The perovskite PbCrO 3 is an antiferromagnetic insulator. However, the fundamental interactions leading to the insulating state in this single-valent perovskite are unclear. Moreover, the origin of the unprecedented volume drop observed at a modest pressure of P = 1.6 GPa remains an outstanding problem. We report a variety of in situ pressure measurements including electron transport properties, X-ray absorption spectrum, and crystal structure study by X-ray and neutron diffraction. These studies reveal key information leading to the elucidation of the physics behind the insulating state and the pressure-induced transition. We argue that a charge disproportionation 3Cr 4+ → 2Cr 3+ + Cr 6+ in association with the 6s-p hybridization on the Pb 2+ is responsible for the insulating ground state of PbCrO 3 at ambient pressure and the charge disproportionation phase is suppressed under pressure to give rise to a metallic phase at high pressure. The model is well supported by density function theory plus the correlation energy U (DFT+U) calculations. Fermi energy in a partially filled band system to give rise to a Mott insulator without changing the translation symmetry (1). However, how to justify the insulating ground state in the cubic perovskite PbCrO 3 with a 2/3-filled band remains controversial (2-11). An unphysically large U needs to be used in the density function theory (DFT) calculation (10) to open a gap, indicating that electron-electron correlations alone is insufficient to account for the insulator phase. More surprisingly, the structure undergoes a first-order transition at P = 1.6 GPa to another cubic phase with an extremely large volume drop (6). To clarify the fundamental interactions leading to the cubic insulating state and whether the pressure-induced volume collapse is accompanied with an insulator-metal transition, we carried out a suite of high-pressure experiments including structural characterization, measurements of resistivity, and X-ray absorption near edge structure (XANES) under high pressure and performed DFT with Hubbard U correction (DFT+U) calculations. Detailed information about the experiments, the DFT calculation, and the simulation for XANES is provided in SI Text.The PbCrO 3 perovskite was known to be stabilized under high pressure and high temperature (HPHT) in the 1960s (2). Structural studies by X-ray and neutron diffraction revealed that it crystallizes as a cubic Pm-3m perovskite with a lattice constant of a 0 ∼ 4.00 Å and exhibits a type G antiferromagnetic (AFM) order below T N = 240 K in contrast to the type C AFM order of CaCrO 3 below T N = 90 K and that of the tetragonal phase of SrCrO 3 . The magnetic moment on Cr 4+ as refined from neutron diffraction is 1.9 μ B , which is very close to the spin-only moment of 2 μ B expected for localized d 2 electrons. In comparison with other Cr 4+ -containing perovskites, ACrO 3 (A = Ca, Sr) (9, 12), PbCrO 3 exhibits peculiar structural and physical properties. The unit-cell volume V 0 of PbCrO 3 is significantly larger than expec...
All single-valent spinels are insulators. The relatively small activation energy in the temperature dependence of resistivity in vanadate spinels led to a speculation that the spinels are near the crossover from localized to itinerant electronic behavior and the crossover could be achieved under pressure. We have performed a number of experiments and calculations aimed at obtaining information regarding structural changes under high pressure for the whole series of vanadate spinels, as well as transport and magnetic properties under pressure for MgV 2 O 4 . We have also studied the crystal structure under pressure of wide-gap insulators ACr 2 O 4 (A= Mg, Mn, Fe, Zn) for comparison. Moreover, the relationship between the bulk modulus and the cell volume of
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