Anisotropic Polarization Switching in Tungsten Bronze Ferroelectrics in Binary (Bi1/2Na1/2)Nb2O6--BaNb2O6 System

• Published in: Jpn J Appl Phys 2010-09, Vol.49 (9), p.09MD12-09MD12-4
• Main Authors: Watanabe, Takayuki, Hayashi, Jumpei, Miura, Kaoru
• Format: Article
• Language: eng ; jpn
• ISSN: 0021-4922; 1347-4065
• DOI: 10.1143/JJAP.49.09MD12

Randomly oriented ($1-x$)BaNb 2 O 6 --$x$(Bi 1/2 Na 1/2 )Nb 2 O 6 ceramics were prepared by conventional solid state sintering. The ceramics ($0.2\leq x\leq 0.7$) sintered at 1250 \mbox{ \circ C} consisted of a single phase of a tungsten bronze structure, while the end-member materials did not crystallize into the tungsten bronze structure. The crystal systems of tungsten bronze phases were orthorhombic for $0.2\leq x\leq 0.25$ and tetragonal for $0.3\leq x\leq 0.7$. The temperatures for the maximum dielectric constant ($T_{\text{m}}$) measured for orthorhombic crystals were 300 and 190 \mbox{ \circ C} for $x=0.2$ and 0.25, respectively, and showed no frequency dispersion. However, the $T_{\text{m}}$ for tetragonal crystals was suppressed below 160 \mbox{ \circ C} and showed an obvious frequency dispersion. To evaluate the orientation of spontaneous polarization, (001)- and ($hk0$)-oriented fiber-textured ceramics were prepared for $x=0.2$, 0.25, and 0.3 by slip casting under a strong magnetic field. Clear hysteresis loops of polarization--electric field were observed for (001)-oriented orthorhombic ceramics ($x=0.2$ and 0.25) at room temperature, while the polarization induced in the ($hk0$)-oriented or tetragonal ceramics ($x=0.3$) varied proportionally to the external electric field. It was concluded that the tungsten bronze crystals, ($1-x$)BaNb 2 O 6 --$x$(Bi 1/2 Na 1/2 )Nb 2 O 6 ($x=0.2$ and 0.25), are ferroelectric at room temperature with the spontaneous polarization parallel to the $c$-axis.