Ferroelectric and Piezoelectric Properties of KNbO₃ Ceramics Containing Small Amounts of LaFeO₃

Dense KNbO₃ ceramics have been successfully synthesized by pressure-less sintering under optimized heat-treatment conditions using a small amount of La₂O₃ and Fe₂O₃ additives. KNbO₃ forms (K_1-xLa_x)(Nb_1-xFe_x)O₃ solid solutions and changes in the crystal system, depending on the additive content, from orthorhombic to tetragonal at x of 0.020, and from tetragonal to cubic at x of 0.200 or higher. When only 0.002 mol of La₂O₃ and Fe₂O₃ (x=0.002) was added into KNbO₃, the highest value (98.8%) of the theoretical density was obtained. This specimen showed orthorhombic symmetry with a high Curie temperature of 420°C, and demonstrated a well-saturated ferroelectric hysteresis loop with large remanent polarization (P_r) of 18 µC/cm², which is comparable to the value reported for pure KNbO₃ ceramics fabricated by hot pressing. Furthermore, the x=0.002 specimen showed a planar electromechanical coupling ratio (k_p) of 0.17 and piezoelectric d₃₃ constant of 98 pC/N, regardless of the unsaturated poling state.