Experimental and theoretical study of the phase transition between perovskite polytypes of ${\mathrm{BaPtO}}_3$ under high pressure and temperature

${\mathrm{APtO}}_3$ (A = Ca, Sr, Ba) is a unique family in the complex transition metal oxides with the ${\mathrm{AMO}}_3$ formula. Most $3d$ and $4d{\mathrm{AMO}}_3$ compounds adopt the perovskite structure when synthesized at ambient or high pressure; however, ${\mathrm{APtO}}_3$ 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 ${\mathrm{BaPtO}}_3$ by performing the in situ x-ray diffraction over a broad range in the pressure ($P$)–temperature ($T$) phase diagram. The perovskite ${\mathrm{BaPtO}}_3$ 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 ${\mathrm{BaPtO}}_3$ match the structure predictions from the first-principles calculations reasonably well.