Relaxed Exciton Luminescence Induced by the Structural Phase Change in Ammonium Bromide

Abstract

When the tetragonal III phase of NH₄Br crystals is “frozen” upon rapid cooling from room temperature down to 10 K, a new luminescence appears at 3.0 eV in addition to the 4.18 eV luminescence arising from self-trapped excitons in the cubic IV phase. This luminescence band is found to decrease in intensity with a complementary increase of the 4.18 eV band as the crystal is warmed above 50 K. Measurements of the excitation spectrum, decay time and linear polarization confirm that the 3.0 eV luminescence is due to a radiative transition from the triplet state of self-trapped excitons in the III phase structure involving the displacement of Br⁻ ions from their cubic lattice sites.