Comparative analysis of optical properties $C$–Gd$_2$O$_3$ : Eu$^{3+}$ single crystal and nanocrystalline sample

Single crystals (SC) of $C$–Gd$_2$O$_3$ were grown from a solution in a Li$_6$Gd(BO$_3$)$_3$+Gd$_2$O$_3$+Eu$_2$O$_3$-flux with seed using Czochralski method. Growing was carried out at a relatively low temperature of 1145$^\circ$C. A sample of nanocrystallites powder (NCP) was obtained by the sol-gel method from aqueous solutions of Gd(NO$_3$)$_3$ and Eu(NO$_3$)$_3$ with NaOH-precipitant by dispersed spraying of these solutions. The obtained SC and NCP $C$–Gd$_2$O$_3$ : Eu$^{3+}$ samples were analyzed by X-ray powder diffraction (XRD) to confirm the $C$–Gd$_2$O$_3$ cubic phase formation and to determine both the sizes (59 nm) and the deformation stresses (0.06%) of nanoparticles. It has been shown, when the photoluminescence (PL) excitation energy changes over the series 250, 280, and 300 nm, then low-energy PL bands at 620 and 710 nm decrease in intensity relative to the intensity of main PL band at 611 nm. For high-energy bands at 595, 550, 480, 470 and 420 nm, an increase in their intensity relative to the 611 nm band is observed. High-energy bands are not visible against the background noise for NCP. A study of diffuse reflection (DR) spectra showed the charge transfers along PL radiative transitions are not direct and are carried out with the participation of phonons at energy of 0.29 eV for the bulk part of the SC and 0.26 eV for the damaged surface layer of the SC formed during mechanical polishing. Higher phonon energy of 0.49 eV is observed for NCPs, this is due to the large surface area of the particles and that correlates with the broadening of the XRD reflections at half maximum.