Density functional theory study of the structural, electronic, mechanical, and thermal properties of Hf₆Ta₂O₁₇

The structural, electronic, mechanical, and thermal properties of Hf₆Ta₂O₁₇ are studied using density functional theory. The Helmholtz free energy is computed for various symmetry constraints on the Hf₆Ta₂O₁₇ structure during lattice structure optimization and atomic configuration relaxation. The Helmholtz free energies calculated from Γ-point phonon frequencies demonstrate a preference for slightly lowered symmetry, preferring triclinic symmetry rather than the experimentally determined orthorhombic symmetry. Elastic constants are obtained for the symmetry-enforced (i.e., orthorhombic) structure and are used to calculate the structure's mechanical and thermal properties including its bulk, shear, and Young's moduli, Poisson ratio, Pugh ratio, longitudinal and shear sound velocities, and Debye temperature. Furthermore, the Born criteria predicts that the symmetry-enforced structure is mechanically stable. Finally, the electronic band structure for Hf₆Ta₂O₁₇ is presented and found to exhibit a large indirect band gap.