Formation of nanoneedles and nanoplatelets of KNbO3 perovskite during templated crystallization of the precursor gel

Highly anisotropic nanocrystallites in the form of nanoneedles and nanoplatelets of KNbO3 perovskite were prepared by crystallization of the amorphous gel. The gel was prepared by the polymerizable-complex method based on the Pechini-type reaction route. The nanoneedles were typically 50 mn thick and approximately 10 mu m long and had surfaces parallel to the {100} planes of the pseudocubic perovskite. The formation of these pseudocubic KNbO3 perovskite crystallites of an unusual and highly anisotropic shape was studied by electron microscopy in combination with X-ray diffractometry and thermal analyses. A layer-structured compound, K4Nb6O17, appeared as the first crystalline phase during the heating of the amorphous precursor gel at temperatures around 490 degrees C. When this compound reacts to form the cubic perovskite at higher temperatures (around 540 degrees C), its highly anisotropic, layered structure defines the anisotropic shape of the product.