METAL-INSULATOR-TRANSITION IN THE THALLIUM STRONTIUM COBALTITE TLSR2COO5

The recently discovered thallium strontium cobaltite T1(1-delta)Sr(2)Co(1-tau)(3+)Co(tau)(4+)O(5-epsilon) exhibits a structure closely related to that of the Tl-1201 cuprate. It undergoes structural, magnetic and metal insulator-transitions at room temperature. In the high temperature metallic form carriers occupy a less than half filled o* (x(2)-y(2)) band and the three other unpaired electrons of the high spin Co3+ ions are localized. The resulting magnetic moments are coupled,led by strong ferromagnetic interactions ascribed to indirect exchange via itinerant electrons. The semiconducting behavior of the low temperature phase and the drop of magnetic susceptibility are attributed to a spin equilibrium transferring electrons from the sigma* (x(2)-y(2)) levels into an e(g) (xz, yz) level in a D-4h symmetry.