Prediction of hexagonal Ta2O5 structure by first-principles calculations

The undetermined crystal structure of hexagonal Ta2O5 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 cia 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 Angstrom and c=3.831 Angstrom, 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.