Derivation of Interatomic Potentials from Ab-initio Calculations for Molecular Dynamics Simulations of Na_XCoO₂

抄録

A parameter set for Buckingham-type potentials to describe interatomic bonding between Na⁺-O^2-, Co³⁺-O^2-, Co⁴⁺-O^2-, and O^2--O^2- in Na_XCoO₂ thermoelectric materials having a high figure of merit was determined by ab-initio calculations. Parameters for pairs other than O^2--O^2- were chosen so that they reproduce the lattice constants and elastic constants of NaCoO₂ obtained by ab-initio calculations. Fitting was also performed using the corresponding binary oxides as well as Na_XCoO₂ in order to improve transferability and accuracy. Lattice constants calculated using the potential were in good agreement those of the binary oxides and the reproducibility of elastic constants was also satisfactory. The potential is robust enough to simulate atomic vibrations in molecular dynamics even at elevated temperatures. In-plane phonon thermal conductivities calculated by perturbed molecular dynamics with the potential are in good agreement with experimental values and the reduction of the thermal conductivity with increasing temperature due to phonon-phonon scattering is also reproduced well. Molecular dynamics simulations with this potential set enable features unobservable by experiment to be revealed, such as Co valence distribution at elevated temperature.