Experimental and theoretical study of the phase transition between perovskite polytypes of BaPtO3 under high pressure and temperature

APtO3 (A = Ca, Sr, Ba) is a unique family in the complex transition metal oxides with the AMO3 formula. Most 3d and 4dAMO3 compounds adopt the perovskite structure when synthesized at ambient or high pressure; however, APtO3 perovskites have not been reported, even if their geometric tolerance factor, t, falls in the range suitable for the perovskite structure. Herein, we studied the crystal structures of BaPtO3 by performing the in situ x-ray diffraction over a broad range in the pressure (P)-temperature (T) phase diagram. The perovskite BaPtO3 can be stabilized at P>35 GPa and T>1100 K. A variety of crystal structures, including the perovskite structure and its hexagonal polytypes, observed in the P-T phase diagram of BaPtO3 match the structure predictions from the first-principles calculations reasonably well.