EPR and electronic absorption studies of vanadyl ions in the Cd(NH4)2(SO4)2⋅6H2O single crystals

Abstract

EPR studies of vanadyl (VO2+) impurity ions in the Cd(NH4)2(SO4)2⋅6H2O) (CdASH) crystals are reported at 9.4 GHz. The VO2+ ions are found to substitute the divalent cadmium site and have fixed orientations along Cd–Ow bond directions. The angular variation of the EPR spectra in the a*b, bc, and ca* planes are used to determine the principal g and A values and their direction cosines for the V=O orientations with different populations. A correlation between the two V=O orientations and the metal–water bond directions is observed. The observed superhyperfine structure with the intensity ratio 1:4:6:4:1 is attributed to the interaction of a single unpaired electron with the two pairs of equivalent protons through the oxygen ligands and it indicated the amount of delocalization of unpaired spin density from the vanadium nucleus. The electronic absorption spectrum of VO2+ ions in the CdASH lattice was studied at 80 and 300 K. The characteristic spectrum of the VO2+ ion had four absorption bands at 12 158, 13 888, 18 640, and 28 409 cm−1. From the analysis of the spectrum, the first three bands were attributed to d–d transitions and the last band to characteristic charge transfer for the VO2+ ion. The results indicate a tetragonal site symmetry for VO2+ ions in this lattice. The band positions were calculated using the energy expressions and compared with the observed band positions to confirm the above transitions. The best fit values of the crystal field (Dq) and tetragonal (Ds and Dt) parameters were evaluated from the observed band positions.