Predicting Oxynitrides High-pressure Phases and VON

For NbON, the data (not shown for reasons of brevity) predict a very similar high-pressure behavior. This is not unexpected because both Ta + and Nb have identical Shannon ionic radii (seven-fold coordination) of 0.69 A, as given in Table 1.1. Beyond 27 GPa, NbON also adopts the cotunnite structure - as expected - but the density-of-states is nonzero at the Fermi level, such that cotunnite-like NbON will have metallic properties [382]. Similar cases of metalization for insulating (or semiconducting) materials imder pressure have already been reported [383]. 

23) The small moment seems strange when thinking about the large effective moments of and V + (2.8 and 3.8 but one needs to keep in mind the metallic nature of this phase. 

Indeed, the quantum-chemical structure optimizations confirm the preference of V + for lower coordination numbers. Under the assumption of zero pressure, the lowest energy is found for the case of a-cristobalite (four-fold coordination, molar volume = 41.1 cm /mol), followed by quartz (four-fold, 33.2 cm /mol), followed by anatase (six-fold, 19.8 cm /mol), followed by bad-deleyite (seven-fold, 16.6 cm /mol) and several others. In order to arrive at stable structures, however, a minimum thermochemical requirement is that the total energy of VON is lower than those of its educts. Here we rely on the plausible reaction 

Summarizing, the pseudopotential GGA calculations allow the prediction of unknown phases for the oxynitrides of V, Nb, and Ta, and they also allow the provision of quantitative data for the required synthetic conditions in terms of pressure. Similar predictions would have been impossible to make using purely traditional approaches. 

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