The correlation between the structure and the dielectric properties of KxBa1−xGa2−xGe2+xO8 ceramics

A study of KxBa1−xGa2−xGe2+xO8 ceramics revealed that these solid solutions undergo a monoclinic-to-monoclinic P21/a ⇔ C2/m phase transition. The temperature of this phase transition decreases with an increase in x, in a similar way to the sintering temperature, which decreases from 1100 °C (x = 0) to 970 °C (x = 1). The temperature of the P21/a ⇔ C2/m phase transition is below the sintering temperatures of KxBa1−xGa2−xGe2+xO8 (0.67 ≤ x ≤ 1) solid solutions, whereas the compositions at lower x (x = 0.4 and 0) remain in the P21/a modification over a wide temperature range above the sintering temperature. Compared to the C2/m modification of KxBa1−xGa2−xGe2+xO8 (0.67 ≤ x ≤ 1), with a permittivity of 6.2–6.9, the P21/a modifications exhibit permittivities of 5.9–7.0 and three-to-eight times higher Q × f values of ∼100,000 GHz (at ∼12 GHz). The temperature coefficient of the resonant frequency is ∼−25 ppm/K, regardless of the composition.