Structure of the C Absorption Band of Tl+-Type Centers in Alkali Halides Due to the Jahn-Teller Effect

The C band ( 1 A 1 g → 1 Tl 1 u ) of KI:Tl type phosphors has been studied at various temperatures. Its triplet structure (C 1 , C 2 , C 3 ) has been confirmed in 24 combinations of impurity ions (Ga + , In + , Tl + , Sn 2+ , Sb 2+ ) and alkali halide hosts with the NaCl and CsCl type lattices. Since the separation between C 2 and C 3 is proportional to \(√T\) at high temperatures as exemplified clearly in the case of KI:Ga, KI:Sn and CsBr:In, the structure is concluded to be certainly caused by the dynamical Jahn-Teller effect. Therefore, except its asymmetry which becomes prominent at high temperatures, it is qualitatively understood in terms of Toyozawa, Inoue and Cho's theory based on the linear interaction. Importance of quadratic interactions is discussed in connection with the asymmetry; the difference in force constants between the 1 T 1 u and 1 A 1 g energy surfaces causes the asymmetry. Lastly, Kristoffel's theory for the structure is criticized; the breakdown of the Condon approximation is not essential for the appearance of the structure observed.