Platinum dioxide phases: Relative thermodynamic stability and kinetics of inter-conversion from first-principles

PtO2 phases are studied here with first-principles-based thermodynamics. The phase transformations are then studied with first-principles-based kinetics calculations. Three polymorphs are found to be nearly energy degenerate. The α-(CdI2) structure is the thermodynamically stable phase at low pressure while the β-(CaCl2) structure is stable at high pressure. The β′-(rutile) structure represents an unstable fixed point on the potential energy surface, or is barely bound. The conversion β- → α-PtO2 is found to involve a large energy barrier, rendering the decay of β- to α-slow over a wide temperature range. These results reconcile seemingly contradictory findings and answer longstanding questions about PtO2.