New 5d Perovskite BaPtO₃ Synthesized under High Pressure

APtO₃ (A = Ca, Sr, Ba) is a unique family in the complex transition metal oxides with the AMO₃ formula. Whereas most 3d and 4d AMO₃ compounds adopt the perovskite structure when synthesized at ambient or high pressure, there have been no reports in the literature about APtO₃ perovskites, even if their geometric tolerance factor t falls in the range suitable for the perovskite structure. BaPtO₃ in the APtO₃ family has a t > 1 and it adopts the polytype structure if made under ambient pressure. High-pressure synthesis is needed to convert the polytype structures to the perovskite structure which has a higher density. By treating the polytype BaPtO₃ under high pressure and high temperature with a laser-heated diamond anvil cell (DAC) above 40 GPa and 1100 K, we have for the first time successfully obtained the perovskite phase BaPtO₃ denoted as 3C BaPtO₃. The bulk modulus of the 3C BaPtO₃ phase of 230 ± 20 GPa is near the high limit of all perovskites. The preferred local structure of the face-shared octahedra in the polytype BaPtO₃ appears to be responsible for the unusually high stabilization pressure of the BaPtO₃ perovskite. The threshold pressure of 40 GPa for stabilizing BaPtO₃ perovskite matches the prediction from density functional theory calculations.