EPR OF VO-2+ IN CD(NH4)2(SO4)2.6H2O/AND/MG(NH4)2(SO4)2.6H2O SINGLE-CRYSTALS - LIGAND SUPERHYPERFINE INTERACTION AND BONDING COEFFICIENTS

Detailed X-band electron paramagnetic resonance (EPR) studies of the VO2+ ion have been carried out in the single crystals of Tutton salts Cd(NH4)2(SO4)2·6H2O (CASH) and Mg(NH4)2(SO4)2·6H2O (MASH) at 295, 80 and 4.2 K. F or both samples the data are indicative of the presence of two magnetically inequivalent, but physically equivalent, VO2+ sites in the unit cell, each consisting of three magnetically inequivalent VO2+ ions, with one of them being present with a very small probability. Each vanadyl hyperfine line is characterized by an anisotropic quintet superhyperfine (SHF) splitting, with the intensity ratios 1:4:6:4:1. For any of the two samples investigated, the maximum separations of the SHF splitting for the two largely populated magnetically inequivalent VO2+ ions, corresponding to any one site, are observed when the orientations of the external magnetic field (B) are along the M2+-H2O(7), and the M2+-H2O(8) directions (M2+ = divalent metal ion, Cd2+ or Mg2+). The temperature dependence of the SHF splitting was found to be different for the two samples. The principal values and direction cosines of the principal axes of the g̃2 and Ã2 tensors are evaluated from a simultaneous fitting of the various EPR line positions, using a least-squares fitting program. The following details apply to both the samples: an impurity model of the vanadyl ion, described by the [VO(H2O)5]2+ complex, is found to explain the data adequately. The orientations of the V4+-O2- bonds have been deduced to lie close to either the M2+-H2O(7) and M2+-H2O(8) directions. The SHF structure is explained to be due to the interaction of the unpaired electron of the VO2+ ion with the four protons of its two nearest-neighbor H2O(9) molecules. The principal values and orientations of the principal axes of the SHF interaction tensor have been estimated. Using the reported optical absorption- and the present EPR data, the bonding coefficients of the [VO(H2O)5]2+ complex have been estimated. © 1990.