Two spin-state transitions in LaCoO3

Neutron-diffraction measurements have revealed a new anomalous thermal lattice expansion of LaCoO3 near 500 K that indicates the existence of a second spin-state transition; in addition to the one previously established near 100 K. The anomalous expansion and the temperature dependence of the Co magnetic moments are successfully interpreted in a wide temperature range based on a simple model assuming low-spin (LS, S = 0); intermediate-spin (IS, S = 1), and high-spin (HS, S = 2) states of Co atoms. The first spin transition, near 100 K, is from LS to IS, and the second, near 500 K. is hom IS to a mixed state of IS and HS. The fitted model parameters indicate that the initially-large energy difference between IS and HS states decreases towards zero as the second transition proceeds. The large drop in resistivity associated with the latter transition appears to be correlated with the population of the HS state.