Raman spectroscopy and microwave properties of CaTiO3-based ceramics

x CaTiO 3 −(1−x) Sr ( Mg 1/3 Nb 2/3 ) O 3 (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 R3̄c space group, consistent with an a−a−a− tilt configuration. Sr(Mg1/3Nb2/3)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.