Crystal structure, vibrational spectroscopy, and microwave dielectric properties of CaAl4O7 ceramics with low permittivity

Abstract

CaAl4O7 ceramics with low permittivity are prepared via a standard solid-state reaction method and their microwave dielectric properties are revealed for the first time. X-ray diffraction (XRD) patterns and Rietveld refinement results indicate that CaAl4O7 ceramics crystallized in monoclinic crystal symmetry with space group of C2/c. The lattice parameters are calculated as a = 12.89382 A, b = 8.8905 A, c = 5.44898 A , and β = 106.84771°. er increased monotonously with increasing sintering temperature owing to the enhancement of densification, while the variation of τf value was indistinctive. Qf value was improved to the maximum value of 25,500 GHz and then deteriorated slightly. The variation trend of Qf value could be explained from the extrinsic parameters. Moreover, the intrinsic phonon parameters obtained from the infrared reflectivity spectra provided possible routes for further improvement of Qf values. The optimal microwave dielectric properties of CaAl4O7 ceramics were obtained when sintered at 1575 °C (er = 8.45, Qf = 25,500 GHz, τf = − 98 ppm/°C).