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Comparative study of the crystal-field splitting of trivalent neodymium energy levels in polycrystalline ceramic and nanocrystalline yttrium oxideThe absorption spectrum of C -Gd 2 0, was recorded at room temperature up to 40 600 cm -'. 47 Stark levels of the 4/' configuration were obtained. The fluorescence spectra from the ·P m level were recorded for Gd'+ in Y,O, and Yb20,. The crystal-field splittings are large. Because of the peculiarities ofthe half-filled shell. the experimental results (32 Stark levels) were fitted with a C 2, -type crystal field Hamiltonian restricted to operators of rank 2 and 4. applied on six different basis sets for J = 3/2. 5/2. 7/2 (two matrices). and 9/2 (two matrices) limited by a truncation choice depending on free-ion wave vectors and ignoring J mixing. Real crystal field parameters of rank 2 and 4 were obtained for the C 2 site of C -Gd 2 0, and are in agreement with those previously determined for Eu3+. The fluorescence data yielded a large value ( -1700 cm -') for the B g parameter oftheS. site in C-Y 2 0,.