Novel p-V relationship in ReO3 under pressure

High-precision strain measurements on the metallic perovskite ReO3 across the pressure-induced phase transformation at ptr=5 kbar are reported. The crystals soften exponentially as the pressure approaches ptr from below. The high-pressure phase exhibits large volume strain, proportional to (p-ptr)23 up to at least 30 kbar pressure. The abnormal p-V relationship reflects either highly nonlinear restoring forces counteracting the rotations of the ReO6 octahedra or the dominance of electron redistribution among subbands as in a Lifshitz transition.