The shape of the precursor particles must be preserved during the topochemical reaction, which occurs in the reaction stage (see 3.2). In the reaction stage, two reaction processes are possible; the solution-precipitation and solution-diffusion processes. If the former process is dominant, the precursor particles dissolve and the objective material particles precipitate with an intrinsic shape. Therefore, anisometric particles cannot be obtained. The reaction conditions must be selected to make the solution-diffusion process dominant. Platelike KNbO3 particles are obtained via platelike K4Nb6O17 particles (Saito & Takao, 2007). Figure 13(a) shows platelike K4Nb6O17 particles obtained by the reaction between Nb2O5 and (COOK)2∙H2O in KCl. KNbO3 is obtained by the reaction of the platelike K4Nb6O17 particles with (COOK)2∙H2O, but the particle shape depends on the particle size of (COOK)2∙H2O (Sakurai, 2011). When (COOK)2∙H2O particles of tens of micrometer are used, KNbO3 particles with an equiaxed shape are obtained (Fig. 13(b)). When (COOK)2∙H2O particles of several micrometers are used, then, polycrystalline platelike particles are obtained (Fig. 13(c)). The (COOK)2∙H2O particle size determines the dissolution rate of K2O in the molten salt and the process of the formation of the KNbO3 particles. Usually, K2CO3 is used as the potassium source, but it is hygroscopic and difficult in handling. (COOK)2∙H2O is not hygroscopic and it can be used in the preparation of the particles (Fig. 13); (COOK)2∙H2O decomposes to K2CO3 upon heating at about 370ºC