Funct. Mater. 2025; 32 (3): 429-440.

In this paper, the electroconductive properties of solid non-stoichiometric phases formed in the BaF₂–LaF₃–SnF₂. The Ba_0.86-xLa_xSn_1.14F_4+x and Ba_0.8-xLa_xSn_1.2F_4+x phases containing up to 0.12 mole fractions lanthanum were synthesized by the method of co-precipitation of BaF₂ and LaF₃ followed by sintering with SnF₂. The crystal lattice of the synthesized compounds corresponds to the tetragonal syngony (space group P4/nmm) and is isostructural to BaSnF₄. The electrical conductivity of the synthesized phases depends on the pressure at which the samples were prepared. Samples made at a pressure of 9.8 MPa have the highest conductivity. The electrical conductivity of the synthesized solutions of heterovalent substitution Ba_0.86-xLa_xSn_1.14F_4+x and Ba_0.8-xLa_xSn_1.2F_4+x is higher than the conductivity of the initial non-stoichiometric phases Ba_0.86Sn_1.14F_4.00 and Ba_0.80Sn_1.20F_4.00, and the activation energy of electrical conductivity is lower. An increase in the substituent amount: to 0.12 for Ba_0.86-xLa_xSn_1.14F_4+x and to 0.10 for Ba_0.8-xLa_xSn_1.2F_4+x positively affects the electrical conductivity of the obtained electrolytes. Analysis of the isothermal conductivity surface of the fluoride conducting phases Ba_1-y-xLa_xSn_1+yF_4+x x≤0.12, y≤0.2 indicates that the conductivity of solid electrolytes increases with increasing concentration of lanthanum and tin in the material. The electronic component of conductivity for the samples with different cationic composition, and the charge transfer numbers of fluorine anions were estimated. The conductivity of all synthesized phases is determined by fluorine anions, the charge transfer numbers are close to unity.