Holger Wolff

First-principles and molecular-dynamics study of structure and bonding in perovskite-type oxynitrides ABO2N (A = Ca, Sr, Ba; B = Ta, Nb)

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 Pmc21 to optimize metal–nitrogen bonding; nonetheless, there is no contradiction with a macroscopic cubic description of the structures of BaTaO2N and BaNbO2N adopting space group Pm

A Density-functional Study of Nitrogen and Oxygen Mobility in Fluorite-type Tantalum Oxynitrides

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γ-TaON: A Metastable Polymorph of Tantalum Oxynitride†

m. Additionally, we find that the anionic sublattice is ordered in all compounds studied over a wide temperature range.

A new anatase-type phase in the system Mg–Ta–O–N

In this contribution we present results of our theoretical studies of nitrogen mobility in solid M–Ta–O–N systems. Periodic supercell calculations at density-functional level have been performed to investigate the local structure of N-doped oxynitrides and the anion-diffusion mechanisms. The migration pathways and activation barriers were calculated using the nudged elastic band method with the climbing-image enhancement. We show that the defect migration is mainly caused by the diffusion of oxygen anions. The activation energy can be lowered by increasing the defect concentration, and it is, to a large extent, depending on the dopant size.