The influence of tetrahedral ordering on the microwave dielectric properties of Sr0.05Ba0.95Al2Si2O8 and (M = Al, Ga, M′ = Si, Ge) ceramics

The feldspars Sr0.05Ba0.95Al2Si2O8, BaAl2Ge2O8 and BaGa2Si2O8 with S.G. I2/c, and BaGa2Ge2O8 with S.G. P21/a, were studied by means of crystal structural and microstructural analyses and dielectric measurements. All the investigated densely sintered single-phase feldspars exhibited a permittivity (ɛ) of 7–8 and a temperature coefficient of resonant frequency (τf) from −20 to −30 ppm/°C. In contrast to the ɛ and τf the dielectric losses were found to be dependent on the annealing conditions. In Sr0.05Ba0.95Al2Si2O8 the Qxf values increased from 42,500 to 92,600 GHz when the annealing time at 1400 °C was increased from 1 to 162 h. Such a difference in the Qxf values as a result of various annealing conditions was attributed to different degrees of tetrahedral ordering. In contrast to aluminosilicate feldspars, Ge-containing feldspars can be sintered and ordered at low temperature. In BaAl2Ge2O8 the Qxf values decreased when the sintering temperature exceeded the order-disorder I2/c ↔ C2/m phase-transition temperature. The BaGa2Si2O8 and BaGa2Ge2O8 feldspars exhibited a rapid decrease of Qxf values when the annealing temperature approached the melting point. However, the BaAl2Ge2O8 and BaGa2Ge2O8 can regain their high Qxf values by annealing at 1000 °C. The BaGa2Ge2O8 stood out from the other investigated feldspars, with a sintering temperature of 1100 °C, Qxf values of 100,000–150,000 GHz and a τf of −26 ppm/°C.