Structure property relations in La(Mg1/2Ti1/2)O3-based solid solutions

Ceramics based on (1-x)La(Mg1/2Ti1/2)O3 (LMT)-xATiO3 solid solutions, where A=Ca, Sr, and Ba, have been investigated to study the effect of average A-site ionic radius on structure and microwave dielectric properties. X-ray diffraction (XRD), transmission electron microscopy (TEM), and Raman spectroscopy were used to study the structural changes and planar defects and thereby interpret changes in microwave properties. 1:1 rocksalt ordering between Mg2+ and Ti4+ in LMT was destroyed for all samples in which x>0.1. XRD and TEM revealed that all (1-x)LMT-xCaTiO3 (LMTCT), x≤0.9, were distorted through a combination of antiphase and in-phase octahedral rotations (tilted). For (1-x)LMT-xSrTiO3 (LMTST) this combination is present until x=0.5, and for x>0.5 only antiphase tilting exists. For (1-x)LMT-xBaTiO3 (LMTBT), in-phase and antiphase octahedral distortions were only present for x≤0.3, x=0.5 was tilted in antiphase and x≥0.7 was untilted. The temperature coefficient of resonant frequency (τf) varied linearly with relative permittivity (εr) for LMTCT and LMTST but a discontinuity was observed for LMTBT at x≅0.7, the composition at which octahedral rotations are no longer present at room temperature. Zero τf was achieved at x≅0.5 in all solid solution series. In general, the microwave dielectric quality factor (Qfo) decreased with increasing x to a minimum at x=0.5 for all solid solution series followed by an increase at x=0.7 despite εr being higher for x=0.7 than for 0.5. Qfo then continued to decrease as x=1 was approached and εr increased