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

Abstract

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.