Ab Initio Lattice Dynamics Calculations Research Articles

High-temperature ab initio calculations on FeSi and NiSi at conditions relevant to small planetary cores

The Fe–Ni–Si system is potentially a very important component of terrestrial planetary cores. However, at present, even the behaviour of the FeSi and NiSi end members is poorly understood, especially at low to moderate pressures—the data for FeSi are contradictory and NiSi has been little studied. For FeSi, there is general agreement that there is a phase transition from the ε-FeSi to the CsCl structure with increasing pressure, but, in experiments, there is disagreement as to the position and slope of the phase boundary and the range of coexistence of the two phases. In this paper we have used ab initio lattice dynamics calculations to determine the phase boundary between the ε-FeSi and CsCl structures as a function of pressure and temperature in both FeSi and NiSi. For FeSi, we find that the transition pressure at zero Kelvin is ~11 GPa and that the boundary between the ε-FeSi and CsCl phases varies little with temperature, having a slight negative Clapeyron slope, going from ~11 GPa at 300 K to ~3 GPa at 2000 K. For NiSi, there is much greater variation of the transition pressure with temperature, with a much shallower negative Clapeyron slope, going from ~156 GPa at 300 K to ~94 GPa at 2000 K.

Nanoscale and Microscale Heat Transfer V (NMHT-V) EUROTHERM seminar No 108

Nanoscale and Microscale Heat Transfer V (NMHT-V) EUROTHERM seminar No 108

Lattice dynamics and elasticity of SrCO3

The lattice dynamics and elasticity of synthetic SrCO3 have been investigated by a combination of ab initio lattice dynamics calculations, microcalorimetry, Raman spectroscopy, X-ray thermal diffuse scattering and high-resolution inelastic X-ray scattering. The results of density functional based calculations were in all cases in good agreement with experiment. For the spectroscopic investigations, peak positions and intensities are well reproduced by the density functional theory model. Experimentally determined intensity distributions in thermal diffuse scattering maps differ from the theoretical distribution only in the (HK0) plane, a fact that is attributed to stacking disorder. As the model is accurate and reliable, the complete elastic stiffness tensor is predicted and, on the basis of these results, the anisotropy of the sound velocities is discussed, also in relation to the anisotropy in other carbonate systems.

Phasego 3.0: Automatic analysis of synthesis and decomposition conditions for compounds

In this paper, we present a revised version of Phasego 2.0 toolkit which is a package for automatic phase diagram analysis and anharmonic effect corrections. In this new version, the automatic synthesis/decomposition conditions analysis functionality is added to predict the stable field of compounds based on the quasi-harmonic approximation (QHA) or the anharmonic free energy corrections. The analysis about the synthesis/decomposition conditions provides the vital information for new materials design under high pressure and temperature. New version program summaryProgram Title:Phasego3Program Files doi:http://dx.doi.org/10.17632/xp7gvmrxrg.1Licensing provisions: GNU General Public License, version 3Programming language: Python (versions 2.4 and later)Journal reference of previous version: Computer Physics Communications, 197 (2015) 341Does the new version supersede the previous version?: YesExternal libraries: Numpy [1], Scipy [2], Matplotlib [3]Nature of problem: A compound can be synthesized from different components and at different pressure and temperature conditions. It can also decompose into several possible products at specific pressure and temperature conditions according to the thermodynamic equilibrium relations. There are many possibilities in products when pressure and/or temperature vary. However, it is very complicated to analyze which chemical process can really proceed according to the Gibbs free energy criteria.Solution method: All kinds of possible combinations of reactants/products before/after the synthesis/decomposition of a compound can be automatically analyzed according to the atom number equality of each atom species on the two sides of the chemical equation. The comparison and analysis of the Gibbs free energies among all the possible combinations of reactants/products are automated in the updated version. The boundaries between the compound and the possible combinations of reactants/products can also be plotted and labeled automatically. The reactants for a compound or the decomposition products of a compound can be predicted and located accurately.Reasons for the new version: We have improved the package considerably to perform the synthesis/decomposition conditions analysis for compounds. In the new version, the phase boundaries between the compound and the possible combinations of reactants/products can also be plotted automatically. The reactants for a compound and the decomposition products of a compound can be predicted and located accurately. These functionalities are very helpful to the researchers in the new materials design community.Summary of revisions:• The functionality of synthesis/decomposition conditions analysis is added.• The automatic chemical equations search functionality has been added according to the atom number equality of each atom species on the two sides of a chemical equation.• The phase diagram can be analyzed by taking into account the synthesis/decomposition conditions.• The energy and volume now are treated with per formula unit (f.u.) instead of per atom.• The previously calculated phonon Helmholtz free energies can be imported for further calculations.• Some parameters are renamed or added.• A number of bugs have been corrected.Restrictions: The restrictions are from the phonon DOS calculations. The accuracy of the previously calculated phonon DOS also affects that of the final results.Unusual features: The synthesis/decomposition conditions of compounds can be analyzed and plotted automatically.Additional comments: The new version of this package can perform the synthesis/decomposition conditions analysis based on the free energies calculated from the phonon density of states data from many methods, e.g. the molecular dynamics (MD) simulations [4], the self-consistent ab initio lattice dynamics (SCAILD) calculations [5], or other simulation methods.Running time: The examples provided in the distribution take less than 5 minute to run.

