Anisotropic polarization switching in tungsten bronze ferroelectrics in binary (Bi_1/2Na_1/2)Nb₂O₆-BaNb ₂O₆ system

Randomly oriented (1-x)BaNb₂O₆-x(Bi ^1/2Na_1/2)Nb₂O₆ ceramics were prepared by conventional solid state sintering. The ceramics (0.2 ≤ x ≤ 0.7) sintered at 1250°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 ≤ x ≤ 0.25 and tetragonal for 0.3 ≤ x ≤ 0.7. The temperatures for the maximum dielectric constant (T_m) measured for orthorhombic crystals were 300 and 190°C for x = 0.2 and 0.25, respectively, and showed no frequency dispersion. However, the T_m for tetragonal crystals was suppressed below 160°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)BaNb₂O₆-x(Bi _1/2Na_1/2)Nb₂O₆ (x = 0.2 and 0.25), are ferroelectric at room temperature with the spontaneous polarization parallel to the c-axis.