Infrared reflectivity and intrinsic dielectric behavior of RETiTaO 6 (RE = Y, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, an
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oreiraa) Departamento de Física–Instituto de Ciências Exatas-Universidade Federal de Minas Gerais, Belo Horizonte (Minas Gerais), 30161-970 Brazil
K.P. Surendran and M.T. Sebastian Ceramics Technology Division, Regional Research Laboratory, Trivandrum, 695 019 India (Received 22 September 2004; accepted 12 January 2005)
In this work, we performed infrared reflectivity measurements of RETiTaO6 dielectric ceramics for rare-earth (RE) ⳱ Y, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Yb. The infrared spectra were analyzed through the four-parameter semi-quantum model based on two orthorhombic (aeschynite and euxenite) structures presented by the ceramics. We discerned the strongest phonon modes that contribute to the intrinsic dielectric behavior of the two families. Then the unloaded quality factors and the dielectric permittivities were estimated at the microwave region. The variations of the intrinsic dielectric response of both systems with RE ion substitution are discussed in terms of changes in their dielectric polarizabilities. It is proposed that the covalency of the RE–O bonds play an important role in the evolution of dielectric permittivity as well as in the sign of the temperature coefficient of the resonant frequency.
I. INTRODUCTION
Advances in materials and processing technologies have strongly contributed to the rapid evolution of wireless telecommunication and satellite broadcasting; once optimized, materials are used in miniaturized circuitry and devices. In particular, for dielectric resonators (DRs) and filters acting in microwave region, miniaturization is promoted by the development of devices and components made of ceramics with high dielectric permittivity (⑀⬘), since the resonator size is inversely proportional to √⑀⬘. Besides, these materials should satisfy two other requirements: they must present high unloaded quality factors (Qu), propitiating selectivity, and low temperature coefficient of the resonant frequency (f), which provides good thermal stability. Different classes of materials are being developed and used as DRs in microwave circuitry. Among them, it is worth citing BaTi4O9, the first used microwave ceramic,1 followed by Ba2Ti9O20 and its solid solutions.2 Later on, several other materials became widely used such as the (Zr,Sn)TiO4,3 BaO–TiO2–Nb2O5/Ta2O5,4 and BaO– TiO2–RE2O5 systems (RE ⳱ La, Nd, Sm and Pr).5 On a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0163 1164
J. Mater. Res., Vol. 20, No. 5, May 2005
the other hand, microwave filters are commonly made of complex perovskites of the type A(B⬘1/3B⬙2/3)O3 (Refs. 6–12) and A(B⬘1/2B⬙1/2)O3 (Refs. 13–16), once these materials show very high quality factors. Recently, several materials of the RETi(Ta,Nb)O6 system, such as SmTiNbO6, EuTiNbO6, ErTiTaO6, EuTiTaO6, and YTiTaO6 showed rather adequate microwave parameters, so they can be considered good candidates for applications as DRs.17–19 An interesting feature of these compounds is that they present either an aeschynite o
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