To describe the metal-insulator transition in the oxide ${\mathrm{TlSr}}_2{\mathrm{CoO}}_5,$ we investigate the electronic structure of its high-temperature tetragonal phase by the local density approximation (LDA) and model Hartree-Fock calculations. Within the LDA we find a homogeneous metallic and ferromagnetic ground state; however, when including the strong Coulomb interaction in the $3d$ shell more explicitly within the Hartree-Fock approximation, we find an insulating state of lower energy that exhibits both spin and orbital order. The instability of the metallic state toward the insulating one is driven by orbital degeneracy and a near degeneracy in energy of states of intermediate $\mathrm{}(s=1\mathrm{})\mathrm{}$ and high $\mathrm{}(s=2\mathrm{})\mathrm{}$ spin. We also interpret our results in terms of a simple model.

• Received 9 October 2000

DOI:https://doi.org/10.1103/PhysRevB.64.075104