Local structure of perovskites ReO3 and ScF3 with negative thermal expansion: interpretation beyond the quasiharmonic approximation

We propose an approach beyond the quasiharmonic approximation for interpretation of EXAFS and XRD data and for ab initio calculations of electronic and vibration properties of materials with negative thermal expansion. Ab initio electronic structure and lattice dynamics calculations for cubic and distorted ScF3 were performed using the linear combination of atomic orbitals (LCAO) method. The band gap obtained in calculations for ScF3 is equal to 10.54 eV and agree well with the expected value. The calculated infrared spectra of F displaced (FD) cubic ScF3 allow us to predict that its mean Sc-F-Sc angle within NTE deviates from 180 degree.

Lattice dynamics and thermal expansion behavior in the metal cyanidesMCN(M=Cu, Ag, Au): Neutron inelastic scattering and first-principles calculations

We report measurement of temperature dependence of phonon spectra in quasi one-dimensional metal cyanides MCN (M=Cu, Ag and Au). Ab initio lattice dynamics calculations have been performed to interpret the phonon-spectra as well as to understand the anamolous thermal expansion behavior in these compounds. We bring out the differences in the phonon mode behavior to explain the differences in the thermal expansion behavior among the three compounds. The chain-sliding modes are found to contribute maximum to the negative thermal expansion along c axis in the Cu and Ag compounds, while the same modes contribute to positive thermal expansion in the Au compound. Several low energy transverse modes lead to positive thermal expansion along a and b axis in all the compounds. The calculated elastic constants and Born effective charges are correlated with the difference in nature of bonding among these metal cyanides.

Synthesis and High-Pressure Study of Corundum-Type In2O3

This work reports the high-pressure and high-temperature (HP-HT) synthesis of pure rhombohedral (corundum-type) phase of indium oxide (In2O3) from its most stable polymorph, cubic bixbyite-type In2O3, using a multianvil press. Structural and vibrational properties of corundum-type In2O3 (rh-In2O3) have been characterized by means of angle-dispersive powder X-ray diffraction and Raman scattering measurements at high pressures which have been compared to structural and lattice dynamics ab initio calculations. The equation of state and the pressure dependence of the Raman-active modes of the corundum-type phase are reported and compared to those of corundum (α-Al2O3). It can be concluded that rh-In2O3 is stable under compression up to 14 GPa and it gradually transforms to the orthorhombic Rh2O3–III structure with Pbca space group (N. 61) between 14 and 26 GPa. The bulk modulus, axial compressibilities, and the pressure range of stability of the corundum-type phase in group IIIA sesquioxides A2O3 (A = Al, Ga,...

