Effect of hydroxyl group on global and local structures of hydrothermally grown KNbO₃ nanorods

We report the annealing temperature dependence of global and local structures of hydrothermally grown KNbO₃ nanorods. With increasing annealing temperature, the amount of hydroxyl groups in the KNbO₃ nanorods decreased and finally disappeared at about 800 °C. Morphology of the nanorods seemed not to change significantly, however, X-ray intensity ratio between (0 2 2) and (2 0 0) planes, i.e., I_{0 2 2}/I_{2 0 0}, increased with decreasing lattice hydroxyl group contents. This result could be attributed to the fact that the hydroxyl groups were desorbed more effectively along the [0 1 1] direction than in other directions due to the elongated nanorod along the [0 1 1] direction. The frequencies of external and bending modes, besides stretching mode, showed red-shift with decreasing lattice hydroxyl group contents. This result implied that the lattice hydroxyl groups existed as interstitial defects near K ions.