Ab initio calculations of the structure and mechanical properties of vanadium oxides

VO, V₂O₃, VO₂, V₆O₁₃, V₄O₉, V₃O₇ and V₂O₅ have been investigated in terms of structure, bulk modulus B and elastic constant C₄₄ using ab initio calculations. The C₄₄ values for V₆O₁₃, V₄O₉, V₃O₇ and V₂O₅ are significantly lower than those for V₂O₃ and VO₂. As the V valency is increased from 3 to 5, C₄₄ decreases by 83%, whereas the bulk modulus decreases by 61%, leading to an increase in the B/C₄₄ ratio from 1.4 to 3.4. This is consistent with calculated decohesion energies for cleavage in VO₂ and V₂O₅. When cleaving V₂O₅, decohesion energies are considerably lower than those of VO₂. This behaviour may be understood based on V valency induced changes in the crystal and electronic structure as well as in the chemical bonding. As the V valency is increased, the bond strength decreases. The phases with a V valency >4 exhibit low C₄₄ values, large anisotropy and possess weak ionic bonding between the layers. The formation of easily plastically deformable structures is enabled by the screened Coulomb potential. The largest distance and therefore weakest bond strength is observed for V₂O₅ in the (002) plane.