Absence of spin-orbit coupling induced effects on the lattice dynamics inCePt3Si

Motivated by model calculations, for the heavy fermion superconductor $\mathrm{CeP}{\mathrm{t}}_{3}\mathrm{Si}$ predicting phonon anomalies because of antisymmetric spin-orbit coupling, we performed a detailed experimental study of the lattice dynamical properties of $\mathrm{CeP}{\mathrm{t}}_{3}\mathrm{Si}$. In particular, we investigated the dispersion of transverse acoustic and low energy optic phonon branches along the [110] direction using inelastic neutron scattering. In these branches we found deviations from our ab initio lattice dynamical calculations, which overall give a good description of the phonon dispersion in $\mathrm{CeP}{\mathrm{t}}_{3}\mathrm{Si}$. However, the agreement for the [110] transverse modes can be improved if we neglect the Ce $4f$ states, done in an additional calculation. We conclude that the lattice dynamics of $\mathrm{CeP}{\mathrm{t}}_{3}\mathrm{Si}$ are conventional and that the observed deviations are not related to effects of antisymmetric spin-orbit coupling. More likely, ab initio calculations overestimate the exchange between different phonon branches, particularly in the presence of $4f$ electron states. Our results imply that the ASOC plays less a role in noncentrosymmetric superconductors than commonly believed.

Phasego 2.0: Counting full anharmonic effects from high-temperature phonon density of states

In this paper, we present a revised version of Phasego 1.0 toolkit. The automatic anharmonic effects analysis functionality is added to perform the full anharmonic corrections for the quasi-harmonic approximation (QHA) results. The anharmonic free energies are extracted from the high-temperature phonon density of states (DOS), and then the phase boundaries and other properties can be automatically corrected by taking into account full anharmonic effects. New version program summaryProgram title: Phasego2Catalogue identifier: AEVQ_v2_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEVQ_v2_0.htmlProgram obtainable from: CPC Program Library, Queen’s University, Belfast, N. IrelandLicensing provisions: GNU General Public License, version 3No. of lines in distributed program, including test data, etc.: 5867093No. of bytes in distributed program, including test data, etc.: 56696690Distribution format: tar.gzProgramming language: Python (versions 2.4 and later).Computer: Any computer that can run Python (versions 2.4 and later).Operating system: Any operating system that can run Python.RAM: 50 M bytesClassification: 7.8.Catalogue identifier of previous version: AEVQ_v1_0Journal reference of previous version: Comput. Phys. Comm. 191 (2015) 150External routines: Numpy [1], Scipy [2], Matplotlib [3]Does the new version supersede the previous version?: YesNature of problem: The anharmonic lattice vibrations have more and more contributions to Gibbs free energy when temperature is beyond Debye temperature and goes up to melting temperature. The quasi-harmonic approximation (QHA) only includes part of the anharmonic effects due to the volume variation of frequency. At high temperature, the neglect of phonon–phonon interactions in the QHA will introduce errors and even arrive at wrong conclusions. However, the calculation of the anharmonic free energy from the phonon–phonon interactions is very complicated to perform.Solution method: The contribution of phonon–phonon interactions to free energy can be naturally extracted from the high-temperature phonon density of states (DOS). The extraction and inclusion of anharmonic free energy are simplified and automated in the updated version. The thermal properties of materials are then corrected by including full anharmonic effects with respective to the QHA results.Reasons for new version: We have improved the package considerably to include full anharmonic contributions of phonon–phonon interactions. The calculations of thermal properties of materials are automated and easy to implement after anharmonic corrections.Summary of revisions:•The anharmonic free energy extraction functionality is added. Now the Gibbs free energy can include full anharmonic effects.•The thermal expansion coefficients are deduced by taking into account full anharmonic contributions.•The bulk moduli, the heat capacities, the thermal pressures can include and reflect the contributions of phonon–phonon interactions.•The Hugoniot pressure–volume–temperature relations, the Grüneisen parameters, and the Debye temperatures are corrected by anharmonic effects.•The Polynomial curve fitting of Helmholtz free energy is added.•The unit of Gibbs free energy in the output files is changed.•A number of bugs have been corrected.Restrictions: The restrictions are from the high-temperature phonon DOS calculations. The smaller interval of temperatures at which the phonon DOS are calculated will yield more accurate results. The accuracy of the previously calculated phonon DOS also affect that of the final results.Unusual features: The Gibbs free energy of phonon–phonon interactions are automatically extracted from the high-temperature phonon DOS. The anharmonic effects of all the thermal properties are automatically corrected.Additional comments: The new version of this package can treat the high-temperature phonon density of states data from many methods, including the molecular dynamics (MD) simulations [4], the self-consistent ab initio lattice dynamics (SCAILD) calculations [5], or other simulation methods.The distribution file for this program is over 56 Mbytes and therefore is not delivered directly when download or Email is requested. Instead a html file giving details of how the program can be obtained is sent.Running time: The examples provided in the distribution take less than 5 minute to run.

