A DFT geometry optimization with the PBEsol functional was conducted on these input structures and the formation energies for the one-dimensional chemical space K2SnClx (5 ≤ 𝑥 ≤ 6) are shown in Figure . Note that the termination at x = 5 is rather arbitrary. The plot suggests phase-separating behavior between the phases at x = 5 and the pristine phase at x = 6. These theoretical results were confirmed by geometry optimizations with the advanced SCAN functional on a set of 24 vacancy structures, which had turned out to be energetically most favorable in the previously conducted PBEsol calculations (Figure ). Figure displays the most stable structures resulting from the SCAN geometry optimization for 5 ≤ 𝑥 < 6. The characteristic SnCl6 octahedra becomes partly reduced upon the introduction of the vacancies but keeps their overall structure during the geometry optimization, i.e., no decomposition of the initial structure results.