Indirect nuclear exchange coupling and electronic structure of the layered semiconductor TlS: NMR study and band structure calculation

We have studied the electronic structure of the layered semiconductor TlS by means of 203Tl and 205Tl NMR and band structure calculation. The crystal structure of this compound is built from the metal-chalcogen layers formed of linked Tl3+S42- tetrahedra and Tl+ ions located between the layers. Our experiments show significant interlayer indirect exchange coupling of thallium nuclei. Since the distances between thallium ions essentially exceed the sum of their ionic radii, it is concluded that such coupling is realized due to the overlap of the Tl electron wavefunctions across the intervening S atom. The exchange interaction has been evaluated by means of the quantitative spectral analysis. The band structure calculation shows that the top of the valence band is composed of mixed Tl+ 6s6pz-S 3s3pz-Tl3+ 6s6pz states, yielding overlap of the Tl+-S-Tl3+ type. Such electronic structure gives rise to the effective interlayer coupling of Tl nuclear spins observed in the experiment. This effect is discussed along with the recently obtained NMR results in the chain thallium sulfide and selenide