Structure and dynamics of the fullerene polymerLi4C60studied with neutron scattering

The two-dimensional polymer structure and lattice dynamics of the superionic conductor ${\mathrm{Li}}_{4}{\mathrm{C}}_{60}$ are investigated by neutron diffraction and spectroscopy. The peculiar bonding architecture of this compound is confirmed through the precise localization of the carbon atoms involved in the intermolecular bonds. The spectral features of this phase are revealed through a combination of ab initio lattice dynamics calculations and inelastic neutron scattering experiments. The neutron scattering observables are found to be in very good agreement with the simulations which predict a partial charge transfer from the Li atoms to the ${\mathrm{C}}_{60}$ cage. The absence of a well-defined band associated with the Li atoms in the experimental spectrum suggests that this species is not ordered even at the lowest temperatures. The calculations predict an unstable Li sublattice at a temperature of $\ensuremath{\sim}200$ K, which we relate to the large ionic diffusivity of this system: Low-frequency optic modes of the Li ions couple to the soft structure of the polymer.

Anisotropic lattice dynamics and intermediate-phase magnetism in delafossiteCuFeO2

Hyperfine interactions and Fe-specific lattice dynamics in ${\mathrm{CuFeO}}_{2}$ were investigated by nuclear resonance scattering methods and compared to ab initio lattice dynamics calculations. Using nuclear forward scattering the collinear spin structure at temperatures below about 11 K could be confirmed, whereas the nuclear forward scattering results in the intermediate temperature range between about 11 K and 14 K are incompatible with the assumption of a sinusoidal distribution of spins parallel to the $c$ axis of ${\mathrm{CuFeO}}_{2}$. The critical behavior of the average hyperfine field at the phase transition at about 14 K further supports a three-dimensional model for the magnetism in this compound. Moreover, using nuclear inelastic scattering by the ${}^{57}\mathrm{Fe}$ M\ossbauer resonance, Fe-specific lattice dynamics are found to be strongly anisotropic with stiffer bonds in the $ab$ plane of the crystal. The powder averaged, Fe partial density of phonon states can be well modeled using ab initio calculations and low-energy phonons are found to deviate from classical Debye-like behavior, indicating spin-phonon coupling in this compound. Besides, the theoretical phonon spectrum exhibits typical characteristics for delafossite-type material.

