Optical properties of Tl2+ centres in alkali halides: Theoretical investigation

The properties due to substitutional Tl2+ ions in KX (X = Cl, Br, I) lattic es have been explored by means of density functional and MS-X alpha calcula tions carried out on clusters involving up to 81 atoms. Density functional calculations strongly indicate the existence of a relevant inwards relaxati on (similar to 12%) for KCl:Tl2+. The present results support that the lowe st optical absorption bands in KCl :Tl2+; peaked at 3.40 and 4.22 eV, can b e associated with the t(1u)(pi) --> a(1g)* and t(1u)(sigma) --> a(1g)* char ge transfer (CT) transitions of the (TlCl6)(4-) unit. Also the 130 meV red shift undergone by the first CT transition on passing from KCl :Tl2+ to RbC l Tl2+ is shown to reflect an increase of the equilibrium Tl2+-Cl- distance of only about 2pm.