Phase transition and host spin-lattice relaxation narrowing in the EPR of Mn(II)-doped single crystals of CoNbOF<inf>5</inf> · 6H<inf>2</inf>O

The EPR of Mn(II) doped in single crystals of diamagnetic ZnNbOF5 · 6H2O and paramagnetic CoNbOF5 · 6H2O is studied at different temperatures. Observation of sharp resonance of Mn(Il) at 300 K and broadening of the resonance lines beyond detection below 120 K in the paramagnetic host is interpreted in terms of the random modulation of the dipolar interaction between Mn2+ and Coil ions by the fast spin-lattice relaxation of Co2+ ions. An estimate of the spin-lattice relaxation rate of the Co2+ ions is made by measuring the linewidth of the dopant Mn(II). The temperature dependence of the EPR parameters of Mn(Il) in CoNbOF5 · 6H2O further indicates a structural phase transition at 238 K. The reduction in symmetry in the low-temperature phase as revealed by the site splitting and spin-Hamiltonian parameters is in accordance with the structural aspects of a well studied phase transition in the isomorphous lattice CoSiF6 · 6H2O. © 1984.