A structural and dielectric characterization of NaxCa1−xAl2−xSi2+xO8 (x = 0 and 1) ceramics

Albite and anorthite, the end-members of the plagioclase feldspar structural family (NaxCa1−xAl2−xSi2+xO8), were synthesized under sub-solidus conditions using the solid-state reaction technique. The structural investigations revealed that both the plagioclase-phase formation and sintering temperature decreased from the anorthite (CaAl2Si2O8; x = 0), which started to form at 1000 °C and sintered at 1300 °C, to the albite (NaAlSi3O8; x = 1), which started to form at 800 °C and sintered at 1000 °C. tric measurements in the microwave (MW) frequency region revealed that a high temperature (1500 °C) and an extended heat-treatment time were needed to obtain low-loss anorthite ceramics (Qxf = 10 000–12 000 GHz). In contrast, the albite attained Qxf = 11 200 GHz at a lower temperature (1025 °C). A temperature coefficient of resonant frequency (τf) close to zero (−5 ppm/°C) was another advantage of the albite over the anorthite, which exhibited τf = −130 ppm/°C. The dielectric measurements also revealed that a slow cooling rate considerably improved the Qxf values of the anorthite and albite.