METAL-INSULATOR-TRANSITION IN THE THALLIUM STRONTIUM COBALTITE TLSR2COO5

The recently discovered thallium strontium cobaltite (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 temperat ure 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+ i ons are localized. The resulting magnetic moments are coupled,led by s trong ferromagnetic interactions ascribed to indirect exchange via iti nerant electrons. The semiconducting behavior of the low temperature p hase 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.