Denis Gryaznov

Phonon calculations in cubic and tetragonal phases of SrTiO3: A comparative LCAO and plane-wave study

The atomic, electronic structure and phonon frequencies have been calculated in a cubic and low-temperature tetragonal SrTiO3 phases at the ab initio level. We demonstrate that the use of hybrid exchange-correlation PBE0 functional gives the best agreement with experimental data. The results for the standard PBE and hybrid PBE0 are compared for the two types of basis sets: a linear combination of atomic orbitals (LCAO, CRYSTAL09 computer code) and plane waves (PW, VASP 5.2 code). Relation between cubic and tetragonal antiferrodistortive (AFD) phases and the relevant AFD phase transition observed at 110 K is discussed in terms of group theory and illustrated with analysis of calculated soft mode frequences at the {\Gamma} and R points in the Brillouin zone. Based on phonon calculations, the temperature dependences of the Helmholtz free energy and heat capacity are in a good agreement with experiment.

Hybrid density-functional calculations of phonons in LaCoO3

g0 → a 1g 2 e g 4 e g 0 , i.e., S=0, here splitting of 3d levels of Co in cubic and rhombohedral crystal fields, respectively, is shown to excited high-spin state HS: t 2g 4 e g 2 → a 1g 2 e g 2 e g 2 , i.e., S=2 or intermediate spin state IS: t 2g 5 e g 1 → a 1g 2 e g 3 e g 1 , S=1. 3,4 Also, the semiconductor-metal transition at 500 K is still not properly understood. 5 As the standard densityfunctional-theory DFT exchange-correlation functionals predict LaCoO3 magnetic and metallic at low temperatures opposite to experiments see discussion below, obviously a careful theoretical treatment is required. The majority of publications discussing the ab initio calculation results for LaCoO3 are based on the so-called LDAGGA+ U method, which was shown to reproduce correctly a nonmagnetic behavior of LaCoO3 at low temperatures. 6,7 A commonly used approach is to find the U parameter by fitting materials properties to experimental values. 8 The U values so far discussed in the literature for LaCoO3 vary in the range between 3 and 8 eV depending on the basis and Hamiltonian. These techniques demonstrated a LS ground state and an IS state to be the lowest excited state and, thus, proposed an explanation to controversy on the magnetic phase transition appearing at temperatures close to 90 K. 6 Recent quantum chemical nonperiodic embedded cluster linear combination of atomic orbitals LCAOs calculations did not confirm the stabilization of IS state against the HS state. 9 In these calculations the restricted Hartree-Fock method and atom-centered Gaussian-type orbitals were applied. However, the LS state was found to be stabilized only if dynamical electron correlation effects were included.

An improved procedure for determining grain boundary diffusion coefficients from averaged concentration profiles

Whipple’s solution of the problem of grain boundary diffusion and Le Claire’s relation, which is often used to determine grain boundary diffusion coefficients, are examined for a broad range of ratios of grain boundary to bulk diffusivities Δ and diffusion times t. Different reasons leading to errors in determining the grain boundary diffusivity (DGB) when using Le Claire’s relation are discussed. It is shown that nonlinearities of the diffusion profiles in lnCav−y6∕5 plots and deviations from “Le Claire’s constant” (−0.78) are the major error sources (Cav=averaged concentration, y=coordinate in diffusion direction). An improved relation (replacing Le Claire’s constant) is suggested for analyzing diffusion profiles particularly suited for small diffusion lengths (short times) as often required in diffusion experiments on nanocrystalline materials.

First Principles Simulations on Surface Properties and Oxidation of Nitride Nuclear Fuels

Uranium mononitride (UN) is an advanced material for the non-oxide nuclear fuel considered as a promising candidate for the use in Generation-IV fast nuclear reactors to be in operation in the next 20-30 years [1, 2]. UN reveals several advantages over a traditional UO2–type fuel (e.g., higher thermal conductivity and metal density as well as high solubility in nitric acid in the case of fuel reprocessing [2]). However, one of important problems with actinide nitrides is their effective oxidation in oxygen-containing atmosphere which can affect nuclear fuel performance [3, 4]. Thus, it is important to understand the mechanism of the initial stage of UN oxidation and to find proper solutions, in order to improve in the future the fabrication process of this nuclear fuel.

A Comparative Hybrid DFT Study of Phonons in Several SrTiO3 Phases

Three SrTiO3 phases (cubic, tetragonal antiferrodistortive and tetragonal ferroelectric) are studied at the ab initio level. Lattice dynamics was calculated via the PBE0 and B3PW hybrid DFT exchange-correlation functionals within the LCAO formalism using the direct frozen-phonon method. The soft-mode-induced phase transitions and thermodynamics (heat capacity and Helmholtz free energy) are discussed. The results obtained are in good agreement with experiments, except for reproduction of the phase transition temperature. The study represents an important first step in considering defective and more complicated perovskite systems.