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

概要

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The origin of the improvement of the ferroelectric and piezoelectric properties of KNbO3 ceramics by the addition of small amounts of La and Fe has been investigated. The La/Fe-codoped KNbO3 ceramics formed a single (K1-xLax)(Nb1-xFex)O3 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 NbO6 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 $k_{\text{t}}$ of 0.48 which corresponds to approximately 70% of the $k_{\text{t}}$ value predicted for a KNbO3 single crystal as its maximized property. Furthermore, La/Fe-codoped KNbO3 ceramics with MnO2 addition were prepared to observe the effect of multivalent ion (Fe and Mn) doping into the KNbO3 structure on the properties. Synchrotron near-edge X-ray absorption spectroscopy (XANES) study revealed a mixture of $k$-edge spectra consisting of Mn4+/Mn3+/Mn2+, while Fe3+ maintained its valence state. The influence of valence fluctuation on the ferroelectric and piezoelectric properties of KNbO3 ceramics was discussed.
2004-09-15