Crystallization of Manganese(V) Oxides by Hydroflux Synthesis: Control of Anisotropic Growth and Electrochemical Stability

Despite intriguing optical, magnetic, and redox properties, inorganic materials containing pentavalent manganese (Mn^V) are rare and could never be designed as shape-controlled crystals, which limits the ability to tune properties. Herein, we explore alkali hydroxide mixtures with controlled water content, namely, hydrofluxes, to demonstrate phase, shape, and nanostructure control of Mn(V) oxides. We demonstrate speciation among KSrMn^VO₄, Sr₅(Mn^VO₄)₃OH, and SrMn^IVO₃ with the water and strontium content and the nature of the alkali cation of the hydroxide salt. We then provide evidence of the key role of water in enabling shape and nanostructure control, which we relate to the preferential interaction of water with specific crystal facets of the hydroxyapatite Sr₅(Mn^VO₄)₃OH, and to the impact of water on precursor solubility in water-poor hydrofluxes. We then show that nanostructured Mn(V) hydroxyapatite possesses an acid-base redox stability window, enabling electrochemical operation in strongly oxidative conditions. By correlating the fundamental knowledge of hydrofluxes with crystallization mechanisms, this work sheds light on the possibilities offered by hydrofluxes for crystal shape, size, and property control.