Synthesis of polycrystalline platelike NaNbO₃ particles by the topochemical micro-crystal conversion from K₄Nb₆O₁₇ and fabrication of grain-oriented (K_0.5Na_0.5)NbO₃ ceramics

Topochemical microcrystal conversion (TMC) method is a powerful tool to synthesize platelike microcrystal particles with a regular-perovskite crystal structure, which is difficult to be fabricated by conventional flux techniques. By using the TMC method, polycrystalline rectangular-platelike NaNbO₃ particles with a orthorhombic perovskite structure were able to be synthesized from platelike precursor particles of layer-structured K₄Nb₆O₁₇ at 1000°C in molten NaCl-salt. The TMC-synthesized NaNbO₃ particles preserved the shape of precursor particles, and had a thickness of about 1 micron and a width of 5-10 microns. However TMC-synthesized platelike NaNbO₃ particles had a polycrystalline morphology having a preferred pseudo-cubic {100} orientation. Oriented particulate layer X-ray diffraction (OPL-XRD) analysis revealed that, during the TMC reaction, the crystallographic {010} plane of K₄Nb₆O₁₇ is converted to the most of {001} plane of polycrystalline NaNbO₃ particles in spite of polycrystalline morphology. Using the polycrystalline platelike NaNbO₃ particles as a template in the reactive templated grain growth method (RTGG), {001} grain-oriented (K_0.5Na_0.5)NbO₃—1 mol% CuO ceramics having a {001} orientation degree (Logering’s factor) of 45% could be fabricated. The result indicates that not only single crystalline particles, which were generally used, but also the polycrystalline particles can be act as template in the RTGG process. The availability of polycrystalline particles will give a new design of synthesizing templates for texturing of various kinds of perovskite crystal-structured ceramics.