A3Y2Ge3O12 (A = Ca, Mg): Two novel microwave dielectric ceramics with contrasting τf and Q x f

( )Two garnet-structured microwave dielectric ceramics of A(3)Y(2)Ge(3)O(12) (A = Ca, Mg; called CYG and MYG, respectively) were synthesized. CYG was crystallized in a normal cubic garnet structure with Ca-2(+) fully occupying the dodecahedral (A) site, whereas MYG was an inverse garnet with mixed distribution of Mg-2(+)/Y-3(+) at the A site. The difference in ionic occupation resulted in anomalies in microwave dielectric properties, with dielectric constant (epsilon(r)) = 14.1, quality factor (Q x f) = 12,600 GHz and positive temperature coefficient of resonant frequency (tau(f)) =120.5 ppm/degrees C for MYG ceramic and epsilon(r) = 10.8, Q x f = 97,126 GHz and tau(f) = - 40.6 ppm/degrees C for CYG ceramic. The large deviations in measured epsilon(r) from theoretical epsilon(th), possibly resulted from the garnet structural constraints leading to 'rattling' Mg-2(+) and Y-3(+) in A site of MYG. Infrared reflectivity spectra analysis revealed ion polarization contributed mostly to the permittivity of both ceramics in microwave frequency ranges.