The thermally induced insulator-metal transition (IMT) has been investigated for single crystals of undoped and lightly doped ${\mathrm{LaCoO}}_3$ by measurements of the optical-conductivity spectra, the resistivity, and the Hall coefficient up to 800 K. The IMT around 500 K, where the spin-state transition from the nonmagnetic $\mathrm{}(S=0\mathrm{})\mathrm{}$ ground state to the intermediate-spin $\mathrm{}(S=1\mathrm{})\mathrm{}$ state is nearly completed, is characterized by a large energy-scale change of the electronic structure as well as by a steep increase of carrier number up to a typical metallic value due to closing of the charge gap. These behaviors are strikingly parallel to those characteristic of the Mott transition.

• Received 14 April 1998

DOI:https://doi.org/10.1103/PhysRevB.58.R1699