A microscopic model for the temperature coefficient of the resonant frequency (τ_f) in complex perovskites used for microwave filter

Abstract

A microscopic interpretation of the behaviour of the thermal coefficient of the relative permittivity as a function of temperature has been developed. This behaviour has been found to be qualitatively the same for all the studied Ba- and Sr-based complex perovskites. The tilting of the O-octahedra seems to play the most important role because it allows a steady thermal expansion with little modification of the volume of the octahedra and allows the “virtual tolerance factor” (a value of tolerance factor which takes into account both the temperature and structure of the compound) to be continuously adjusted to a value close to 1. The model describes the temperature behaviour of the electronic polarizability and the ionic polarizabilities at the sites A and B, which compose the total polarizability.