  We present a mechanism able to show intrinsic bistable behaviour involving
single Yb3+ ions embedded into bromide lattices, in which intrinsic optical
bistability (IOB) has been observed. The mechanism is based on the
experimentally found coupling between the Yb3+ ion and the totally symmetric
local mode of vibration of the [YbBr6]3- coordination unit. The model
reproduces the IOB observed in CsCdBr3:1% Yb3+ and allows to understand the
experimentally found presence of the phenomenon in the other bromides, but its
absence in Cs3Lu2Cl9:Yb3+.
