Charge disproportionation and the pressure-induced insulator–metal transition in cubic perovskite PbCrO3

The perovskite PbCrO₃ 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 3Cr⁴⁺ → 2Cr³⁺ + Cr⁶⁺ in association with the 6s-p hybridization on the Pb²⁺ is responsible for the insulating ground state of PbCrO₃ 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.Electron–electron correlations can open up a gap near the 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 PbCrO₃ 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 PbCrO₃ 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 a₀ ∼ 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 CaCrO₃ below T_N = 90 K and that of the tetragonal phase of SrCrO₃. The magnetic moment on Cr⁴⁺ 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 d² electrons. In comparison with other Cr⁴⁺-containing perovskites, ACrO₃ (A = Ca, Sr) (9, 12), PbCrO₃ exhibits peculiar structural and physical properties. The unit-cell volume V₀ of PbCrO₃ is significantly larger than expected based on the A cation size r_A (13) in the A²⁺Cr⁴⁺O₃ family and the prediction with the software SPuDs (14). PbCrO₃ also shows a much higher resistivity and activation energy E_a than those found in CaCrO₃ and SrCrO₃ (9). These peculiarities of PbCrO₃ may be attributed to the sterochemical effect of the Pb 6s² lone-pair electrons, but this effect normally reduces the symmetry from cubic. A comparative study of the electron energy loss spectroscopy (EELS) of ACrO₃ (A = Ca, Sr, Pb) perovskites by Arévalo-Lópe et al. (4) has indeed shown a subtle difference in the O-K edge spectra as a result of different A–O interactions. In the calculation by Ganesh and Cohen (7), the peculiar electronic configuration on Pb²⁺ makes Pb off-centering energetically favorable to form a tetragonal structure at ambient pressure as occurs in PbTiO₃. High pressure favors the cubic phase by suppressing the Pb off-centering displacement. However, the metallic phase predicted with this model does not match the experimental result for the phase at ambient condition. Moreover, a tetragonal phase is not confirmed by experiments.     

Vasa nervorum of epicardial nerves and valves of small veins investigated in porcine heart by two types of vascular injections

The morphology of the vascular supply of peripheral branches of cardiac nerves has not been systematically described until now. The aim of this study was to describe the architectonics of the vasa nervorum of epicardial nerves in porcine hearts by using two injection techniques. Twenty-three hearts from young healthy pigs were used. In 10 hearts India ink solution was injected into the origin of the anterior interventricular branch. In another 10 hearts India ink solution was injected retrogradely through the coronary sinus. The hearts were then analyzed using a magnifying glass and light microscopy. The arterial injection showed the entirety of the rich venous components of the vasa nervorum, which often consisted of paired veins accompanying the epicardial nerves. The thickness of the nerves ranged from 50 to 815 μm. The vasa nervorum drained into larger subepicardial veins. In seven of the hearts prepared with venous injections the vasa nervorum of epicardial nerves were visualized in the same detail as in the arterial preparations and India ink solution filled the right ventricle via the smallest cardiac veins. The histological analysis of these seven hearts showed complete dehiscence and functional insufficiency of small and larger veins valves. In the other three hearts prepared with venous injections the valves were competent, which prevented retrograde filling of larger and smaller veins. The results obtained expand the current knowledge on epicardial nerves vasa nervorum and provide anatomical evidence behind the mechanism of retrograde application of cardioplegic solutions in cardiac surgery.