Ultraviolet luminescence of ScPO4, AlPO4 and GaPO4 crystals.

The luminescence of self-trapped excitons (STEs) was previously observed and described for the case of tetragonal-symmetry ScPO4 single crystals. The subject band in this material is situated in the UV spectral range of ∼210 nm or ∼5.8 eV. In the present work, we are both expanding this earlier luminescence study and seeking to identify similar luminescence phenomena in other orthophosphate crystals, i.e., AlPO4 and GaPO4. These efforts have proven to be successful--in spite of the structural differences between these materials and ScPO4. Specifically we have found that for AlPO4 and GaPO4, in addition to an α-quartz-like STE, there is a UV luminescence band that is similar in position and decay properties to that of ScPO4 crystals. Potentially this represents an STE in AlPO4 and GaPO4 crystals that is analogous to the STE of ScPO4 and other orthophosphates. The decay kinetics of the UV luminescence of ScPO4 was studied over a wide temperature range from 8 to 300 K, and they exhibited some unusual decay characteristics when subjected to pulses from an F2 excimer laser (157 nm). These features could be ascribed to a triplet state of the STE that is split in a zero magnetic field. A fast decay of the STE was detected as well, and therefore, we conclude that, in addition to the slow luminescence corresponding to a transition from the triplet state, there are singlet-singlet transitions of the STE. Time-resolved spectra of the slow and fast decay exhibit a small shift (∼0.15 eV) indicating that the singlet-triplet splitting is small and the corresponding wavefunction of the STE is widely distributed over the atoms of the ScPO4 crystal where the STE is created.