| Electric Field Induced Antiferroelectric - Ferroelectric Phase Transition on NaNbO3-Based Lead-Free Ceramics |
| Hiroyuki SHIMIZU |
| Taiyo Yuden Co., Ltd. |
|
Keywords: NaNbO3, antiferroelectric, tolerance factor, electric field induced phase transition
Sodium niobate NaNbO3 (NN) has been known structurally to be antiferroelectric (AFE) phase with Pbma at room temperature.[1] However, a number of groups have already reported an electric field induced metastable ferroelectric (FE) phase with P21ma,[2] and the observed electrical properties can be attributed to a metastable FE phase. As for polarization-electric field (P-E) characteristics, lead-containing AFE ceramics such as PbZrO3 and La doped PZT95/5 (PLZT) feature double P-E loops, marking a reversible AFE¡êFE phase transition.[3-5] In contrast, square P-E loops, typical characteristic of ferroelectrics, are usually observed in polycrystalline NN ceramics at room temperature once large polarizations are developed. In this work, we stabilized the AFE phase in NN ceramics in terms of designing tolerance factor and electronegativity in the perovskite structure, and thereby demonstrated clear double P-E loops under ac field and small capacitance change with dc field. Tolerance factor can be generally categorized into two regions of AFE and FE distortive, as shown in Fig. 1. Considering polarizability in systems that have a tolerance factor decrease, AFE cations such as Zr4+ and Hf4+ can be expected enough for lowering tolerance factor. In that case, divalent cations such as Sr2+ and Ca2+ should be needed for charge neutrality. These combinations between 2+ and 4+ ions chemically derive stabilized antiferroelectricity according to increased average-electronegativity-difference in the perovskite structure. From these expectations, (Na1-xCax)(Nb1-xZrx)O3 (x=0.00, 0.02, 0.04, 0.05, 0.06 and 0.10, abbreviated as CZNNx) ceramics were prepared by conventional solid state reaction. Figure 2 shows close view of XRD spectra around AFE {1 1 3/4} superlattice peak in the sintered ceramics. The normalized intensity obviously increased comparing to CZNN0.00 ceramic although solid state limitation seems to be around x=0.06. This indicates that antiferroelectricity in NN is enhanced as expected. Figure 3 shows the AFE¡êFE phase transition behavior at high temperature. The CZNN0.00 ceramic showed a square P-E loop resulting from induced metastable FE phase as mentioned above, on the other hand, the CZNN0.02, CZNN0.04 and CZNN0.05 ceramics showed clear double P-E loops. Remarkably, the induced polarization decreased and switching field (EAFE to FE) increased with amount of Ca and Zr. Reflecting this behavior, the CZNN0.05 ceramic showed much smaller capacitance change and loss under dc field below the switching field at high temperature. This work will provide new opportunities for lead-free AFE application like high temperature-voltage capacitors as well as PLZT of lead-containing AFE ceramics.
References:
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[2] V. A. Shuvaeva, M. Yu. Antipin, R. S. V. Lindeman, O. E. Fesenko, V. G. Smotrakov, and Yu. T. Struchkov, Ferroelect., 141 [1], 307–31 (1993).
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[5] X. Tan, C. Ma, J. Frederick, S. Beckman, and K. G. Webber, J. Am. Ceram. Soc., 94, 4091 (2011). |
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