Prediction of hexagonalTa2O5structure by first-principles calculations

Abstract

The undetermined crystal structure of hexagonal ${\mathrm{Ta}}_{2}$${\mathrm{O}}_{5}$ is predicted by the first-principles ultrasoft pseudopotential calculations. Among the several hypothetical structures, the structure which is energetically favorable and consistent with the available experimental data is determined by minimizing the total energy with respect to the volume, the c/a ratio, and the atomic configurations. The predicted structure has the space group of P6/mmm with two formula units in the unit cell. The calculated lattice constants, a=7.191 \AA{} and c=3.831 \AA{}, are in good agreement with the x-ray-diffraction measurement if a is assumed to be twice as large as the measured value due to the extinction rule. The coordination number of O atom to Ta atom is 8 for one Ta atom and 6 for the other three Ta atoms.