Photochemical and thermochemical reactions of MnO₄^- ions in NH₄ClO₄ and NH₄BF₄ crystals: An EPR study

UV photochemical and thermochemical reactions of the MnO₄^- ions doped into single crystals of NH₄ClO₄ and NH₄BF₄ have been investigated via the method of electron paramagnetic resonance (EPR). An isotropic EPR spectrum displaying a ⁵⁵Mn hyperfine sextet has been observed in both reactions. The EPR spectra of the Mn(II) species produced by UV irradiation and by thermal treatments are identical and identical to that exhibited by the doped Mn²⁺ ions. The g-factor is g = 2.004 and the hyperfine constant is A⁵⁵ = 96.6 G in NH₄ClO₄ for the Mn²⁺ ions produced by thermal treatments, by UV irradiation, and by doping. The corresponding values are g = 2.008 and A⁵⁵ = 94.2 G for NH₄BF₄. The observed Mn^VIIO₄^- to Mn(II) reduction can be regarded as the reduction part of an overall redox reaction between MnO₄^- and the NH₄⁺ group. We discuss the mechanism of the solid-state redox reaction.