Jahn‐Teller Effect and Partially Covalent Bond as Causes of Low Symmetry of Ga+, In+, and Tl+ Centres in CsCl‐Type Alkali Halides

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Abstract
Partially polarized luminescence is observed for Ga+, In+, and Tl+ centres in alkali halides of CsCl‐type structure at liquid nitrogen temperature. The symmetry of excited Ga+, In+, and Tl+ centres is established to be tetragonal in the most crystals investigated. The symmetry of Tl+ centres in CsI:Tl crystals is trigonal. The possible causes of the low symmetry are discussed. The interaction of the optical electrons of the activator with tetragonal vibrations of the lattice may be the main cause for the low symmetry of the excited Ga+, In+, and Tl+ centres in most of the alkali halides. The partial covalent bond between thallium and some of the surrounding iodine ions seems to be essential for CsI:Tl crystals.