Collective particle dynamics of molten NaCl by inelastic x-ray scattering

An inelastic x-ray scattering experiment has been performed on molten NaCl over wide wave vector and energy transfer ranges. Data of high statistical quality are analyzed using a memory function approach within a generalized Langevin equation. The approach with two relaxation times for the memory function provides a very good data description over the whole wave vector range beyond the hydrodynamic regime. A slow thermal and a fast structural relaxation process in the memory function completely define the density fluctuations in molten NaCl and evidences the thermal-viscoelastic model as the minimal description for collective particle dynamics in molten alkali halides. The obtained excitation frequencies demonstrate a large positive dispersion effect, which can be related to the viscoelastic reaction of the molten salt. A transition from the viscoelastic to a hydrodynamic response of the molten salt at small wave vectors is observed. In the hydrodynamic regime the resulting thermal diffusivity agrees well with values obtained through light scattering. The modeling indicates some deficiencies at small wave vectors and large energy transfers and the spectra of the current correlation function evidences additional intensity at high frequency. The frequency of these additional modes approach a non-zero value at zero wave vector and indicates a non-acoustic character of these excitations. The frequency center of this additional inelastic intensity coincides with optic-type modes in molten NaCl predicted by simulations.

Coherent phase equilibria of systems with large lattice mismatch

In many metallurgical applications, an accurate knowledge of miscibility gaps and spinodal decompositions is highly desirable. Some binary systems where the main constituents of the same crystal structures have similar lattice parameters (less than 15% difference) reveal a composition, temperature shift of the miscibility gap due to lattice coherency. So far, the well-known Cahn's approach is the only available calculation method to estimate the coherent solid state phase equilibria. Nevertheless, this approach shows some limitations, in particular it fails to predict accurately the evolution of phase equilibria for large deformation, i.e. the large lattice parameter difference (more than 5%). The aim of this study is to propose an alternative approach to overcome the limits of Cahn's method. The elastic contribution to the Gibbs energy, representing the elastic energy stored in the coherent boundary, is formulated based on the linear elasticity theory. The expression of the molar elastic energy corresponding to the coherency along both directions [100] and [111] has been formulated in the small and large deformation regimes. Several case studies have been examined in cubic systems, and the proposed formalism is showing an appropriate predictive capability, making it a serious alternative to the Cahn's method. The present formulation is applied to predict phase equilibria evolution of systems under other stresses rather than only those induced by the lattice mismatch.

Alchemical screening of ionic crystals

We introduce alchemical perturbations as a rapid and accurate tool to estimate fundamental structural and energetic properties in pure and mixed ionic crystals.

Charge-transfer potentials for ionic crystals: Cauchy violation, LO-TO splitting, and the necessity of an ionic reference state

In this work, we study how including charge transfer into force fields affects the predicted elastic and vibrational Γ-point properties of ionic crystals, in particular those of rock salt. In both analytical and numerical calculations, we find that charge transfer generally leads to a negative contribution to the Cauchy pressure, P(C) ≡ C12 - C66, where C12 and C66 are elements of the elastic tensor. This contribution increases in magnitude with pressure for different charge-transfer approaches in agreement with results obtained with density functional theory (DFT). However, details of the charge-transfer models determine the pressure dependence of the longitudinal optical-transverse optical splitting and that for partial charges. These last two quantities increase with density as long as the chemical hardness depends at most weakly on the environment while experiments and DFT find a decrease. In order to reflect the correct trends, the charge-transfer expansion has to be made around ions and the chemical (bond) hardness has to increase roughly exponentially with inverse density or bond lengths. Finally, the adjustable force-field parameters only turn out meaningful, when the expansion is made around ions.

Strong renormalization of the electronic band gap due to lattice polarization in the GW formalism

The self-consistent GW band gaps are known to be significantly overestimated. We show that this overestimation is, to a large extent, due to the neglect of the contribution of the lattice polarization to the screening of the electron-electron interaction. To solve this problem, we derive within the GW formalism a generalized plasmon-pole model that accounts for lattice polarization. The resulting GW self-energy is used to calculate the band structures of a set of binary semiconductors and insulators. The lattice contribution always decreases the band gap. The shrinkage increases with the size of the longitudinal-transverse optical splitting and it can represent more than 15% of the band gap in highly polar compounds, reducing the band-gap percentage error by a factor of 3.

SrRietveld: a program for automating Rietveld refinements for high-throughput powder diffraction studies

SrRietveldis a highly automated software toolkit for Rietveld refinement. Compared to traditional refinement programs, it is more efficient to use and easier to learn. It is designed for modern high-throughput diffractometers and is capable of processing large numbers of data sets with minimal effort. The software currently uses conventional Rietveld refinement engines, automatingGSASandFullProfrefinements. However, as well as automating and extending many tasks associated with these programs, it is designed in a flexible and extensible way so that in the future these engines can be replaced with new refinement engines as they become available.SrRietveldis an open-source software package developed in Python.

A density-functional approach to polarizable models: A Kim-Gordon response density interaction potential for molecular simulations

A combined linear-response-frozen electron-density model has been implemented in a molecular-dynamics scheme derived from an extended Lagrangian formalism. This approach is based on a partition of the electronic charge distribution into a frozen region described by Kim-Gordon theory [J. Chem. Phys. 56, 3122 (1972); J. Chem. Phys. 60, 1842 (1974)] and a response contribution determined by the instantaneous ionic configuration of the system. The method is free from empirical pair potentials and the parametrization protocol involves only calculations on properly chosen subsystems. We apply this method to a series of alkali halides in different physical phases and are able to reproduce experimental structural and thermodynamic properties with an accuracy comparable to Kohn-Sham density-functional calculations.

Nanoindentation: Toward the sensing of atomic interactions

The mechanical properties of surfaces of layered materials (highly oriented pyrolytic graphite, InSe, and GaSe) and single-crystal ionic materials (NaCl, KBr, and KCl) have been investigated at the nanometer scale by using nanoindentations produced with an atomic force microscope with ultrasharp tips. Special attention has been devoted to the elastic response of the materials before the onset of plastic yield. A new model based on an equivalent spring constant that takes into account the changes in in-plane interactions on nanoindentation is proposed. The results of this model are well correlated with those obtained by using the Debye model of solid vibrations.

Dielectric dispersion and the structures of ionic lattices

Dielectric and spectroscopic measurements have been made on the alkali halides and several thallium and silver halides, at 2 and 290 °K, in order to determine, for each, values for the parameters which characterize the dispersion of the dielectric constant due to lattice vibrations. These have then been used to assess critically the current theoretical treatments and interpretations of the electronic structures and interionic forces in ionic crystals. The measurements were of audio and radio-frequency dielectric constants, of far infrared transmission spectra and of refractive indices through the visible spectrum.