Microstructural control of microwave dielectric properties in CaTiO 3-La (Mg 1/2 Ti 1/2) O 3 ceramics

Abstract

High-density xCaTiO3- (1-x) La (Mg12 Ti12) O3 ceramics (x=0.0-1.0) were prepared by solid-state reaction. The products were characterized by scanning electron microscopy, x-ray diffraction, and transmission electron microscopy. CaTiO3-La (Mg12 Ti12) O3 ceramics formed solid solutions throughout the compositional range with no evidence of a secondary phase. For most ceramics the symmetry was orthorhombic with Pbnm space group. The La (Mg12 Ti12) O3 end member exhibited 1:1 ordering of Mg2+ and Ti4+ cations (in octahedral sites) superimposed on the perovskite distortions leading to monoclinic symmetry with P21n space group. Long-range 1:1 order of Mg2+ and Ti4+ cations was destroyed at x=0.3. The degree of distortion, unit-cell volume, and crystal lattice stress decreased with increasing CaTiO3 content. Twin and antiphase domains, generated by structural phase transitions on cooling, were observed in all the ceramics. Twin domain density increased with CaTiO3 content but the size of individual domains decreased. The twin domains were classified according to the symmetry elements at the twin boundaries as {112} and (110) twin types. The microwave dielectric properties (at 3.5-5.5 GHz) showed a strong dependence on composition. The relative permittivity (εr) and temperature coefficient of resonant frequency (TCf) increased nonlinearly with CaTiO3 content (εr 29-170; TCf -50 to +710 ppm°C). In contrast Qf (GHz) values decreased approximately linearly with CaTiO3 content (48 000-5790). Near zero TCf (1.06 ppm°C) was attained at x=0.55, where εr=44.6 and Qf=32 000 GHz. The increase in relative permittivity and TCf with CaTiO3 content correlated with the reduction in composition-induced lattice stress in association with increasing macroscopic polarizability. © 2005 American Institute of Physics.