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

A study of K x Ba 1− x Ga 2− x Ge 2+ x O 8 ceramics revealed that these solid solutions undergo a monoclinic-to-monoclinic P 2 1 / a ⇔ C 2/ 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 P 2 1 / a ⇔ C 2/ m phase transition is below the sintering temperatures of K x Ba 1− x Ga 2− x Ge 2+ x O 8 (0.67 ≤ x ≤ 1) solid solutions, whereas the compositions at lower x ( x = 0.4 and 0) remain in the P 2 1 / a modification over a wide temperature range above the sintering temperature. Compared to the C 2/ m modification of K x Ba 1− x Ga 2− x Ge 2+ x O 8 (0.67 ≤ x ≤ 1), with a permittivity of 6.2–6.9, the P 2 1 / 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.