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 (K1-xLax)(Nb1-xFex)O-3 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 420degreesC, and demonstrated a well-saturated ferroelectric hysteresis loop with large remanent polarization (P-r) of 18 muC/cm(2), 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 d(33) constant of 98 pC/N, regardless of the unsaturated poling state.