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

Abstract

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.