Electronic structure of ScPO4 single crystals: Optical and photoelectric properties

Initial investigations of the intrinsic optical properties of ScPO4 have been carried out in which detailed luminescence spectra were observed. An optical gap of 7 eV was determined that depended on temperature according to the Urbach-Toyozawa rule. The band gap is equal to 7.2 eV, and in the narrow range between the optical and band gaps, excitons are created by light. The self-trapped exciton (STE) luminescence band at 5.6 eV can be excited at energies of 7 eV and higher. This luminescence is strongly polarized (P = 70%) along the optical axes of the crystal, and the luminescence-decay time constant (tau) is about 1 ms near 4.2 K. The decay kinetics exhibit two ranges in which tau decreases strongly with increasing temperature: a low-temperature range associated with the zero-field splitting of the STE triplet states and a second region where decreases in both tau and P arise from reorientations of the STE. The thermal-quenching energy of the STE luminescence is 0.22 eV with a rate of 5 x 10(5), and the reorientation activation energy is 0.02 eV with a rate of only 200. A detailed study of the ScPO4 luminescence bands and the development of a model for the STE are presently being carried out.