Thermal and Transport Properties, Ion Mobility, and Phase Transitions in Compounds (NH4)6CsZr4F23 and (NH4)6CsHf4F23

Abstract

Thermal, electrophysical properties, ion mobility, and phase transitions (PT) in compounds (NH₄)₆CsZr₄F₂₃ (I) and (NH₄)₆CsHf₄F₂₃ (II) have been studied by ¹H, ¹⁹F NMR, X-ray powder diffraction, DSC, and impedance spectroscopy. The types of ion motions in the fluoride and ammonium sublattices of both compounds have been determined in temperature range 150–450 K and their activation energy has been assessed. Phase transitions in compounds I and II has been revealed in temperature range 380–404 and 384–412 K to form high-temperature modifications with diffusion of fluoride and ammonium ions as the main type of ion motions. The high ionic conductivity of compounds I and II (higher 10^–4 S/cm at 463 K) allows one to refer these fluoro complexes to the class of superionic conductors.