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 degrees C using SEM, EDS and XRD, and a subsolidus phase equilibrium diagram was constructed. It was demonstrated that the addition of the La1/3NbO3 to the La2O3-TiO2 mixture completely stabilizes a perovskite-La2/3TiO3 compound, which is otherwise not stable at stoichiometric composition. On the other hand, a mixture La2O3:TiO2 =1:3 dissolves in La1/3NbO3 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 La3Ti5Nb10O39.5-La2Ti2Nb5O27 were proposed. In the binary subsystem LaTiNbO6-La0.462Ti0.356Nb0.614O3, 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/La0.462Ti0.356Nb0.614O3 molar ratio of 0.27.