Electronic Structure Manipulation of the Mott Insulator RuCl₃ via Single‐Crystal to Single‐Crystal Topotactic Transformation

The core task for Mott insulators includes how rigid distributions of electrons evolve and how these induce exotic physical phenomena. However, it is highly challenging to chemically dope Mott insulators to tune properties. Herein, we report how to tailor electronic structures of the honeycomb Mott insulator RuCl 3 employing a facile and reversible single‐crystal to single‐crystal intercalation process. The resulting product (NH 4 ) 0.5 RuCl 3 ⋅1.5 H 2 O forms a new hybrid superlattice of alternating RuCl 3 monolayers with NH 4 + and H 2 O molecules. Its manipulated electronic structure markedly shrinks the Mott–Hubbard gap from 1.2 to 0.7 eV. Its electrical conductivity increases by more than 10 3 folds. This arises from concurrently enhanced carrier concentration and mobility in contrary to the general physics rule of their inverse proportionality. We show topotactic and topochemical intercalation chemistry to control Mott insulators, escalating the prospect of discovering exotic physical phenomena.