Features of valence band structure of TlAlF₄

Modulation of the upper edge of valence band of ionic solids by 6s² occupied states of Tl⁺ is a primary origin of the changes in optical and electrical properties of host materials containing Tl⁺. Semiquantitative estimation of mixing of 6s² states on Tl⁺ ions with 2p⁶ on F^- ions in the TlAlF₄ crystal was carried out by photoemission, optical absorption, photoluminescence and photocurrent measurements. Photoemission spectra were measured with excitation energies of 60, 130 and 1486.7 eV. The separation between the uppermost two bands was found to be 2.5 eV for TlAlF₄ and T1F, which were considerably greater than the corresponding values for TlCl and TlBr. This was interpreted as due to the fact that the energy level of F 2p is deeper than Cl 3p and Br 4p. It was concluded from the excitation energy dependences of the intensities of the respective peaks in the valence band region that the contribution of Tl 6s to the top edge of the valence band of TlAlF₄ and TlF was several tens of a per cent. A large Stokes shift of about 2 eV and the long lifetime (1.2 μs) of fluorescence suggested that the 6s² levels of Tl⁺ ions have a tendency to localize and the top of the valence band did not form such an extended state of TlBr. This conclusion was confirmed by the observation that the photocurrent of TlAlF⁴ was one fifth of that of TlBr.