High temperature phase transition and multiphase state formation in K3H(SO4)2 superprotonic conductor

EPR spectra of K3H(SO4)2 (KHS) doped with VO2+ or Mn2+ recorded at the temperature higher than about 440 K are time-dependent and indicate on anomalously slow kinetics of the paraelastic/ferroelastic phase transition which takes place far below the temperature proposed by Baranov et al. (TII = 463 K). The formation of a multidomain state upon heating below TII is manifested as irreversible changes in EPR spectra after sample cooling to room temperature. The angular dependence of EPR spectra indicates that K3H(SO4)2 undergoes a ferroelastic phase transition between the point groups 2/m and 3¯m. The EPR spectra of KHS doped with VO2+ or Mn2+ are rapidly changed into the pseudo-powder ones after annealing above TI = 480 K. This points to a decomposition of the KHS crystal and a multiphase state formation at TI = 480 K, rather than at superprotonic phase transition, which is typical for the majority of compounds of the M3H(XO4)2 type.