Tuning electronic and magnetic properties of Mn-mullite oxide sub-nanoclusters via MnOn polyhedron units

Abstract

Ternary oxide nano-clusters compared to unary metallic and binary ones potentially exhibit more remarkable properties due to their higher stoichiometric flexibility in addition to cluster size variations. Herein, by combining with the structural searching scheme CALYPSO, we have built a series of Mn-mullite oxide clusters (Sm_xMn_yO_z)_n {(xyz) = (125); (115); n = 1–4, 8} prior to investigation of their geometric and electronic structures via first-principles calculations. In small size regime (n < 4), (Sm_xMn_yO_z)_n prefer nonstoichiometric (Sm₁Mn₁O₅)_n phases composed of nonmagnetic MnO₄ tetrahedrons. When n ≧ 4, the clusters tend to develop as stoichiometric (Sm₁Mn₂O₅)_n species, including magnetic MnO_n polyhedrons and Mn–Mn dimers, which contribute 3d-orbitals (d_z² and/or d_x²−y²) around the Fermi levels. The different magnetic behaviors of nonstoichiometric and stoichiometric species originate from the distinct couplings of MnO_n polyhedronal units, wherein Mn atoms experience different ligand fields and thus display different spin states. Such findings enable the tuning of electronic properties and potential applications in heterogeneous catalysis, electrochemical catalysis, and the related fields via engineering cluster size and stoichiometry.