Lattice dynamics of α-cristobalite and the Boson peak in silica glass

The lattice dynamics of the silica polymorph -cristobalite has been investigated by a combination of diffuse and inelastic x-ray scattering and ab initio lattice dynamics calculations. Phonon dispersion relations and vibrational density of states are reported and the phonon eigenvectors analyzed by a detailed comparison of scattering intensities. The experimentally validated calculation is used to identify the vibration contributing most to the first peak in the density of vibrational states. The comparison of its displacement pattern to the silica polymorphs -quartz and coesite and to vitreous silica reveals a distinct similarity and allows for decisive conclusions on the vibrations causing the so-called Boson peak in silica glass.

Powder X-ray Diffraction Electron Density of Cubic Boron Nitride

Conventionally, the core electron density (ED) of atoms in molecules is considered to be virtually unperturbed by chemical bonding effects. Here we report a combined experimental and theoretical investigation of the ED of cubic boron nitride including a detailed modeling of the core ED. By modeling structure factors obtained from very-high-resolution synchrotron powder X-ray diffraction data, it is possible to model not only the valence ED but also the response of the core ED to the effects of chemical bonding. The biggest challenge when studying the core ED is the deconvolution of the thermal motion from the experimental structure factors, since the thermal motion is strongly correlated to core ED deformation. However, atomic displacement parameters could be estimated from a full pattern Rietveld-multipolar refinement, and they are shown to be in good correspondence with ab initio lattice dynamics calculations. The corresponding extended multipole model including both core and valence ED refinement sugge...

Low-energy phonon dispersion inLaFe4Sb12

We studied the vibrational dynamics of a single crystal of ${\mathrm{LaFe}}_{4}{\mathrm{Sb}}_{12}$ by three-axis inelastic neutron spectroscopy. The dispersion of phonons with wave vectors $\mathbf{q}$ along $[xx0]$ and $[xxx]$ directions in the energy range of eigenmodes with high amplitudes of lanthanum vibrations, i.e., at $\ensuremath{\hbar}\ensuremath{\omega}\ensuremath{\lesssim}12$ meV is identified. Symmetry-avoided anticrossing dispersion of phonons is established in both monitored directions and distinct eigenstates at high-symmetry points and at the Brillouin-zone center are discriminated. The experimentally derived phonon dispersion and intensities are compared with and backed up by ab initio lattice dynamics calculations. Results of the computer model match well with the experimental data.

Structural and Vibrational Study of Pseudocubic CdIn2Se4 under Compression

We report a comprehensive experimental and theoretical study of the structural and vibrational properties of α-CdIn2Se4 under compression. Angle-dispersive synchrotron X-ray diffraction and Raman spectroscopy evidence that this ordered-vacancy compound with pseudocubic structure undergoes a phase transition (7 GPa) toward a disordered rocksalt structure as observed in many other ordered-vacancy compounds. The equation of state and the pressure dependence of the Raman-active modes of this semiconductor have been determined and compared both to ab initio total energy and lattice dynamics calculations and to related compounds. Interestingly, on decreasing pressure, at ∼2 GPa, CdIn2Se4 transforms into a spinel structure which, according to calculations, is energetically competitive with the initial pseudocubic phase. The phase behavior of this compound under compression and the structural and compressibility trends in AB2Se4 selenides are discussed.

Lattice Dynamics Study of Nanocrystalline Yttrium Gallium Garnet at High Pressure

This work reports an experimental and theoretical lattice dynamics study of nanocrystalline Y3Ga5O12 (YGG) garnet at high pressures. Raman scattering measurements in nanocrystalline Tm3+-doped YGG garnet performed up to 29 GPa have been compared to lattice dynamics ab initio calculations for bulk garnet carried out up to 89 GPa. Good agreement between the theoretical vibrational modes of bulk crystal and the experimental modes measured in the nanocrystals is found. The contribution of GaO4 tetrahedra and GaO6 octahedra to the different phonon modes of YGG is discussed on the basis of the calculated total and partial phonon density of states. Symmetries, frequencies, and pressure coefficients of the Raman-active modes are discussed. Moreover, the calculated infrared-active modes and their pressure dependence are reported. No pressure-induced phase transition has been observed in nano-YGG up to 29 GPa. This is in agreement with theoretical results, which show a mechanical instability of YGG above 84 GPa, si...

