Phase transitions and microwave dielectric properties in the perovskite-like Ca(Al0.5Nb0.5)O3-CaTiO3 system

Phase transitions and microwave dielectric properties in the (1-x)Ca(Al(0.5)Nb(0.5))O(3)-xCaTiO(3) system were analyzed using x-ray and neutron powder diffraction, transmission electron microscopy, Raman spectroscopy, and dielectric measurements at microwave frequencies (2-8 GHz). Rietveld structural refinements demonstrated that both end compounds exhibit similar octahedral tilted frameworks, while in Ca(Al(0.5)Nb(0.5))O(3), tilting is superimposed onto NaCl-type ordering of Al and Nb on the B sites. Accordingly, the room-temperature structures of CaTiO(3) and Ca(Al(0.5)Nb(0.5))O(3) are described by orthorhombic Pbnm and monoclinic P2(1)/n symmetries, respectively, with similar lattice parameters, root 2a(c)x root 2a(c)x2a(c) (where a(c) is the lattice parameter of cubic perovskite). The (1-x)Ca(Al(0.5)Nb(0.5))O(3)-xCaTiO(3) system features both cation ordering and octahedral tilting phase transitions. The Ca(Al(0.5)Nb(0.5))O(3) structure remains ordered at least up to 1625 degreesC. However, the temperature of the order/disorder transition decreases rapidly with increasing Ti content, which correlates with a progressive increase of cation disorder in the specimens. A disordered structure is attained at x=0.5. For the "solid solutions," the nonlinear dependence of both permittivity epsilon and the temperature coefficient of the resonant frequency tau (f) on Ti content corresponds to a linear dependence of the macroscopic polarizability on composition; that is, the oxide additivity rule was closely obeyed. Therefore, this rule can be used to predict epsilon and tau (f) for any intermediate composition from the permittivities and temperature coefficients of permittivity of the end compounds. A zero temperature coefficient of the resonant frequency occurs at the composition x approximate to0.5 with a relative permittivity of 50 and a Qf value of approximately 30 000 GHz (@4 GHz). (C) 2001 American Institute of Physics.