Potassium tantalate (KTaO3) crystals were grown by directional solidification from various compositions of K2CO3-Ta2O5 melt. When raw materials with the stoichiometric composition (K2CO3:Ta2O5=50:50 mol%) were used, K6Ta10.8O30 crystallized as the primary phase, indicating that KTaO3 melts incongruently. K6Ta10.8O30 is newly found in the K2CO3-Ta2O5 phase diagram and it is stably synthesized at temperatures above 1200 degrees C. In contrast, KTaO3 crystallized when the ratio of K2CO3 was more than 51.5 mol%. There were three kinds of color in the KTaO3-crystallized portions: milky white, colorless transparent and pale yellow. Based on these results, the phase diagram of the K2CO3-Ta2O5 system around the stoichiometric composition of KTaO3 is proposed. (C) 2013 Elsevier B.V. All rights reserved.