Physicochemical prerequisites of the synthesis of new ionic conductors based on complex oxides with a ramsdellite-type structure

Abstract

This paper reports on the results of experimental investigations of solid solutions that have a ramsdellite structure and crystallize in the Li2O-Fe2O3-SnO2-TiO2, Li2O-Fe2O3-TiO2, and Li2O-Cr2O3-TiO2 systems. The concentration boundaries of existence of these solid solutions are determined, and their thermal and electrical properties are studied. It is established that the ramsdellite phases crystallizing in the Li2O-Fe2O3-SnO2-TiO2 and Li2O-Fe2O3-TiO2 systems transform into a metastable hexagonal phase at temperatures of ∼ 650–700 and 550°C, respectively. The results of investigations of this phase transformation by differential thermal analysis, differential scanning calorimetry, and high-temperature X-ray diffraction, as well as measurements of the electrical conductivity of the ramsdellite phases in the Li2O-Fe2O3-SnO2-TiO2 system, indicate that this transition is a first-order phase transition. Solid solutions of ramsdellite phases crystallizing in the Li2O-Cr2O3-TiO2 system do not undergo the phase transformation under consideration. These solid solutions are stable during multiple dynamic heating to 1000°C (under heating-cooling conditions at a rate of 10–15°C/min). Long-term heat treatments at 1000°C (for more than 3 h) lead to their decomposition with the formation of ramsdellite phases of the unknown composition and the Li2TiO3 compound. The electrical conductivities of the solid solutions formed by the ramsdellite phases crystallizing in the Li2O-Fe2O3-TiO2 and Li2O-Cr2O3-TiO2 systems at a temperature of 500°C are evaluated to be σ ≈ 10−1.5 S/cm. The transformation of the ramsdellite phases in the Li2O-Fe2O3-TiO2 system into the metastable hexagonal phases is accompanied by a decrease in the electrical conductivity by several orders of magnitude. The electrical conductivity of Li1.9CrxTi3.025 − 0.75xO7 (0 ≤ x ≤ 0.8) solid solutions remains unchanged upon multiple dynamic heating to 1000°C (with subsequent cooling).