Solid-Solution Structure and Piezoelectric Property of KNbO₃ Ceramics Doped with Small Amounts of Elements

Abstract

The origin of the improvement of the ferroelectric and piezoelectric properties of KNbO 3 ceramics by the addition of small amounts of La and Fe has been investigated. The La/Fe-codoped KNbO 3 ceramics formed a single (K 1- x La x )(Nb 1- x Fe x )O 3 solid solution, and their unit-cell volume decreased significantly with an x of 0.002 mol, while they recovered at x =0.004. In this region, the lattice parameters of the a - b basal plane decreased and the elongation in the c direction toward apical oxygen in NbO 6 octahedra occurred. These results were correlated with the enhanced polarization and piezoelectric properties measured for the x =0.002 specimen. The x =0.002 specimen demonstrated a k t of 0.48 which corresponds to approximately 70% of the k t value predicted for a KNbO 3 single crystal as its maximized property. Furthermore, La/Fe-codoped KNbO 3 ceramics with MnO 2 addition were prepared to observe the effect of multivalent ion (Fe and Mn) doping into the KNbO 3 structure on the properties. Synchrotron near-edge X-ray absorption spectroscopy (XANES) study revealed a mixture of k -edge spectra consisting of Mn 4+ /Mn 3+ /Mn 2+ , while Fe 3+ maintained its valence state. The influence of valence fluctuation on the ferroelectric and piezoelectric properties of KNbO 3 ceramics was discussed.