Chemical Substitution in Ba(RE_1/2Nb_1/2)O₃ (RE = La, Nd, Sm, Gd, Tb, and Y) Microwave Ceramics and Its Influence on the Crystal Structure and Phonon Modes

Abstract

Raman spectroscopy was employed to evaluate the crystal structure and phonon modes of chemically substituted Ba(RE1/2Nb1/2)O3 microwave ceramics (RE = La, Nd, Sm, Gd, Tb, and Y). It was verified that these materials could be divided into tetragonal (ceramics with RE = Y, Tb, and Gd) and orthorhombic (RE = Sm, Nd, and La) structures. Lorentzian lines were used to fit the spectra, which presented 9 bands for the first group and 23 bands for the second group of ceramics. The position and width of the phonon modes were determined, and were correlated to the ionic radii and tolerance factors for the different atoms substituted in the B‘-site. It is believed that simple rotational distortions of the oxygen octahedra led to the occurrence of structures other than cubic, which is very difficult to detect by X-ray diffraction or even spectroscopic techniques.