Fig. 7.

Chemical potential map of Co–Sn–Te: (a) elemental chemical potential planes. Each plane corresponds to a pure elemental phase: Co (orange), Sn (purple), and Te (blue). The axes define the deviation from these reference planes. (b) Binary and ternary competing phases represent planes that intersect with the reference planes in (a). The skutterudite Co₄Sn₆Te₆ exists as a small facet (labeled SKD) indicating limited compositional stability, consistent with experimental findings. (c) Zoomed view of the Co₄Sn₆Te₆ stability region. The invariant three-phase points are labeled A, B, and C. At A, SKD is in equilibrium with SnTe and CoSn, at B, with CoSn and CoTe, and at C, with CoTe and SnTe. Coordinates for all three points can be found in the ESI (ES7^†).