Crystal Structure and Infrared Reflection Spectra of SrLn2Al2O7 (Ln = La, Nd, Sm) Microwave Dielectric Ceramics

SrLn(2)Al(2)O(7) (Ln = La, Nd, Sm) ceramics belonging to Ruddlesden-Popper phase with n = 2 in space group I4/mmm have been prepared by a standard solid-state reaction process. Their crystal structure refinement and infrared reflection spectra analysis have been carried out to investigate the correlations among the crystal structure, polar phonon modes, and microwave dielectric characteristics. Heavy deviations of cation-oxygen bonds in (Sr, Ln)O-12, (Sr, Ln)O-9, and AlO6 polyhedra are observed, and the deviations significantly affect the microwave dielectric characteristics of the present ceramics. The polar phonon modes with lower frequencies due to translation vibration of Sr/La ions against the AlO6 octahedron give the primary contribution to dielectric constant and intrinsic dielectric loss at microwave frequencies. The calculated Q x f value is significantly larger than the measured one, and this result is very obvious for SrLn(2)Al(2)O(7) ceramics where the calculated one (130,300 GHz) is about 2 times of the measured one (68,800 GHz).