Structural Dependence of Microwave Dielectric Properties of SrRAlO4 (R = Sm, Nd, La) Ceramics: Crystal Structure Refinement and Infrared Reflectivity Study

The crystal structure refinement and infrared reflectivity study were carried out for SrRAlO4 (R = Sm, Nd, La) ceramics with K2NiF4 structure to investigate the correlations between the crystal structure, polar-phonon mode parameters, and microwave dielectric properties. Fourier transform infrared reflectivity spectra in the range of 50−4000 cm−1 were measured and evaluated by means of classical oscillator fit. The data were extrapolated below the measured frequency range to estimate the intrinsic microwave losses. Heavy distortions of (Sr,R)O9 dodecahedra and AlO6 octahedra are observed in the crystal structure and they should attribute to interlayer electric polarization and interlayer size mismatch. The two lowest-frequency polar-phonon modes, which give primary contributions to the microwave complex permittivity, are the bending and stretching vibrations of (Sr,R)-AlO6, so that the distortion of (Sr,R)O9 dodecahedra has a great impact on the dispersion parameters of these two modes and consequently affects polarizabilities and intrinsic dielectric losses of SrRAlO4 greatly. The above findings provide the general guideline for modifying the intrinsic microwave dielectric properties for SrRAlO4 ceramics. Moreover, the calculated Q × f values are 25 000−85 000 GHz higher than the measured ones, and this suggests the great opportunity to improve Q × f through optimizing the microstructures.