3.4. Topochemical micro-crystalline conversion
In materials with low crystallographic symmetry, particles with an anisometric shape can be easily prepared by molten salt synthesis. In contrast, for materials with high crystallographic symmetry, particles with a large aspect ratio (plate or needle) are difficult to prepare directly from the constituent materials, because this shape is far from the equilibrium form. In this case, a precursor particle is used and, subsequently, it is converted to the objective material. The precursor particle must have an anisometric shape and a topotactic relation with the objective material. Molten salt synthesis can easily make the solution-diffusion mechanism dominant (mechanism 2 in Fig. 7); the particles of the objective material are formed by the diffusion of the supplementary material into the precursor particles, thus, preserving the outer shape of the precursor particles. This method is called topochemical micro-crystalline conversion, and is divided into three groups based on the relation between the crystal structures of the precursor and the objective materials; (1) the precursor with a structure similar to the objective material; (2) the precursor with a structure different from the objective material; and (3) the conversion accompanied by by-product.