Raman Spectroscopy and Microwave Properties of CaTiO₃-Based Ceramics

H. Zheng, University of Sheffield,
H. Bagshaw, University of Sheffield,
G. D.C. Csete de Györgyfalva, University of Sheffield,
I. M. Reaney, University of Sheffield,
Rick Ubic, Boise State University
J. Yarwood, Sheffield Hallam University

Abstract

xCaTiO₃–(1–x)Sr(Mg_1/3Nb_2/3)O₃ (CTSMN) and yCaTiO₃–(1–y)NdAlO₃ (CTNA) microwave ceramics have been studied by x-ray diffraction (XRD), transmission electron microscopy (TEM), and Raman spectroscopy. TEM and XRD revealed that all compositions underwent octahedral tilt transitions on cooling, generally resulting in an a^–a^–c⁺ tilt system. The exception was the NdAlO₃ end member, which had the R¯3c space group, consistent with an a^–a^–a^– tilt configuration. Sr(Mg_1/3Nb_2/3)O₃ (SMN), (x = 0) also exhibited +/–1/3{hkl} reflections in x-ray and electron diffraction patterns associated with 1:2 long-range ordering of the B-site cations. For x≥ 0.2, no 1:2 ordered reflections were observed. The long-range B-site ordering in SMN gave rise to sharp Raman bands at 391 and 825 cm^–1. The 391 cm^–1 band disappeared for x≥ 0.2 and the width of the 825 cm^–1 band became broader as x increased. It was concluded that for samples with x≥ 0.2, only short-range ordering remained which decreased in correlation length as x increased. In CTNA solid solutions, a broad Raman band occurred at ~800 cm^–1 (absent for y = 0 and 1 and strongest for y = 0.5). The position of this band suggested that its origin was similar in nature to the ~825 cm^–1 band observed in CTSMN and therefore related to local short-range cation ordering. A relation between the presence of strong local order and poor Q in zero-temperature coefficient of the resonant frequency CTSMN (x ≈ 0.2) is postulated.