Optical and far-IR spectroscopy techniques have shown that the spectroscopic properties of Np(VI) and Pu(VI) compounds formed with different tetrahedral anions XO42- (X = S, Se, Cr) are depending on the nature of the central atom X in the oxoanions. Changes in electronic transition energies within the ground state configuration are mainly due to crystal field perturbations induced by oxygen charge distribution variations in the actinide ion surrounding depending upon the nature of X. Changes may be related to covalence variation in An-O bonding or distortions in the ligand arrangement.
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