A specimen having a stoichiometric composition of KSbO3 center dot(KSb) calcined at 800 degrees C has an R (3) over bar rhombohedral structure ( RS), and changes to a Pn (3) over bar cubic structure ( CS) when calcined at 1100 degrees C. Finally, a <111>-oriented rhombohedral phase is formed in the specimen calcined at 1230 degrees C. K/Sb ratio decreases from 1.0 in RS, 0.93 in CS, and finally to 0.85 in <111>-oriented rhombohedral phases. On the other hand, a specimen having a K-excess composition of K1.1SbO3 calcined at 800 degrees C shows a RS that is maintained in the K-excess specimen calcined at 1230 degrees C. The composition of these specimens is very close to KSb. Therefore, the RS with a space group of R (3) over bar is a stable form of KSbO3. The formation of Pn (3) over bar cubic and < 111>-oriented R (3) over bar phases can be explained by the evaporation of K2O during the calcination process at temperatures above 1100 degrees C.