High resolution MCD spectroscopy of transition metal ions in fluoride crystals

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Abstract
Measurements of the MCD spectra of Ni2+ in KMgF3 and KZnF3 have confirmed and extended previous assignments of spin-orbit structure based on similarities of vibrational patterns. The measurements also reveal the presence of a 1 A 1g (1 G) state which interacts strongly with the 3 T 1g b (3 P) state. The MCD spectra of the tetragonal crystals K3Ni2F7 and K2NiF4 have been measured for the transitions 3 A 2g →3 T 1g a , 1 Eg . The spectrum of K3Ni2F7 is characterized by the appearance of electric dipole zero phonon lines, consistent with the C 4v site symmetry of the Ni2+ in the paramagnetic phase. An analysis of the MCD spectrum allows a positive assignment of six of the seven states (from 3 T 1g a and 1 Eg ) to which transitions are allowed by symmetry in the axial spectrum. A complete tetragonal crystal-field analysis has been made which shows that the distortion is an axial compression of the fluoride octahedron. The absorption spectrum of K2NiF4 shows much less vibrational structure, but the MCD spectrum allows a definitive assignment of six states (from 3 T 1g a and 1 Eg ) through a 2u vibrations in the usual vibronic mechanism. False origins based on eu phonons have not been identified. The tetragonal field is larger than for K3Ni2F7 and corresponds to an axial compression, in agreement with the known crystal structure.