Large volume collapse in the phase transition in cubic PbCrO3

Recent HPCAT experiments have lead to an observation of an isostructural phase transition with 9.8% volume collapse at ~1.6 GPa between Phases I (ambient pressure) and II of PbCrO3 perovskite.  The small inset plot in the figure refers to the P-V relation for Phase I. The rare isostructural phase transitions (without change of symmetry) induced by high pressures are usually considered to originate from the electronic structural change. In the case of PbCrO3 this seems to be improbable due to specific electronic configurations of ions Cr4+ and Pb2+. The unit cell volume of phase I seems to be abnormally large from the point of view of crystal chemistry and its bulk modulus estimated from P-V data is much smaller than typical value for oxide perovskites.
 Proc. Natl. Acad. Sci., 107, 14026