Tuyển tập báo cáo khoa học trường đại học Quốc Gia Hà Nội đề tài: Effect of Cr$^{3+}$ concentration on structural and optical properties of ZnAl$_2$O$_4$:Cr$^{3+}$ | VNU Journal of Science Mathematics - Physics 26 2010 37-42 Effect of Cr3 concentration on structural and optical properties of ZnAl2O4 Cr3 Trinh Thi Loan Le Hong Ha Nguyen Ngoc Long Faculty of Physics Hanoi University of Science VNU 334 Nguyen Trai Thanh Xuan Hanoi Received 10 February 2010 Abstract. Zn Al .Cr . 2 Ị powders with Cr3 contents ranging from X to have been synthesized by sol-gel method using the following precursors zinc nitrate Zn NC 3 2 6H2O aluminium nitrate A1 NC 3 3 9H2O chrome nitrate Cr NC 3 3 9H2O and citric acid. The effect of the Cr3 concentration on structural and optical properties of the synthesized samples has been studied. For the samples with low Cr3 -dopant concentrations the photoluminescent PL spectra consist of sharp lines assigned to the 2E 2G 4A2 4F transition of ions Cr3 in strong octahedral field. But for the samples with high Cr3 -dopanl concentrations the PL spectra consist of a broad band centered at longer wavelengths assigned to the 4T2 4F 4A2 4F transition of ions Cr3 in weak octahedral field. 1. Introduction The optical properties of Cr3 ion located in octahedral coordination of materials such as ruby and spinel was extensively studied for several decades and these studies played an important role in the development of ligand-fieId theory. For the weak ligand-fieId the lowest excited state is the 4T2 t2g2 es1 state having the same spin multiplicity as the 4A2 t2g3 ground state meanwhile for the strong ligand-field the lowest excited state is the 2E t2g3 state having a different spin multiplicity but the same orbital occupancy as the 4A2 t2g3 ground state 1 . The optical properties of Cr3 -dopcd spinel ZnAl2O4 were studied by Luc et al. 2 Wood et al. 3 Mikenda et al. 4 Nie et al. 5 . They found that the optical properties of the ZnAl2O4 Cr3 were characterized by sharp emission lines associated with the 2E 2G 4A2 4F transitions of different Cr3 sites in the strong ligand-field. However the broad luminescence band
