EPR OF A VO2+-DOPED FE(NH4)2(SO4)2.6H2O SINGLE-CRYSTAL - VO2+-FE2+ EXCHANGE INTERACTION AND SPIN-LATTICE RELAXATION-TIME OF HOST FE2+ IONS

X-band (almost-equal-to 9.5 GHz) electron paramagnetic resonance (EPR) studies of the VO2+ ion are carried out on a single crystal of the Tutton salt Fe(NH4)2(SO4)2.6H2O (FASH) at 295,80, and 4.2 K. The data indicate two physically-equivalent VO2+ sites in the unit cell, each consisting of three magnetically-inequivalent VO2+ ions. The principal values and direction cosines of the principal axes of the g2 and A2 tensors are evaluated, using a least-squares-fitting program. An impurity [VO(H2O)5]2+ complex model of the vanadyl ion is found to explain the data well. The V4+-O2- bond directions for the VO2+ ions are deduced. The VO2+-Fe2+ exchange-interaction constant in FASH is estimated. The host-ion (Fe2+) spin-lattice relaxation time is estimated from VO2+ EPR linewidth.