Deciphering the Atomic Patterns Leading to MnO2 Polymorphism

A fundamental understanding of phase transition mechanisms for polymorphic materials is the prerequisite for developing rational synthesis strategies toward phase homogeneity. Here, we target polymorphic MnO2 as the prototype system and reveal the atomic mechanisms governing the phase selection among various MnO2 tunnel structures that are hydrothermally synthesized from a layered MnO(2 )precursor. A topotactic layer-to-tunnel (L-T) transition mechanism featuring solid-state Mn migration and structure rearrangement is discovered. The transition exhibits multi-step kinetics with the formation of intermediate large tunnels, which introduces structural complexity into the end product with significant phase and compositional heterogeneity within single MnO2 particles. Localized valence analysis further reveals the significant effect of Jahn-Teller Mn3+ ordering on the L-T transition kinetics. We expect these findings to assist the understanding of polymorphism evolution and be further applied to rationalize material synthesis strategies toward phase homogeneity and to establish an accurate synthesis-structure relationship.