Pressure-induced phase transitions and electronic properties of Cd 2 V 2 O 7 .

We report a density-functional theory study of the structural and electronic properties of Cd 2 V 2 O 7 under high-pressure conditions. The calculations have been performed by using first-principles calculations with the CRYSTAL program. The occurrence of two structural phase transitions, at 0.3 and 10.9 GPa, is proposed. The crystal structure of the different high-pressure phases is reported. Interestingly a cubic pyrochlore-type structure is predicted to stabilize under compression. The two phase transitions involve substantial changes in the coordination polyhedra of Cd and V. We have also determined the compressibility and room-temperature equation of state of the three polymorphs of Cd 2 V 2 O 7 . According to our systematic electronic band-structure calculations, under ambient conditions Cd 2 V 2 O 7 is an indirect wide band-gap material with a band-gap energy of 4.39 eV. In addition, the pressure dependence of the band gap has been determined. In particular, we have found that after the second phase transition the band gap decreases abruptly to a value of 2.56 eV.