| Summary: | Luminescence properties of Lu<sub>2</sub>SiO<sub>5</sub>:Ce<sup>3+</sup> and Y<sub>2</sub>SiO<sub>5</sub>:Ce<sup>3+</sup> nanocrystals were studied using photo- and X-ray luminescence techniques. The crystal structure of Re<sub>2</sub>SiO<sub>5</sub> nanocrystals (P2<sub>1</sub>/c space group) differs from the crystal structure of Re<sub>2</sub>SiO<sub>5</sub> bulk crystals (C2/c space group) with 9- and 7-oxygen-coordinated cation positions instead of 6- and 7-coordinated ones observed for Re<sub>2</sub>SiO<sub>5</sub> bulk crystals. Two optical centers (Ce1 and Ce2) were observed for Re<sub>2</sub>SiO<sub>5</sub>:Ce<sup>3+</sup> nanocrystals originating from cerium ions substituting 9- and 7-oxygen-coordinated cation sites. Preferential substitution of larger cation sites by cerium ions leads to higher photoluminescence intensity of Ce1 centers, however, Ce2 centers are the main centers for electron-hole recombination, so only Ce2 band is observed in X-ray luminescence spectra. The features of oxygen coordination of Ce1 and Ce2 centers and high content of oxygen vacancies in Re<sub>2</sub>SiO<sub>5</sub>:Ce<sup>3+</sup> nanocrystals can provide preferential trapping of electrons near Ce2 centers, and therefore, the dominant role of Ce2 band in X-ray luminescence spectra.
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