X-ray diffraction studyrevealing phase coexistence in barium titanate stannate
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Hans-Peter Abicht Fachbereich Chemie, Martin-Luther-Universität Halle, D-06120 Halle, Germany
Christian Eisenschmidt Fachbereich Physik, Martin-Luther-Universita¨t Halle, D-06120 Halle, Germany (Received 24 March 2004; accepted 14 April 2004)
In this paper, the results of a temperature dependent x-ray diffraction (XRD) study on BaTi0.95Sn0.05O3 (BTS-5) ceramics are compared with dielectric measurements. The orthorhombic-tetragonal phase transition at T2 ⳱ 306 K is found to proceed in a considerably wider temperature range than expected from the dielectric anomaly. Although the macroscopic properties of BTS-5 indicate a rather sharp ferroelectric phase transition at Tc ⳱ 358K, we observe anomalous XRD-patterns in a 25 K wide temperature range. This is interpreted in terms of mechanically clamped tetragonal and cubic phase, coexisting in the vicinity of Tc in grains with inhomogeneous Sn-distribution.
I. INTRODUCTION
Solid solutions BaTi1−xSnxO3 (BTS-x) may be considered as model system for Pb-free perovskites displaying the diffuse ferroelectric phase transition.1,2 Due to their interesting dielectric, pyroelectric, and electromechanical properties,3 these compositions may be of interest as environmentally friendly materials for various technical applications. The isovalent substitution of Ti4+ by Sn4+ ions in barium titanate (BT) leads to a reduced stability range of the tetragonal phase, both due to a decrease of the ferroelectric cubic-tetragonal phase transition temperature Tc, and due to the increasing temperature T2 of the tetragonal–orthorhombic phase transition. The stability limit of the tetragonal phase is reached at x ⳱ 0.1, above which the paraelectric cubic phase transforms directly into the rhombohedral phase. In addition, Snsubstitution gradually shifts the first order character of the phase transition in BT towards a second order transition. In BTS-5, the transition is close to second order, as shown by the very small anomaly of the latent heat.4 The character of the ferroelectric phase transition in BTS strongly depends on the Sn-level x. Differential thermo-Raman data indicate a rather sharp transition for x 艋 0.05.5 The transition becomes increasingly diffuse a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0386 2834
http://journals.cambridge.org
J. Mater. Res., Vol. 19, No. 10, Oct 2004 Downloaded: 17 Aug 2014
if x is further increased, as confirmed by dielectric measurements revealing significant deviations from the Curie–Weiss law.4,5 The observation of ferroelectric domains below the permittivity maximum temperature provided evidence of ferroelectric long-range order for x 艋 0.13,6 whereas indication for the relaxor state of BTS was obtained for compositions x 艌 0.2.4,7 The composition-controlled crossover from a sharp ferroelectric phase transition into a relaxor ferroelectric state in BTS is of high scientific interest. Ferroelectric relaxors have initially been considered as inhomogeneous ferroelectrics with distributed transitio
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