Electronic structure of cubic and orthorhombic phases ofZrW2O8

ZrW 2 O 8 has a negative thermal expansion, and exists in two crystalline forms. The stable cubic a phase transforms into an orthorhombic γ-phase at a low pressure of 0.21 GPa. A further increase in pressure beyond 1.5 GPacan induce an amorphous phase. The band structures of both α and y phases of ZrW 2 O 8 are studied using a first-principles method based on density-functional theory. α-ZrW 2 O 8 has an indirect band gap of 2.84 eV. while γ-ZrW 2 O 8 has a direct band gap of 2.17 eV. The electronic structure and bonding are discussed in the context of atom-resolved and orbital-resolved partial density of states, the Mulliken effective charges, and the bond orders. It is shown that γ-ZrW 2 O 8 has a larger total crystal bond order than α-ZrW 2 O 8 , due to the formation of additional W-O bonds. For the a phase, the calculated bulk modulus of 63 GPa is found to be in good agreement experimental values.