The in-situ high-temperature single-crystal X-ray diffraction experiments on KNbO3 (KN) and Na0.5K0.5NbO3 (NKN) revealed a series of structural evolution associated with two transitions among orthorhombic, tetragonal and cubic forms. Upon heating, the pseudocubic unit cell volume collapsed discontinuously at these transition points, whereas the reverse took place upon cooling, indicating that both transitions were of the first-order. The coordination numbers of Nb showed a decreasing tendency with decreasing temperature, i.e., 6 in cubic, 5+1 in tetragonal, and 4+2 in orthorhombic. The difference in ionic size between Na and K gave the Na atom an attribute of rattling in the cavity surrounded by corner-sharing NbO6 octahedra in NKN. The small Na not only vibrated with large amplitude, but also shifted further from the centre of the cavity than K. The coordination number of Na was always small compared with K at all temperatures, and showed an decreasing tendency upon cooling, i.e., 8+4 in tetragonal and cubic, and 7+5 in orthorhombic. The structural phase transition of KN occurred in a similar way as NKN, though the transition temperatures were shifted slightly toward the high temperature side.
|ジャーナル||IOP Conference Series: Materials Science and Engineering|
|出版ステータス||Published - 2011|
|イベント||3rd International Congress on Ceramics, ICC3 - Osaka, Japan|
継続期間: 2010 11 14 → 2010 11 18
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