Symmetric stretching vibrations of two-coordinate oxygen bridges as a cause for negative thermal expansion in ZrVxP2-xO7 and AW(2)O(8) (A=Zr, Hf) at high temperature

To examine the origin of negative thermal expansion in oxides ZrVxP2-xO7 (x = 0-2) and AW(2)O(8) (A = Zr, Hf), ab initio self-consistent-field molecular orbital calculations were carried out for a model compound (HO)(3)P-O-P(OH)(3)(2+). Our study suggests that symmetric stretching vibrations of two-coordinate oxygen bridges are strongly coupled with their bending vibrations and are more important for negative thermal expansion at high temperature. The oxygen bridges accomodate a large stretching of the A-O (A = Zr, Hf) and M-O (M = P, V) bonds at high temperature by decreasing the bond angles of the oxygen bridges. When the bond angles increase, the bonds are shortened due to the pi-type orbital interactions associated with the oxygen lone pair orbitals. (C) 1997 Academic Press.