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Abstract: Nanocrystalline thin films of mixed rare earth oxides (Y 1-x Er x ) 2 O 3 (0.1 ≤ x ≤ 1) were deposited by electron beam evaporation technique on polished fused silica glass at different substrate temperatures (200-500°C). The effect of the substrate temperature as well as the mixing parameter (x) on the structural and optical properties of these films has been investigated by using X-ray diffraction (XRD), energy dispersive x-ray analysis and optical spectrophotometry. XRD investigation shows that mixed rare earth oxides film (Y 1-x Er x ) 2 O 3 grown at lower substrate temperature (T s ≤300°C) are poorly crystalline, whereas films grown at higher substrate temperatures (T s ≥400°C) tend to have better crystallinity. Furthermore, the mixing parameter (x) was found to stabilize the cubic phase over the entire of 0.1 ≤ x ≤ 1. The crystallite size of the films was found to vary in the range from 25 to 39nm. Optical band gap of the films was deterimined by analysis of the absoprtion coeffifcient. For films deposited at different substrate temperatures direct and indirect transitions occur with energies varied from 5.29 to 5.94eV and from 4.23 to 4.51eV, respectively. However, films of different composition x, give optical band gap varied from 6.14 to 5.86eV for direct transition and from 5.23 to 4.22eV for indirect transitions. Consequently, one may conclude that it is possible to tune the energy band gap by relative fraction of constituent oxides. It was found that optical constants increase with increasing the substrate temperature. Nevertheless, the values of n and k decrease with increasing the mixing parameter, x. [Copyright &y& Elsevier]