Phase transitions and microwave dielectric properties in the perovskite-like Ca(Al0.5Nb0.5)O-3-CaTiO3 system

Phase transitions and microwave dielectric properties in the (1-x)Ca(Al0.5N b0.5)O-3-xCaTiO(3) system were analyzed using x-ray and neutron powder diff raction, transmission electron microscopy, Raman spectroscopy, and dielectr ic measurements at microwave frequencies (2-8 GHz). Rietveld structural ref inements demonstrated that both end compounds exhibit similar octahedral ti lted frameworks, while in Ca(Al0.5Nb0.5)O-3, tilting is superimposed onto N aCl-type ordering of Al and Nb on the B sites. Accordingly, the room-temper ature structures of CaTiO3 and Ca(Al0.5Nb0.5)O-3 are described by orthorhom bic Pbnm and monoclinic P2(1)/n symmetries, respectively, with similar latt ice parameters, root 2a(c)x root 2a(c)x2a(c) (where a(c) is the lattice par ameter of cubic perovskite). The (1-x)Ca(Al0.5Nb0.5)O-3-xCaTiO(3) system fe atures both cation ordering and octahedral tilting phase transitions. The C a(Al0.5Nb0.5)O-3 structure remains ordered at least up to 1625 degreesC. Ho wever, the temperature of the order/disorder transition decreases rapidly w ith 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 permittivit y epsilon and the temperature coefficient of the resonant frequency tau (f) on Ti content corresponds to a linear dependence of the macroscopic polari zability on composition; that is, the oxide additivity rule was closely obe yed. Therefore, this rule can be used to predict epsilon and tau (f) for an y intermediate composition from the permittivities and temperature coeffici ents of permittivity of the end compounds. A zero temperature coefficient o f 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.