Phase equilibria and microwave dielectric properties in the ternary La2O3–TiO2–Nb2O5 system

High-temperature phase relations in the La₂O₃–TiO₂–Nb₂O₅ ternary system were investigated at 1290 °C using SEM, EDS and XRD, and a subsolidus phase equilibrium diagram was constructed. It was demonstrated that the addition of the La_1/3NbO₃ to the La₂O₃:3TiO₂ mixture completely stabilizes a perovskite-La_2/3TiO₃ compound, which is otherwise not stable at stoichiometric composition. On the other hand, a mixture La₂O₃:TiO₂ =1:3 dissolves in La_1/3NbO₃ up to 15 mol% TiO₂. Moreover, a newly defined composition of compound La₃₀Ti₁₆Nb₄O₈₇ formed in the La₂O₃-rich part of the system and a solid solution La₃Ti₅Nb₁₀O_39.5–La₂Ti₂Nb₈O₂₇ were proposed. In the binary subsystem LaTiNbO₆-La_0.462Ti_0.386Nb_0.614O₃, the components exhibit opposite temperature dependence of resonant frequency in the microwave frequency range. Thus, the dielectric properties of the prepared ceramics can be tailored by varying the component concentration and thermally stable dielectric properties can be achieved for a composition with a LaTiNbO₆/La_0.462Ti_0.386Nb_0.614O₃ molar ratio of 0.27.