Structural transitions, ion mobility, and conductivity in CsSbF₃(H₂PO₄)

Highlights: • Studied the phase transitions, ion mobility and conductivity in CsSbF₃(H₂PO₄). • Above 425 K a transition is observed: the crystalline phase→amorphous phase. • The types of ion mobility in CsSbF₃(H₂PO₄) and its amorphous phase have been established. • The main type of ion motion above 300 K in disordered phase is a diffusion H. • The conductivity of the CsSbF₃(H₂PO₄) more than 10^-4 S/cm in 410–425 K region. - Abstract: Structural transitions, ion mobility, and conductivity in CsSbF₃(H₂PO₄) (I) have been investigated by the methods of ¹H, ¹⁹F, ³¹P NMR (including ¹H, ¹⁹F, ³¹P MAS NMR), DSC, X-ray diffraction, and impedance spectroscopy. It was found that the fundamental changes in ¹H, ¹⁹F, ³¹P NMR spectra (above 390 K) were associated with the formation of a crystalline disorder phase I with high ionic mobility in the proton and fluoride sublattices, as a result of a phase transition in the 400–420 K range. In the same temperature range, the transition of PO₂(OH)₂^- anions from the ^rigid lattice to fast reorientations takes place. Above 430 K, there occurs a transition from the crystalline disordered phase to the amorphous one. The types of ion mobility in CsSbF₃(H₂PO₄) and its amorphous phase have been established and temperature ranges of their realization have been determined (150–450 K). According to the NMR data, the diffusion in the proton sublattice of the disordered crystalline and amorphous phases is preserved even at room temperature. The ionic conductivity in CsSbF₃(H₂PO₄) reaches the values of ~ 2.6 × 10^-4 S/cm in the temperature range 410–425 K and decreases down to ~ 2.0 × 10^-5 S/cm upon transition to the amorphous phase (435–445 K).