**4. Conclusions**

We developed K1–*x*Na*x*N–NTK composite lead-free piezoelectric ceramic composed of the two primary phases KNN and NTK. The NTK phase is a dielectric substance with a layered structure and is not piezoelectric. This phase fills the voids between KNN crystalline particles, and a portion of this phase transforms into K2(Ti,Nb,Co,Zn)6O13 and/or CoZnTiO4. Conse‐ quently, a densely sintered ceramic is obtained. This KNN–NTK composite lead-free piezo‐ electric ceramic exhibits enhanced piezoelectric characteristics, such as a planar-mode electromechanical coupling coefficient *k*p = 0.52, a dielectric constant *ε*33T/*ε*<sup>0</sup> = 1600, *N*p = 3170 Hz m, and a high thermal durability.

In this system, KNN forms the single tetragonal phase for *x* < 0.56. However, near *x* = 0.56 appears a two-phase coexisting state containing both the *P*4*mm* tetragonal phase and the *Imm*2 orthorhombic phase, and the fraction of the orthorhombic phase increases with *x*. No XRD peaks corresponding to the *Imm*2 orthorhombic phase are observed for the initial coexisting state; however, superlattice reflections appear in the SAD patterns. This K1–*x*Na*x*N– NTK composite lead-free piezoelectric ceramic, with granular *Imm*2 orthorhombic nanodo‐ mains dispersed in a *P*4*mm* tetragonal matrix, has excellent piezoelectric properties, with a planar-mode electromechanical coupling coefficient *k*p = 0.56 at the onset of the two-phase coexisting state. The phase transition of K1–*x*Na*x*N–NTK composite lead-free piezoelectric ceramic occurs gently, which distinguishes it from the drastic phase transition of the MPB of PZT. However, the diffusivity *γ* of K1–*x*Na*x*N–NTK shows that K1–*x*Na*x*N–NTK composite leadfree piezoelectric ceramic is close to a ferroelectric but is not a relaxor.

### **Acknowledgements**

The synchrotron radiation experiments were performed at SPring-8 with the approval of Japan Synchrotron Radiation Research Institute (JASRI). Part of this work was supported by Japan Fine Ceramics Center (JFCC).
