Structural phase transitions in IrO2 at high pressures

Structural transformations in iridium dioxide (IrO2) were investigated using first-principles calculations up to a pressure of 50 GPa at 0 K. The phase transformation from the rutile-type to the pyrite-type structure was confirmed at 8-15 GPa. Although structures of the CaCl2-type and alpha-PbO2-type are observed in other metal dioxides, such as SiO2, GeO2, and SnO2, our calculations indicated that these structures are metastable in IrO2. Our calculations explain experimental observations which show the direct transformation from the rutile-type to the pyrite-type structures in IrO2. The bulk modulus of the pyrite-type phase calculated in this study is in good agreement with the experimental value. The non-magnetic state is stable relative to the ferromagnetic and antiferromagnetic states in all IrO2 phases. The calculated electronic density of states suggests that the pyrite-type phase is metallic.