The nephelauxetic effect in ZnAl$_2$O$_4$:Cr$^{3+}$ nanocrystals induced by their size

ZnAl$_2$O$_4$ nanocrystallites doped with Cr$^{3+}$ ions with mean sizes ranging from 2 to 16 nm were synthesized by the hydrothermal method. Chromium ions occupy the aluminum positions, which symmetry depends on the crystallite size. The smallest nanocrystals have a much larger unit cell than the bigger ones. The metal to ligand distance increases when the size of the nanocrystals decreases. This causes the nephelauxetic effect, which is for the first time (to our knowledge) observed as a size effect. It was also observed that ZnAl$_2$O$_4$ : Cr$^{3+}$ nanocrystals with size larger than 10 nm possesses the same spectroscopic properties as monocrystal.