Anomalous microwave dielectric behaviour induced by the orthorhombic-tetragonal phase transition in CaLaGaO4 ceramics

Abstract

In this work, orthorhombic (o-) and tetragonal (t-) phase CaLaGaO4 ceramics were synthesized using a conventional solid-state method. Phase evolution and crystal structure were investigated via XRD and TEM, and o-CaLaGaO4 phase with Pna21 space group can be obtained at a low temperature (<1260 °C). With the increase in sintering temperature, o-CaLaGaO4 and t-CaLaGaO4 phases (I4/mmm) coexisted when sintered in the range of 1280 °C – 1300 °C. The microwave dielectric properties of the o-CaLaGaO4 ceramic sintered at 1240 °C were εr = 11.2, Q × f = 63,900 GHz, and τf = −56.6 ppm/℃. By contrast, the phase transformation in the CaLaGaO4 ceramic led to a significantly increased εr (19.5) in t-CaLaGaO4 and a near-zero τf (+2.3 ppm/℃). The theoretical permittivity (εth) was calculated on the basis of the Clausius–Mossotti equation and P-V-L theory. Results indicate that in the low-symmetry-structure o-CaLaGaO4 ceramics, εth can be estimated accurately by the latter.