The nature of photochromism in chlorosodalites from optical data

Methods have been devised for the preparation of pure and doped (S, Se, Te) chlorosodalites. Optical data are presented which are at variance with the accepted mechanism for photochromism in these materials (i.e. the presence of an S 2 2- ion). Intrinsic and extrinsic mechanisms which depend upon the formation of an F centre in conjunction with an O 2- ion on a chlorine vacancy site are proposed. Similar mechanisms involving S, Se and Te may occur, and it is possible that more complex ions are involved.