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 y CaTiO ₃ –(1– y )NdAlO ₃ (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 NdAlO ₃ end member, which had the R ¯3 c space group, consistent with an a ^– a ^– a ^– tilt configuration. Sr(Mg _1/3 Nb _2/3 )O ₃ (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.