Identification of VO(II) at Substitutional and Interstitial Locations in Magnesium Ammonium Phosphate Hexahydrate: A Single Crystal EPR Study

Single crystal EPR studies of VO(II) doped magnesium ammonium phosphate hexahydrate have been carried out at room temperature. The results indicate that the paramagnetic impurity has entered substitutional and interstitial sites in the lattice. The maximum hyperfine coupling value for one site and the minimum hyperfine constant for the other site have occurred at the same angle and vice versa. The spin Hamiltonian parameters obtained from single crystal data for these two sites are: Site 1: g|| = 1.941; g⊥ = 1.994, A|| = 19.23 mT; A⊥ = 7.14 mT; Site 2: g|| = 1.946; g⊥ = 1.997, A|| = 19.07 mT; A⊥ = 7.29 mT. From these observations, it has been concluded that the two-vanadyl impurities are located approximately at right angles to each other. Superhyperfine interaction with the protons of ligand water molecules have been noticed at a few orientations. EPR powder spectrum of the sample showed a set of eight parallel and perpendicular features indicating the presence of only one magnetically distinct site and the spin Hamiltonian parameters calculated from this spectrum matched with single crystal data. The admixture coefficients have also been calculated, and agree well with the reported values.