On the stability of MOPO4 structure types with M: V, Mo, Nb, W, Ta, Sb

Transition metal oxide phosphates MOPO4 have recently attracted interest due to their catalytic and electronic properties, and as electrode material. In order to predict possible polymorphs that have hitherto not been prepared, we present a systematic theoretical study on the stability of four known MOPO4 structure types, beta-VOSO4, alpha-MoOPO4, SbOPO4 and monophosphate tungsten bronzes (MPTB) with six transition and main group metals that are interesting due to their catalytic activity (M: V, Mo, Nb, W, Ta, Sb). Harmonic frequencies and Crystal Orbital Hamilton Populations (COHP) were computed to verify and predict the stability and interpret the bond strengths, respectively, of the optimized structure models. The M-O bond strength has only a minor influence of the polymorph formation, but its vibrational contribution to the thermodynamic functions has a substantial effect. Based on the electronic energy calculated with the hybrid density functional PW1PW, every thermodynamical stable polymorph could be identified. The predictions of polymorph formation are in agreement with the experimental observations of VOPO4, NbOPO4 and TaOPO4, while the theoretical results for MoOPO4 and SbOPO4 show some deviations from the experimentally observed stability order.

Automated summary

Transition metal oxide phosphates MOPO4 have been studied systematically to predict their stability and polymorph formation. Computational methods were used to calculate bond strengths, vibrational frequencies, and crystal orbital Hamilton populations. The predicted polymorphs were in agreement with experimental observations, except for MoOPO4 and SbOPO4.