Revealing and Rationalizing the Rich Polytypism of Todorokite MnO2.

Polytypism, or stacking disorder, in crystals is an important structural aspect that can impact materials properties and hinder our understanding of the materials. One example of a polytypic system is todorokite-MnO2, which has a unique structure among the transition-metal oxides, with large ionic conductive channels formed by the metal oxide framework that can be utilized for potential functionalization, from molecular/ion sieving to charge storage. In contrast to the perceived 3 × 3 tunneled structure, we reveal a coexistence of a diverse array of tunnel sizes in well-crystallized, chemically homogeneous one-dimensional todorokite-MnO2. We explain the formation and persistence of this distribution of tunnel sizes thermochemically, demonstrating the stabilization of a range of coherent large-tunnel environments by the intercalation of partially solvated Mg2+ cations. Based on structural behavior of the system, compared to the common well-ordered alkali-stabilized polymorphs of MnO2, we suggest generalizable principles determining the selectivity of structure selection by dopant incorporation.