Raman spectroscopy and microwave properties of CaTiO3-based ceramics

xCaTiO(3)-(1-x)Sr(Mg1/3Nb2/3)O-3 (CTSMN) and yCaTiO(3)-(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 (3) over barc space group, consistent with an a(-)a(-)a(-) tilt configuration. Sr(Mg1/3Nb2/3)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 xgreater than or equal to0.2, 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 xgreater than or equal to0.2 and the width of the 825 cm(-1) band became broader as x increased. It was concluded that for samples with xgreater than or equal to0.2, only short-range ordering remained which decreased in correlation length as x increased. In CTNA solid solutions, a broad Raman band occurred at similar to800 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 similar to825 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 (xapproximate to0.2) is postulated. (C) 2003 American Institute of Physics.