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

VO, V(2)O(3), VO(2), V(6)O(13), V(4)O(9), V(3)O(7) and V(2)O(5) have been investigated in terms of structure, bulk modulus B and elastic constant C(44) using ab initio calculations. The C(44) values for V(6)O(13), V(4)O(9), V(3)O(7) and V(2)O(5) are significantly lower than those for V(2)O(3) and VO(2). As the V valency is increased from 3 to 5, C(44) decreases by 83%, whereas the bulk modulus decreases by 61%, leading to an increase in the B/C(44) ratio from 1.4 to 3.4. This is consistent with calculated decohesion energies for cleavage in VO(2) and V(2)O(5). When cleaving V(2)O(5), decohesion energies are considerably lower than those of VO(2). 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 C(44) 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 V(2)O(5) in the (002) plane.