Crystal Structure and Molten Salt Environment Cooperatively Controlling the Morphology of the Plate-like CaMnO₃ Template through Topochemical Conversion

In the field of oxide thermoelectrics, perovskite CaMnO₃ ceramics have drawn plenty of attention due to their chemical stability, low cost, and environmental friendliness. By employing Ruddlesden-Poppe phase Ca₃Mn₂O₇ as a precursor, the plate-like CaMnO₃ microcrystals were successfully synthesized by the molten salt method combined with topochemical microcrystal conversion (TMC). The plate-like morphology of CaMnO₃ was coordinately optimized by modulating the crystal structure of MnO₂ and the molten salt environment. Plate-like microcrystals with an average size of ∼14.55 μm and a thickness of ∼2.89 μm were obtained by TMC reaction, demonstrating an obvious anisotropy. When β-MnO₂ was used as the raw material, a length-thickness ratio of 4.77 was obtained, which was attributed to the fact that CaMnO₃ inherited the plate-like morphology of the Ca₃Mn₂O₇ precursor during the TMC. The results confirm that the plate-like CaMnO₃ microcrystals with obvious anisotropy can provide excellent template seeds for high-quality CaMnO₃-based textured ceramics.