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.
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