First-principles and molecular-dynamics study of structure and bonding in perovskite-type oxynitrides ABO(2)N (A =...

A series of perovskite-type phases of alkaline-earth-based tantalum and niobium oxynitrides has been studied using both first-principles electronic-structure calculations and molecular-dynamics simulations, in particular by investigating different structural arrangements and anion distributions in terms of total-energy calculations. The structural properties are explained on the basis of COHP chemical bonding analyses and semiempirical molecular orbital calculations. We provide theoretical proof for the surprising result that the local site symmetries of these phases are lower than cubic because density-functional calculations clearly show that all crystallographic unit cells are better described as being orthorhombic with space group Pmc2(1) to optimize metal-nitrogen bonding; nonetheless, there is no contradiction with a macroscopic cubic description of the structures of BaTaO(2)N and BaNbO(2)N adopting space group Pm3m. Additionally, we find that the anionic sublattice is ordered in all compounds studied over a wide temperature range.