Phase transitions, ion mobility, and conductivity in fluoro complexes (NH4)6RbZr4F23 and (NH4)6RbHf4F23

Ion mobility, phase transitions (PT), and conductivity in compounds (NH4)6RbZr4F23 (I) and (NH4)6RbHf4F23 (II) were studied by 1H and 19F NMR spectroscopy, X-ray powder diffraction (XRD), DSC, and impedance spectroscopy. Ion movement modes in fluoride and ammonium sublattices of both compounds were determined in temperature range 150–480 K and their activation energy was assessed. Phase transitions in compounds I and II were revealed in temperature ranges 400–440 and 419–440 K resulting in formation of α modifications where diffusion of fluoride and ammonium ions becomes the dominant kind of ion movement. High ionic conductivity in fluoride II (σ ~ 7.7 × 10–3 S/cm at 463 K) indicates that this compound belongs to the class of superionic conductors.