A3Y2Ge3O12 (A = Ca, Mg): Two Novel Microwave Dielectric Ceramics with Contrasting τf and Q×f

Abstract Two garnet-structured microwave dielectric ceramics of A3Y2Ge3O12 (A = Ca, Mg; called CYG and MYG, respectively) were synthesized. CYG was crystallized in a normal cubic garnet structure with Ca2+ fully occupying the dodecahedral (A) site, whereas MYG was an inverse garnet with mixed distribution of Mg2+/Y3+ at the A site. The difference in ionic occupation resulted in anomalies in microwave dielectric properties, with dielectric constant (er) = 14.1, quality factor (Q × f) = 12,600 GHz and positive temperature coefficient of resonant frequency (τf) =120.5 ppm/°C for MYG ceramic and er = 10.8, Q × f = 97,126 GHz and τf = −40.6 ppm/°C for CYG ceramic. The large deviations in measured er from theoretical eth possibly resulted from the garnet structural constraints leading to ‘rattling’ Mg2+ and Y3+ in A site of MYG. Infrared reflectivity spectra analysis revealed ion polarization contributed mostly to the permittivity of both ceramics in microwave frequency ranges.