Pressure-induced enhancement of non-polar to polar transition temperature in metallic LiOsO$_3$

LiOsO$_3$ undergoes a continuous transition from a centrosymmetric $Rbar{3}c$ structure to a polar $R3c$ structure at $T_s=140$~K. By combining transport measurements and first-principles calculations, we find that $T_s$ is enhanced by applied pressure, and it reaches a value of $sim$250~K at $sim$6.5~GPa. The enhancement is due to the fact that the polar $R3c$ structure of LiOsO$_3$ has a smaller volume than the centrosymmetric $Rbar{3}c$ structure. Pressure generically favors the structure with the smallest volume, and therefore further stabilizes the polar $R3c$ structure over the $Rbar{3}c$ structure, leading to the increase in $T_s$.