SYMMETRICAL STRETCHING VIBRATIONS OF 2-COORDINATE OXYGEN BRIDGES AS ACAUSE 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-x O7 (x = 0-2) and AW(2)O(8) (A = Zr, Hf), ab initio self-consistent-fie ld molecular orbital calculations were carried out for a model compoun d (HO)(3)P-O-P(OH)(3)(2+). Our study suggests that symmetric stretchin g vibrations of two-coordinate oxygen bridges are strongly coupled wit h 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 te mperature by decreasing the bond angles of the oxygen bridges. When th e bond angles increase, the bonds are shortened due to the pi-type orb ital interactions associated with the oxygen lone pair orbitals. (C) 1 997 Academic Press.