Franckeite is a natural superlattice composed of two alternating layers of different composition which has shown potential for optoelectronic applications. In part, the interest in franckeite lies in its layered nature which makes it easy to exfoliate into very thin heterostructures. Not surprisingly, its chemical composition and lattice structure are so complex that franckeite has escaped screening protocols and high-throughput searches of materials with nontrivial topological properties. On the basis of density functional theory calculations, we predict a quantum phase transition originating from stoichiometric changes in one of franckeite composing layers (the quasihexagonal one). While for a large concentration of Sb, franckeite is a sequence of type-II semiconductor heterojunctions, for a large concentration of Sn, these turn into type-III, much alike InAs/GaSb artificial heterojunctions, and franckeite becomes a strong topological insulator. Transmission electron microscopy observations confirm that such a phase transition may actually occur in nature.

Departamento de Física de la Materia Condensada Instituto Nicolás Cabrera Universidad Autónoma de Madrid Cantoblanco 28049 Madrid Spain

Departamento de Física Universidade Federal do Espírito Santo Vitória ES 29075 910 Brazil

Facultad de Ciencias Físicas and Instituto Plurisciplinar Universidad Complutense de Madrid 28040 Madrid Spain

Instituto de Física Universidade Federal do Rio de Janeiro Caixa Postal 68528 Rio de Janeiro RJ 21941 972 Brazil

Instituto Federal do Espirito Santo Cariacica ES 29150 410 Brazil

J Heyrovský Institute of Physical Chemistry Czech Academy of Sciences Dolejškova 2155 3 182 23 Prague 8 Czech Republic

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