Change from fluorescence to phosphorescence and tuning the pseudo-Jahn-Teller effect with pressure for (NH[sub 4])[sub 3][CrF[sub 6]]

We report here the pressure-induced change of the emitting state of (NH[sub 4])[sub 3][CrF[sub 6]] from a [sup 4]T[sub 2g] fluorescent state to a mixed state by means of the pseudo-Jahn-Teller effect, and then finally to a [sup 2]E[sub g] phosphorescent state over the pressure range 0-12.3 GPa. We believe that this is the first time an emitting state's spin character has been changed completely by the application of pressure. All of the pressure effects reported here are reversible and reproducible on the same or different samples. With the exceptions noted, luminescence spectra from depressurized samples are virtually identical to spectra from samples of (NH[sub 4])[sub 3][CrF[sub 6]] that never were subjected to high pressures. 20 refs., 3 figs.