Lattice dynamics and spin-phonon coupling inCaMn2O4: A Raman study

We present polarized Raman spectra of ${\mathrm{CaMn}}_{2}{\mathrm{O}}_{4}$ (marokite) measured in several exact scattering configurations from oriented single crystals. Most of the 11A${}_{g}$ + 12B${}_{1g}$ + 10B${}_{2g}$ + 9B${}_{3g}$ Raman-allowed modes are identified and assigned to definite atomic motions by comparison to the predictions of ab initio lattice dynamical calculations. The Raman modes at 474 cm${}^{\ensuremath{-}1}$ (${A}_{g}$), 478 cm${}^{\ensuremath{-}1}$ (${B}_{1g}$), and 614 cm${}^{\ensuremath{-}1}$ (${B}_{1g}$) exhibit anomalous softening at temperatures below ${T}_{\mathrm{N}}\ensuremath{\approx}220$ K due to a spin-phonon coupling which depends strongly on the phonon eigenvectors. There are no indications for a structural transition in the temperature range between 80 and 850 K. We argue quantitatively that the mechanism of the phonon softening below ${T}_{\mathrm{N}}$ is similar to that in the undoped manganese perovskites.

Optical phonons in the wurtzstanniteCu2ZnGeS4semiconductor: Polarized Raman spectroscopy and first-principle calculations

The vibrational properties of the wurtzstannite ${\text{Cu}}_{2}$${\text{ZnGeS}}_{4}$ are studied experimentally by polarized Raman scattering in off-resonant and resonant conditions and theoretically by ab initio lattice dynamics calculations. Twenty-nine modes from 45 Raman active theoretically predicted have been experimentally detected and identified, including polar ${\text{A}}_{1}$(TO), ${\text{A}}_{1}$(LO), and ${\text{B}}_{1}$(TO+LO)/${\text{B}}_{2}$(TO+LO) and nonpolar ${\text{A}}_{2}$ symmetry phonon modes from measurements on (2 1 0) and (0 0 1) crystallographic planes of ${\text{Cu}}_{2}$${\text{ZnGeS}}_{4}$ single crystals. The lattice dynamics calculations provide a full picture of the zone center phonon spectrum and allow the assignment of experimentally observed lines to first- and second-order lattice vibrations. Using resonance Raman conditions, a strong enhancement of the ${\text{A}}_{1}$(LO) modes with the highest longitudinal-transversal spiting is observed.

Optical properties of plasma-assisted molecular beam epitaxy grown InN/sapphire

The optical properties of as-grown InN/sapphire films prepared by plasma assisted molecular beam epitaxy (PA-MBE) are characterized by photoluminescence (PL), Raman scattering (RS) and infrared (IR) reflectance techniques. The PL measurements have consistently exhibited lower values of InN band gaps providing clear indications of electron concentration dependent peak energy shifts and widths. The phonon modes identified by RS are found to be in good agreement with the grazing inelastic X-ray scattering measurements and ab initio lattice dynamical calculations. An effective medium theory used to analyze IR reflectance spectra of InN/sapphire films has provided reasonable estimates of free charge carrier concentrations.

Dynamical stability of NiTi under high pressure and high temperature

By using self-consistent ab initio lattice dynamical calculations that go beyond the quasiharmonic approximation, the temperature-dependent phonon spectra are derived by exciting all the lattice vibrations, in which the phonon–phonon interactions are considered. Based on well reproducing the observed high temperature phonon frequencies at zero pressure, we predict the high temperature phonons under high pressure, and obtain the critical temperature, in which the frequencies were promoted from imaginary to real. And then the stable region of austenite NiTi in phase diagram is predicted successfully.