I ran a (mostly) Materials Project compatible calculation of spinel (Fd-3m) Sc2CdS4, and using the Pymatgen/MPRester to build the hull diagram puts my calculation 0.2 meV/atom above the hull, with the MP Sc2CdS4 structure (mp-10953) as the stable decomposition product. However, in the Materials Project, that structure is itself shown as 292 meV/atom above the hull, and so the Pymatgen/MPRester decomposition ought to be CdS + Sc2S3.
It’s possible that this is an error in Pymatgen’s phase diagram, but my first guess is that something is wrong with the hull energy shown in Materials Project; 292 meV/atom above the hull doesn’t seem right for an experimentally synthesized sulfide. Just checking manually on the calculator:
Sc4Cd2S8 -> Sc4S6 + Cd2S2
(-146.5048/2 - 13.9748 + 87.3615)/14 = +0.010 meV/atom, indicating that Sc2CdS4 should be on the hull.