Quantum chemical calculations of transition metal oxynitrides

In this work the chemical properties of oxynitrides of transition metals are investigated. These investigations are performed on a theoretical basis using first-principles calculations. First, the research was focused on the 3d transition-metal oxynitrides Sc-Ni where the structural properties of only cobalt-, vanadium- and chromiumoxynitrid are known. These experimental results could be perfectly reproduced. Moreover, the structural properties of the other oxynitrides are predicted. The structure changes from the rocksalt structure for the early 3d transition-metal oxynitrides Sc-Mn to the zinc blende structure for the later ones Fe-Ni. This phenomenon can be explained in a qualitative way by a detailed chemical bonding analysis with use of the COHP technique. The search for magnetic transition-metal oxynitrides should be focused on manganeseoxynitrid, where the calculated energy difference between the nonmagnetic and ferromagnetic state is large enough to withstand the influence of the anion distribution. Although a disordered anion arrangement will entropically stabilize the oxynitride, all the oxynitrides remain metastable with respect to the corresponding oxides and nitrides. This of course, complicates the synthesis of a 3d transition metal enormously. Second, the stoichiometric oxynitrides of the fifth-group are investigated. Until now there is no report about any stoichiometric vanadium oxynitride (VON). A high-pressure synthesis of VON is proposed, where it should be accessible at pressures beyond 12 GPa. It adopts the baddeleyite structure and is a small band gap semiconductor (0.6 eV). Within the literature there exist two possible phases of TaON, which is one of the most investigated transition-metal oxynitrides. One phase adopts the baddeleyite structure and the other has a complicated hexagonal structure. The calculated results of TaON in the baddeleyite structure match almost perfectly with the experimental data. The hexagonal structure, however, distorts during the relaxation of the lattice parameters and the atomic positions. A detailed investigation of this phase showed that TaON cannot exist in this structure and should therefore be erased from any crystal structure databases. Niobium oxynitride also exhibits the baddeleyite structure, but is less investigated as TaON. Electronic structure calculations on mixed tantalum and niobium containing oxynitrides show a decrease of the band gap. Since our theoretical methods are limited to the ground state, only a qualitative interpretation is allowed here. But it can be said that niobium doped TaON will change its color and therefore may be easily used for band gap tuning, without changing the anionic sublattice. A pressure-induced phase transition of NbON and TaON is suggested to happen at 27 and 31 GPa respectively. The structure changes from baddeleyite to cotunnite, which goes together with an increase of the coordination number of the metal atom. Overall it can be said that this work answers a few questions of the broad field of research of the transition-metal oxynitrides and it hopefully assists to find an appropriate synthesis of some of the calculated oxynitrides.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2006

Author dc:creator

• Lumey, Marck-Willem

Contributors dc:contributor

• Dronskowski, Richard

• info:eu-repo/classification/ddc/540
• Chemie
• Übergangsmetalloxidnitriden
• Dichtefunktionalrechungen
• Hochdruckphasen

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

eng

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-123200%22

OAI identifier oai:identifier

oai:publications.rwth-aachen.de:61545