Polytypism of layered MX2 materials

Abstract

We revisit the problem of polytypism in layered MX2 materials, with a view to reinterpreting the phase space accessible to this family. Our starting point is to develop a simple, constructive, and compact label for the most commonly observed stacking arrangements, similar to the Glazer notation used to label tilt systems in perovskites. The key advantage of this label in the context of MX2 systems is that it contains sufficient information to generate the corresponding stacking sequences uniquely. Using a related approach, we generate a Cartesian representation of the phase space containing all possible MX2 polytypes, with the most common structures appearing as limiting cases. We argue that variation in, e.g., composition, temperature, or pressure may allow navigation of this phase space along continuous paths. This interpretation is shown to be consistent with the structural evolution of stacking-faulted MX2 systems as a function in temperature and pressure. In this way, our study highlights the potential for controlling composition/structure/property relationships among layered MX2 materials in ways that might not previously have been obvious.