Charge disproportionation and the pressure-induced insulator–metal transition in cubic perovskite PbCrO ₃

Abstract

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...

Unusual Mott transition in multiferroic PbCrO 3

“…The β-PbCrO 3 falls into the regime of metal although it has a small negative TCR, which may arise from the grain boundaries effect. The metallic nature of β-PbCrO 3 has also recently been reported (24,29), which supports our conclusion. As expected for a critical behavior, the measured resistance decreases continuously as the pressure crosses the α→β transition at 300 K. This behavior indicates that the electronic transition from an insulator to a "bad" metal is the second order (9), although the structural transition is the first order.…”

“…S2) (27), the α phase should be a pure cubic PbCrO 3 rather than a mixture of "PbCrO 3 and CrPbO 3 ," as suggested previously (22,28). It has come to our attention that the phase-transition mechanism of PbCrO 3 was reported by Cheng et al (29) and Yu et al (30) during the submission of this article. However, in their work, either a charge disproportionation mechanism (i.e., 3Cr 4+ = 2Cr 3+ + Cr 6+ ) or a charge transfer process between Pb and Cr were proposed for α-PbCrO 3 based on a distorted cubic structure (29) or a low-symmetry orthorhombic structure (30).…”

“…It has come to our attention that the phase-transition mechanism of PbCrO 3 was reported by Cheng et al (29) and Yu et al (30) during the submission of this article. However, in their work, either a charge disproportionation mechanism (i.e., 3Cr 4+ = 2Cr 3+ + Cr 6+ ) or a charge transfer process between Pb and Cr were proposed for α-PbCrO 3 based on a distorted cubic structure (29) or a low-symmetry orthorhombic structure (30). Such proposed structures would lead to the presence of 100 peak in neutron diffraction patterns, which is contrary to our experimental observations (Fig.…”

Varied roles of Pb in transition-metal PbMO3perovskites (M= Ti, V, Cr, Mn, Fe, Ni, Ru)

“…2 Å 3 versus 0.2 and 0.44 Å 3 . These observations are important experimental clues in support of a CD model, and a DFT + U calculation gave an independent prediction of the CD model [7]. …”

High Pressure Induced Insulator-to-Semimetal Transition through Intersite Charge Transfer in NaMn7O12

“…Quite often, pressure-induced MIT is associated with symmetry changes, as for instance in SrTiO 3 [8] or BiNiO 3 [9,10], where the suppression of charge disproportionation involving the Bi 3+ /Bi 5+ ions is reported to bring from a triclinic insulating phase to higher-symmetry metallic phase. A similar case is PbCrO 3 [11], where at ambient pressure, Cr 3+ /Cr 6+ charge disproportionation occurs with the 6s-6p hybridization of the Pb 2+ orbitals with the oxygen 2p ones accounting for the insulating properties, while high pressure induces a single Cr 4+ state which gives rise to the metallic phase. In some cases, however, isostructural MIT can take place when the crystallographic modifications induced by applied pressure modulate the structural features without symmetry breaking, as in the case of PrNiO 3 [12], metal osmium [13], FeAs [14] and VO 2 [15].…”