Potassium niobate (KNbO3) crystals were grown in a platinum crucible by directional solidification from stoichiometric starting materials. The solidified boules were approximately 10 mm in diameter by 50 mm in length. It was confirmed that KNbO3 melts incongruently, so that the initial, intermediate and final regions of the boules consisted, respectively, of a mixture of K4Nb6O17 and KNbO3, pure KNbO3, and a mixture of KNbO3, K2CO3 and K8Nb6O19. The KNbO3 crystals in the intermediate region were polycrystalline with a grain size of several millimeters, and the crystal colorations changed from pale red, through milk white to blue with increasing K concentration in the melt. It was concluded that differences in the lattice parameters and the transition temperatures between the pale red, milk white and blue KNbO3 crystals resulted from the existence of non-stoichiometry in KNbO3. (C) 2004 Elsevier B.V. All rights reserved.