  Luminescence characteristics of single crystals (SC), single crystalline
films (SCF), powders and ceramics of YAlO_3 and YAlO_3:Ce have been studied at
4.2-300 K under photoexcitation in the 4-20 eV energy range and X-ray
excitation. The origin and structure of defects responsible for various
exciton-related emission and excitation bands have been identified. The ~5.6 eV
emission of YAlO_3 SCF is ascribed to the self-trapped excitons. In YAlO_3 SC,
the dominating 5.63 eV and 4.12 eV emissions are ascribed to the excitons
localized at the isolated antisite defect Y^{3+}_Al and at the Y^{3+}_{Al}
defect associated with the nearest-neighbouring oxygen vacancy, respectively.
Thermally stimulated release of electrons, trapped at these defects, takes
place at 200 K and 280 K, respectively. The formation energies of various
Y^{3+}_{Al}-related defects are calculated. The presence of Y_{Al}
antisite-related defects is confirmed by NMR measurements. The influence of
various intrinsic and impurity defects on the luminescence characteristics of
Ce^{3+} centers is clarified.
