Energetic Order of Nb2O5 and Ta2O5 Polymorphs

Abstract

Nb2O5 and Ta2O5 are materials with important technical applications, e.g., in electrochemistry as anode materials for Li or Na storage, or as selective oxidation catalysts. The properties of both oxides largely depend on their various polymorphs with different crystal structures. So far, the literature does not agree on which polymorphs of Nb2O5 and Ta2O5 are most stable under standard conditions. Herein, the relative stability of all relevant polymorphs of Nb2O5 and Ta2O5 is calculated using density-functional theory and the recently developed rev2-POB basis sets. First, a benchmark of the atomization-free enthalpies of Nb2O5 is performed with selected generalized gradient approximation and hybrid functionals. The effect of London dispersion on structural and energetic properties is explicitly investigated. A large scattering of atomization energies is observed for the various functionals. The range-separated hybrid functional RSHXLDA without dispersion correction proves to be the method of choice for the systems under investigation. For this reason, the stability order obtained with RSHXLDA is consisdered as most reliable.