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

Abstract

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.