Microstructure and microwave properties of {CaT{i}O}₃–LaGaO₃ {solid solutions}

Abstract

The phase assemblage, microstructure and microwave (MW) properties of xCaTiO₃-(1-x)LaGaO₃ (CT-LG) ceramics prepared by solid state synthesis from raw oxides and carbonates have been investigated. LG was predominantly single phase perovskite with space group, Pnma, but a small quantity of pyrochlore structured La₂Zr₂O₇ second phase was present due to the contamination from the ZrO₂ milling media. At MW frequencies, the temperature coefficient of resonant frequency (τ_{f}) of LG was --80~ppm/${\circ}$C, its permittivity, $\varepsilon_{r} {=}$27 and MW quality factor, $Q^{*}f_{o} {=}$97,000 (@5~GHz). As CT concentration increased, $\tau_{f}$ and $\varepsilon_{r}$ increased to $+$850~ppm/$^{\circ}$C and 160, respectively but $Q^{*}f_{o}$ decreased to $\sim$20,000. Zero $\tau_{f}$ was achieved at $x {\approx}$0.65 with $\varepsilon_{r} {\approx}$47, and $Q^{*}f_{o} {\approx}$40,000, properties comparable with commercial compositions. However, for $x {=}$0.5, a different second phase was observed which was rich in Ca and Ga. Electron diffraction patterns could be indexed according to a body centred cubic lattice, $a {\approx}$12.5 Å. It is suggested that the presence of second phases in the CT-LG compounds may, in part, be responsible for the deterioration in $Q^{*}f_{o}$.