Prediction of the dielectric properties of non-ferroelectric complex perovskites and of the ternary system Ba/SrO-Ln₂O₃-xTiO₂ for microwave applications

Abstract

In the first part of this work an attempt is presented to predict the dielectric properties of solid solutions of non-ferroelectric complex perovskites by means of structural predictions based on the tolerance factor and a microscopic model, an estimation of the optical permittivity based on the Gladstone-Dale relation, and an estimation of total polarisability accordingly to the molecular/ion additivity rule. The objective is to produce materials with increased relative permittivity ε₀ and very low thermal coefficient of the resonant frequency τ_f. The new compositions with predicted dielectric properties are then fabricated in ceramic form and structurally, electrically and optically characterised to verify the model. Results show good agreement with the structural features and optical permittivities. However, the calculated total polarisability for relative permittivities larger than 20 seems to be inaccurate and need further development.

In the second part, the study of the dielectric and structural properties of ceramics based on the ternary system Ba/SrO-Ln₂O₃-xTiO₂ (Ln= Nd, Sm, Gd) with 4 < x < 5 is presented. A preliminary attempt to relate the dielectric behaviour with the structural properties and the chemical and phase composition was made in the case of Ln= Nd, Sm. Special attention was paid to the thermal coefficient of the permittivity τ_ε, because it shows positive values for Ba-Sm-based compounds and negative values for the Ba-Nd-composition and for Ba/Sr-Sm-solid solutions.