Effect of A-site cation on crystal structure and microwave dielectric properties of AGe4O9 (A = Ba, Sr) ceramics

Abstract

AGe4O9 (A = Ba, Sr) ceramics with hexagonal structure were fabricated by solid-phase reaction. BaGe4O9 sintered at 1060 °C had low εr ∼9.8, high Q × f ∼78,400 GHz and τf ∼ -44.2 ppm/°C, whilst SrGe4O9 sintered at 1100 °C exhibited relatively high εr ∼11.2, low Q × f ∼62,500 GHz and near-zero τf ∼ -11.7 ppm/°C. The τf of SrGe4O9 was nearer to zero than that of BaGe4O9 due to the smaller τε value and the smaller distortion of the [GeO6] octahedrons. The ‘rattling’ cations at A-site contribute to comparatively high εr, low Q × f and near-zero τf values in Sr1-xBaxGe4O9 ceramics. This work presents a new idea of adjusting the microwave dielectric performance (MDP) of ceramics by changing the strength of the rattling effect at A-site.