ИСТИНА

A promising basis of materials for alkaline batteries shall be the compounds with tetrahedral anions, primarily phosphates and fluorophosphates. The phase diagram of LiF-Li3PO4 system is studied. The diagram of the eutectic type. Eutectic coordinates are 800±5 °C, 8±1 mol % Li3PO4. The melting point of lithium fluoride 845 °C, lithium phosphate has a polymorphic transition at 1175°C. The eutectic horizontal does not reach the ordinate Li3PO4, which indicates the formation of a solid solution based on lithium phosphate. The maximum concentration of the solid solution at the eutectic temperature is 11±2 mol % LiF. According to the XRD data of the annealed samples, the limiting width of the solid solution region decreases with decreasing temperature, and is 8±1 mol % LiF at 750 °C and 6±1 mol% LiF at 600 °C. The occurrence of lithium fluoride leads to a change in the lattice parameters of γ-Li3PO4. When lithium fluoride is added to phosphate, a slight decrease in the temperature of the thermal effect corresponding to the polymorphous transformation of Li3PO4 from γ to α phase is observed. This indicates the formation of a small region of solid solution based on the high-temperature modification of α-Li3PO4. The suggested scheme of phase equilibria corresponds to metathectic equilibrium. The metathectic coordinates are 98±1 mol % Li3PO4, 1170 °C. We assume that in the system under consideration the tetrahedral ion (PO4)3- is replaced by (LiF4)3- complex. This isomorphism scheme is described by the following reaction: (1-x)Li3PO4+4xLiF → Li3(PO4)1-x(LiF4)x Thus, the presented study results suggest the formation of solid solution based on lithium phosphate as a result of the entry into the crystalline structure of the coordination tetrahedron (LiF4)3-, substituting (PO4)3- anion. Such substitution is a new mechanism of anionic heterovalent isomorphism.