Raman spectroscopy of CaTiO 3 -based perovskite solid solutions
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R. Ubic Department of Materials, Queen Mary, University of London, London E1 4NS, United Kingdom
J. Yarwood Materials Research Institute, Sheffield Hallam University, Sheffield S1 1WB, United Kingdom
M.P. Seabra and V.M. Ferreira Department of Ceramics and Glass Engineering, University of Aveiro, Aveiro, Portugal (Received 29 May 2003; accepted 13 October 2003)
Perovskite-structured solid solutions intended for use as microwave dielectric resonators were studied by Raman spectroscopy. Two distinct categories were investigated: (i) simple perovskite–simple perovskite solid solutions, that is, CaTiO3–SrTiO3 (CTST), CaTiO3–CaZrO3 (CTCZ), CaTiO3–NdAlO3 (CTNA), and CaTiO3–LaGaO3 (CTLG); and (ii) simple perovskite–complex perovskite solid solutions, such as CaTiO3–SrMg1/3Nb2/3O3 (CTSMN). In the latter category, the influence of A-site ion radius was also addressed by examining 0.5CaTiO3– 0.5LaMg1/2Ti1/2O3 (0.5CT–0.5LMT), 0.5SrTiO3 (ST)–0.5LMT, and 0.5BaTiO3 (BT)–0.5LMT. Raman data from the end members and solid solutions are compared, paying particular attention to F2g and A1g mode bands, often associated with ordering of B-site species.
I. INTRODUCTION
Dielectric resonator ceramics are now widely used in miniaturized bandpass filters for mobile telephones and in temperature-stabilized local oscillators for directbroadcasting satellite down converters.1 These applications require a combination of high relative permittivity (⑀r), high quality factor Q (low-dielectric loss tan␦, unloaded Qu ⯝ 1/tan␦), and near-zero temperature coefficient of the resonant frequency (TCF). CaTiO3 (CT) exhibits a high ⑀r ⳱ 160, which is however accompanied by a large positive TCF value of 850 ppm/°C.2 In contrast, other perovskites such as NdAlO3 (NA)3 or complex AB⬘xB⬙1−xO3 type perovskites such as Sr(Mg1/3Nb2/3)O3 (SMN)4 have moderate ⑀r of 20 to 35 combined with small negative TCF values. Thus, potentially useful microwave ceramics with temperature-stable relative permittivities of 40 to 50 can be obtained by forming solid solutions between CT and suitable negative TCF perovskites. Complex perovskites with general formula of A(B1/3⬘ B2/3⬙)O3 are classified as 1:2, whereas the A(B1/2⬘B1/2⬙) O3 types are generally known as 1:1 compounds.5 In these complex or solid-solution perovskites, two or more cations are mixed on the octahedrally coordinated B-sites, and the arrangement of B-site ions can be either “disordered” or “ordered.” If the B⬘ and B⬙ ions randomly occupy 488
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J. Mater. Res., Vol. 19, No. 2, Feb 2004 Downloaded: 12 Jan 2015
the B sites, it is termed a disordered structure. On the other hand, ordered structures may form by the alternate stacking of B⬘ and B⬙ ions on {111} planes in either a 1:1 or 1:2 ratio depending on stoichiometry. In general, an ordered structure is favored if there are large size and charge differences between the two B-site ions.6 The microscopic arrangement of B-site cations (order– disorder behavior) has been a subject of considerable interest. X-ray diffraction (XRD) is
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