Piezoelectric properties and phase transition temperatures of the solid solution of (1 − x)(Bi0.5Na0.5)TiO3–xSrTiO3
Introduction
(Bi0.5Na0.5)TiO3 (BNT) is one of the most investigated lead-free piezoelectric materials.1, 2 It has a perovskite type structure with rhombohedral symmetry (R3C) at room temperature. The depolarization temperature (Td) is at 186 °C and the Curie temperature (Tm) at 335 °C. Td and Tm both can be shifted by doping3 or by the formation of solid solutions with alkaline earth titanates.3, 4 Bi0.5Na0.5TiO3 shows strong ferroelectricity (Pr = 38 μC/cm2), but it also has drawbacks like a large coercive field of about 7 kV/mm, which leads to problems in the poling process. To overcome these problems and to optimize the properties of Bi0.5Na0.5TiO3 solid solutions with other lead-free materials can be used. In combination with compounds of tetragonal symmetry (e.g. BaTiO3 and (Bi0.5K0.5)TiO3)5, 6, 7, 8, 9, 10, 11 the solid solution system with Bi0,5Na0,5TiO3 exhibits a morphotropic phase boundary (MPB). It is known that morphotropic phase boundaries lead to higher values of d33 and k33. For the solid solution (1 − x)(Bi0.5Na0.5)TiO3–xBaTiO3 the morphotropic phase boundary is found at x = 0.06 with a minimum in Td at about 100 °C.
SrTiO3 has a perovskite type structure and a cubic symmetry at room temperature. Below −168 °C a transition to tetragonal symmetry is reported by Jauch and Palmer.12 Hypothetically there is the possibility to observe a phase transition from a rhombohedral to tetragonal phase in solid solutions with Bi0.5Na0.5TiO3. In all known cases of a morphotropic phase boundary in solid solutions of Bi0.5Na0.5TiO3 and a tetragonal phase the depolarization temperature as well as the Curie temperature exhibit a minimum. Out of this reason it is necessary in the system (1 − x)(Bi0.5Na0.5)TiO3–xSrTiO3 to measure at low temperatures to observe a phase transition from a rhombohedral to a tetragonal phase. A phase diagram (Fig. 1) of (1 − x)(Bi0.5Na0.5)TiO3–xSrTiO3 published by Watanabe et al.4 exists up to x = 0.25. Watanabe used the maxima in the permittivity vs. temperature curves measured at a frequency of 10 kHz. The first maximum at lowest temperature, called the depolarization temperature (Td) is connected to the ferroelectric/antiferroelectric phase transition, the maximum at the highest temperature assigned to the Curie temperature (Tm) is associated with the transition from antiferroelectric to paraelectric. Between these two transition temperatures Watanabe identified a third transition temperature, which he called rhombohedral–tetragonal phase transition temperature (Tr–t). Just recently Hiruma et al.13 verified a very small tetragonal distortion in pure BNT above this Tr–t, which supports Watanabes hypothesis.
Hiruma et al.14 also found a strain maximum in the system (1 − x)(Bi0.5Na0.5)TiO3–xSrTiO3 at x = 0.26 and they suggested that a morphotropic phase boundary is the reason for the large strain. Because of obvious differences to the characteristic features of the system BNT-BT we decided to complete the phase diagram over the whole composition range and look for structural and dielectric features that would support the existence of a morphotropic phase boundary.
Section snippets
Experimental procedure
The samples were prepared by a conventional mixed oxide process. In the first step a Bi0.5Na0.5TiO3 master batch was made from bismuth oxide (reagent grade, HEK-Oxide GmbH), sodium carbonate (reagent grade, Merck) and titanium oxide (reagent grade, Bayer). After the first calcination at 850 °C the Bi0.5Na0.5TiO3 was weighted together with strontium carbonate (reagent grade, Solvay) and titanium oxide in the chosen stoichiometry. After milling in a planetary mill (Fritsch Pulverisette 4) a second
Results and discussion
For identification of structural changes throughout the composition range we carried out low temperature XRD-measurements at −243 °C to identify an expected transition between rhombohedral and tetragonal phase. In our XRD-spectra of (1 − x)(Bi0.5Na0.5)TiO3–xSrTiO3 (Fig. 2) we could not detect any tetragonal distortion, even not for pure SrTiO3. We concluded, that the tetragonal distortion in the system (1 − x)(Bi0.5Na0.5)TiO3–xSrTiO3 is too small to be detected within the resolution of the equipment
Conclusion
The phase diagram for (1 − x)(Bi0.5Na0.5)TiO3–xSrTiO3 was completed between x = 0 and x = 1. No tetragonal distortion could be detected in the whole composition range. The lattice parameter (for cubic indexing) has a broad maximum at x = 0.5 contrary to the trend of ionic radii. Relaxor behavior was observed in all solid solutions (A-site relaxor). An antiferroelectric phase exists from pure Bi0.5Na0.5TiO3 to a concentration of approximately x = 0.7. We support the hypothesis that the reason for the
Acknowledgements
This work was supported by EPCOS OHG and Christian Doppler Research Association.
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