The optical spectra of rare‐earth cobalticyanides

Rare earth cobalticyanides Ln[Co(CN) 6 ] · n H 2 O have two cristalline forms, an hexagonal one from La to Nd with n = 5 and an orthorhombic one from Ce to Lu with n = 4. The optical absorption spectra of the Nd 3+ (4f 3 ) and the fluorescence spectra of the Eu 3+ (4f 6 ) in the pure and doped salts are recorded at low temperatures (4.2 or 77 K) in the polycristalline form. The orthorhombic phase exhibits spectra corresponding to a lanthanide in a C 2v site under a weak crystal field (B q 2 , B q 4 < 500 cm −1 ). The spectra for Nd 3+ and Eu 3+ in the hexagonal lanthanum compounds are compatible with a C 3h site symmetry. There is evidence of selection rules for the dipole electric transitions in 4f 3 , and the 4f 6 configuration has levels which are Kramers doublets.