NMR and impedance spectroscopy are used to study the ionic mobility and conductivity in crystalline samples in PbSnF 4 -MF systems (M = Li, Na, K) in a 150-473 K temperature range. The 19 F NMR spectral parameters, types of ionic motion, and ionic conductivity value in the PbSnF 4 compound doped with alkali metal fluoride is found to be determined by the temperature, nature, and concentration of an alkali cation. The specific conductivity of the crystalline samples in PbSnF 4 -MF systems (M = Li, Na, K) is rather high at room temperature and hence, it seems possible to apply them in the development of functional materials with high ionic (superionic) conductivity.One of the aspects in the study of fluoride systems is the examination of the ionic mobility and transport. Ionic conductors with high conductivity through fluoride ions are promising materials for various electrochemical devices, chemical sensors, and so on. Fluoride ions being the lightest of anions (except a hydride ion) have a sufficiently high redox potential, which provides favorable conditions for the use of fluorides in batteries with high specific energy. Fluorides of heavy metals, such as LaF 3 , CeF 3 , PbF 2 , SnF 2 , BiF 3 and materials based on them, can be related to fluoride-containing compounds with high ionic conductivity [1]. This is explained by high polarizability of large-size cations (low ionization energy), which results in a decrease in the activation energy of ion transfer. Among numerous fluoride-containing compounds and solid solutions with high ionic conductivity, SnF 2 -based solid electrolytes are of great interest [2], and among them, compounds of the composition MSnF 4 , in particular, the PbSnF 4 compound, have the best electrophysical properties [2][3][4][5][6][7][8]. Note that the structure and properties of the PbSnF 4 compound (including phase transitions, ionic mobility, transport, and conductivity, etc.) were considered in a number of works [2][3][4][5][6][7][8][9][10][11][12], and some ambiguity in the results of these studies is due to complex polymorphism inherent in this compound [2,7,10].The nature of the outer-sphere cation is known to substantially affect the structure and properties of complex stannate(II) fluorides [2,10,13]. Variation of the cation sublattice composition often leads to the formation of compounds that differ in the structure and energy of ionic motion. It is of interest to answer the question: how does the character of dynamic processes change in the PbSnF 4 compound if part of cations is substituted with other cations? It is exemplified by the results of the analysis of ionic conductivity in tetragonal PbSnF 4 , in which part of Pb 2+ and/or Sn 2+ ions were replaced
