The high pressure perovskite PbCrO3 has been known since the 1960s but the electronic properties were controversial. The average structure is a cubic Pm-3m perovskite but a recent study revealed a charge ordering as Pb2+0.5Pb4+0.5Cr3+O3 with a ~3 x 3 x 3 Pb2+/Pb4+ ordered superstructure over small correlation lengths. A large pressure-induced volume collapse of 9.8% at ~2 GPa is due to a insulator-metal transition accompanying the charge transfer to high pressure configuration Pb2+Cr4+O3. Neutron diffraction is required to study structural and magnetic changes accompanying the unusual insulator to metal change in PbCrO3. Neutrons will show how TN (= 240 K at ambient P) changes with pressure, whether the G-type antiferromagnetism is suppressed at the transition, and whether the length scale for 3-fold local superstructure changes with pressure approaching the transition.

PbCoO3 used to be the missing link in the PbMO3 perovskite series. PbCoO3 was recently synthesized by the Azuma group using high-pressure synthesis at 12 GPa. NPD at JPARC shows that it has the exciting and unexpected valence distribution of Pb2+Pb4+3Co2+2Co3+2O12 where Pb in the A-site and Co in the B-site of perovskite ABO3 both have charge ordering leading to the so-called A-site ordered double perovskite structure. The complex Pb2+0.25Pb4+0.75Co2+0.5Co3+0.5O3 valence distribution is expected to change with perturbations such as pressure and chemical modifications. A preliminary high pressure SXRD experiment found a 0.5 % volume drop at 5 GPa. We think that this corresponds to a Pb-Co charge transfer, but it needs to be confirmed by NPD. We request 3 days on PEARL for an initial room temperature experiment from near ambient up to 10 GPa pressure.