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

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 112; no. 6; pp. 1670 - 1674
• Main Authors: Cheng, Jinguang, Kweon, K. E., Larregola, S. A., Ding, Yang, Shirako, Y., Marshall, L. G., Li, Z.-Y., Li, X., dos Santos, António M., Suchomel, M. R., Matsubayashi, K., Uwatoko, Y., Hwang, G. S., Goodenough, John B., Zhou, J.-S.
• Format: Journal Article
• Language: English
• Published: United States Proceedings of the National Academy of Sciences 10-02-2015
• Subjects: charge disproportionation; CHROMIUM; CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; CRYSTAL; high pressure; insulator-metal transition; LARGE-VOLUME; MICROSTRUCTURE; perovskite; SPECTROSCOPY
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1424431112

The steric activity of the lone pair electrons of Pb
2+
-containing compounds distorts the crystal structure and produces exotic physical properties. In ferroelectric PbTiO
3
and PbVO
3
, the lone-pair electrons hybridizing with the oxygen lead to polarized MO
6
octahedra. In PbRuO
3
, the hybridization induces unprecedented Pb-Ru bonds at high pressure. The sterochemical effect in PbCrO
3
makes Pb bond with oxygen without a long-range periodicity. Under the influence of displaced Pb
2+
, Cr
4+
undergoes a charge disproportionation that opens up a gap. In contrast to the pressure effect on PbTiO
3
and PbRuO
3
, pressure restores the undistorted perovskite structure in PbCrO
3
. This result implies that the sterochemical effect of Pb
2+
in a perovskite depends sensitively on the number and energy of the
d
electrons.
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.