Electronic structure of cubic and orthorhombic phases ofZrW2O8

${\mathrm{ZrW}}_2{\mathrm{O}}_8$ has a negative thermal expansion, and exists in two crystalline forms. The stable cubic α phase transforms into an orthorhombic γ-phase at a low pressure of 0.21 GPa. A further increase in pressure beyond 1.5 GPa can induce an amorphous phase. The band structures of both α and γ phases of ${\mathrm{ZrW}}_2{\mathrm{O}}_8$ are studied using a first-principles method based on density-functional theory. $\alpha -{\mathrm{ZrW}}_2{\mathrm{O}}_8$ has an indirect band gap of 2.84 eV, while $\gamma -{\mathrm{ZrW}}_2{\mathrm{O}}_8$ 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 $\gamma -{\mathrm{ZrW}}_2{\mathrm{O}}_8$ has a larger total crystal bond order than $\alpha -{\mathrm{ZrW}}_2{\mathrm{O}}_8,$ due to the formation of additional W-O bonds. For the α phase, the calculated bulk modulus of 63 GPa is found to be in good agreement experimental values.