Hydrothermal synthesis of single crystalline (K,Na)NbO3 powders

(K1-xNax)NbO3 (x = 0.01, 0.24, 0.89, 0.91, 0.99) single crystalline powders with perovskite structure were synthesized by a hydrothermal method with Nb2O5 in a mixed solution of KOH and NaOH at 220 degrees C for 24 h. XRD investigations showed that the structure of the synthesized (K1-xNax)NbO3 ceramics changed from a KNbO3-type orthorhombic phase (space group Bmm2) to a NaNbO3-type monoclinic phase (space group Pm) with the increase of Na content. The present results confirmed that (K1-xNax)NbO3 exhibited a morphotropic phase boundary (MPB) at around 50 %K separating the orthorhombic phase and monoclinic phase. FE-SEM images showed that the morphological structures of niobates were cubes and nanofingers. The TEM image and the selected area electron diffraction pattern of (K0.01Na0.99)NbO3 showed the as-synthesized powder was single crystalline and indexed to be monoclinic. The phase evolution of the products in the hydrothermal route revealed that the hydrothermal synthesis of (K1-xNax)NbO3 underwent two steps, where an intermediate product (K8-8xNa8x)Nb6O19.center dot H2O was found in the early stage and then pure (K1-xNax)NbO3 was obtained after another 30 min of hydrothermal treatment. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007).