Ultraviolet luminescence of ScPO4, AlPO4 and GaPO4 crystals

Anatoly N Trukhin, Krishjanis Shmits, Janis L Jansons and
Lynn A Boatner

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 -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.

Journal of Physics: Condensed Matter 25 (2013) 385502 (6pp)


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