice dynamical properties of ScB2, TiB2, and V B2 compounds

We have investigated the structural and lattice dynamical properties of XB₂(X=Sc,V,Ti) by using first-principles total energy calculations. Specifically, the lattice parameters (a, c) of the stable phase, the bond lengths of X–B and B–B atoms, phonon dispersion curves and the corresponding density of states, and some thermodynamical quantities such as internal energy, entropy, heat capacity, and their temperature-dependent behaviours, are presented. The obtained results for structural parameters are in good agreement with the available experimental and other theoretical studies.     

First principles lattice dynamical study of the cubic antiperovskite compounds AsNBa3 and SbNBa3

An ab initio pseudopotential plane wave method using linear response approach has been employed to study the lattice dynamics of two cubic antiperovskites AsNBa₃ and SbNBa₃. The bulk properties, elastic constants, phonon dispersion curves, phonon density of states and temperature dependent thermodynamic quantities of both antiperovskites are obtained. The calculated lattice constants, elastic and bulk properties are compared with the available theoretical data. This is the first systematic and quantitative prediction of phonon and thermodynamical properties of these antiperovskite compounds.