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Highly dense (Li,Na,K)NbO3 ceramics were prepared by normal sintering at reduced temperatures by adding excessive Na, and its phase structure and electrical properties were investigated as a function of the compensated Na content. A high piezoelectric coefficient d33 up to 279 pC/N was obtained at an optimized Na content, which was found to correspond to a phase boundary between two kinds of orthorhombic phases. The corresponding Curie temperature and electromechanical coupling factor reached 465°C and 46%, respectively. The phase boundary found in the present (Li,Na,K)NbO3 ceramics like the morphotropic phase boundary existing in the Pb(Zr,Ti)O3 system, but different from the polymorphic phase transition from an orthorhombic phase to a tetragonal one reported previously in most (Li,Na,K)NbO3 ceramics.

Lead-free piezoelectric Na x K 1-x NbO 3 (x = 0.3-0.8) (NKN) ceramics were fabricated by normal sintering at 1060°C for 2 h. Microstructures and electrical properties of the ceramics were investigated with a special emphasis on the influence of Na content. The grain size of the produced dense ceramic was decreased by increasing Na content. A discontinuous change in the space distance was found at the composition close to Na 0.7 K 0.3 NbO 3 ceramic, which indicates the presence of a transitional composition between two different orthorhombic phases, which is similar to the behavior of morphotropic phase boundary (MPB) in Na x K 1−x NbO 3 ceramics. Such MPB-like behavior contributes to the enhanced piezoelectric coefficient d 33 of 122 pC/N, planar-mode electromechanical coupling coefficient k P of 28.6%, and dielectric constant ε r of 703, respectively for the Na 0.7 K 0.3 NbO 3 ceramic. Cubic temperature T C and the transitional temperature T O-T from orthorhombic to tetragonal phase are observed at around 420°C and 200°C, respectively.