Structural evolution and dielectric response of (1-x)BiFeO3-xSrTiO3 ceramics
Introduction
Bismuth ferrite, BiFeO3 (BFO) is a model multiferroic material with very high Curie temperature (825 °C) and large polarization [1], [2]. However, it is hard to synthesize and easy to decompose [3], and with small dielectric permittivity [4]. SrTiO3 (STO) is the so-called quantum paraelectrics with both structural and polar instabilities [5], [6]. It is an important material for dielectric capacitors [7]. At the same time, it is easy to prepare by a simple solid reaction method. Therefore, BFO-STO solid solution has obtained great attention recently as novel multiferroic materials [8], [9], [10], [11], [12], [13], [14], [15], [16].
Some compositions of the solid solution show fantastic electric properties. In ceramics, the spontaneous polarization of 0.3STO is 50 μC/cm2[17], which is higher than that of BFO [18]. In thin films, 0.6STO has a very high electrical energy storage density, e. g., 18.6 J/cm3[9], or 51 J/cm3[19]. These results suggest that the system is very promising for dielectric capacitors, which play a key role in power electronics [20], [21]. However, the dielectric response of the whole solid solution has not been well studied. From a structural view, BFO is a rhombohedral perovskite with oxygen octahedral tilting. STO is a cubic perovskite with strong structural instability [22]. The structural evolution of the solid solution, and its relation to the dielectric response, also have not been explored.
In our previous work, (1-x)BiFeO3-xSrTiO3 (abbreviated as xSTO) up to x = 0.35 were obtained by a solid reaction method without any parasitic phase [8]. Recently, xSTO in the complete solid solution range have been successfully prepared by the same method. We studied the structural evolution, the dielectric response, and the correlations between them. These results are introduced and discussed in this paper.
Section snippets
Experiment
The solid solution was prepared by a conventional solid-state reaction method as described in our previous work [8]. The crystal structures of the powders were investigated by an X-ray diffractometer in the Bragg-Brentano geometry. The dielectric properties of the ceramics with silver electrodes were measured by an impedance analyzer (Keysight E4980A).
Results and discussion
By optimizing processing conditions, the complete solid solution was obtained without any parasitic phase as shown in Fig. 1. Since pure BFO is hard to be prepared by a solid reaction method, the results indicate adding STO can aid to synthesize pure phase. Obtaining pure phase for 0.05STO is remarkable. Because of a small content of doping, multiferroic properties similar to that of BFO can reserve or even enhance in the composition.
Both BFO and STO belong to perovskite structure. BFO is
Conclusions
The solid solution of (1-x)BFO-xSTO has rich structural and dielectric properties. The structure changes from a rhombohedral to a p. c. at x = 0.2–0.4, then to a cubic lattice at x = 0.8. In the p. c. phase, several chemical compositions, e. g., 0.4STO-0.8STO, have enhanced dielectric permittivity, small dielectric loss, and flat temperature coefficients of both dielectric permittivity and loss, making them very promising in electrical energy storage applications at high temperature.
Acknowledgments
The work was supported by National Natural Science Foundation of China (Grant No. 11704242 and 51672226) and Natural Science Foundation of Shanghai, China (Grant No. 17ZR1447200).
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