Temperature- and pressure-driven spin-state transitions in LaCoO3

Crystal structure, spin state, and semiconductor-metal transitions of LaCoO3 in the 10-905 K temperature range were studied by neutron diffraction at high pressures up to 4.5 GPa and x-ray diffraction and Raman spectroscopy at high pressures up to 17.5 GPa. The susceptibility and thermal-expansion anomalies in the studied pressure range can be described successfully by the thermal excitation of two orbitally nondegenerate magnetic states with different electronic configuration, intermediate spin (IS), and high spin (HS) from the low-spin (LS) nonmagnetic ground state. The pressure-induced compression of Co-O bonds leads to a substantial increase of LS-IS and LS-HS energy splittings E1 and E2 with d ln E1 dP=0.37 and d ln E2 dP=0.23 GPa-1. The onset of the semiconductor-metal (S-M) transition in LaCoO3 correlates with the vanishing of several Raman modes forbidden for rhombohedral R 3̄ c symmetry and originating from local distortions due to short-range eg -orbital order of IS Co3+ ions. The S-M transition temperature increases rapidly under pressure with d TS-M dP≈50 K GPa.