Since it has been reported that the instability of these vacancy-ordered perovskites in air is higher in humid atmosphere due to their hygroscopic nature, we also tested storing this 60%-Sn bromide sample in a desiccator with 5% humidity. Under these conditions, the same sample was stable for over a week, while a CsBr diffraction peak became visible after 2 weeks (Figure ). The same experiments were carried out for the iodide alloys with similar trends observed, though the iodides are in general less stable than the bromides. Indeed, only the alloy containing 82% Sn was found to be stable for longer than a few minutes, though after 1 hour a CsI peak was clearly visible in the XRD pattern, even when storing the sample in a desiccator with 5% humidity (Figure ). The thermodynamic stability of these compounds was also computed using hybrid DFT, with the results tabulated in Table . Firstly, we find that all Cs 2 (Sn/Ti)X 6 (X = I, Br) are thermodynamically stable with respect to decomposition into their competing phases within the Cs-Sn/Ti-X chemical space (i.e. stable in vacuum), as indicated by the positive (endothermic) decomposition energies for the intrinsic decomposition reactions, matching experimental observations of stability under inert atmospheres (Figure (a) and (d) and Figures ). In agreement with thermogravimetric analysis (TGA) experiments and semi-local DFT calculations from Ju et al. and Liu et al. for Cs 2 TiI 6 and Cs 2 TiI 6-x Br x respectively, we find that MX 4 and CsX (M = Sn, Ti; X = I, Br) are the lowest energy intrinsic decomposition products. The chemical stability regions showing the competing phases and chemical potentials within the Cs-Sn/Ti-X phase space are plotted in Figure , with much narrower stability regions for the iodides as expected. We note here that van der Waals dispersion interactions between the isolated octahedra are found to contribute significantly to the stability of these compounds. Using hybrid DFT (HSE06) without the D3 vdW dispersion correction to perform geometry relaxations and total energy calculations, we find decomposition energies 378 and 216 meV lower for Cs 2 SnI 6 and Cs 2 TiI 6 respectively. In the presence of oxygen however, we find that oxidation to SnO 2 /TiO 2 is highly thermodynamically-favorable, with negative (exothermic) decomposition energies. In the presence of water, we find that the most favorable aqueous decomposition reaction is Cs 2 MX 6 + 2H 2 O  MO 2 + CsX + 4HX (g) ; M = Sn, Ti; X = I, Br; in agreement with the conclusions of He et al. Due to the formation of gaseous HX (g) (X = I, Br) as a reaction product in this case, the free energy of this decomposition is dependent on the partial pressure of HX (g) in the reaction environment, as shown in Table and plotted in Figure . The effect of the gaseous product partial pressure on the reaction equilibrium indicates that aqueous decomposition will be inhibited in closed containers. Our decomposition energies qualitatively match those calculated by Kaewmeechai et al. for Cs 2 TiX 6 , however their calculations did Table Calculated decomposition reactions and free energies for (a) Cs 2 SnX 6 (X = I, Br) and (b) Cs 2 TiX 6 in vacuum, and in the presence of oxygen and water, at T=300K. For oxidation reactions, the partial pressure of O 2 has been taken as that in air (0.2095 atm). not include gas partial pressure contributions (thus corresponding to p HX = 1 atm for H 2 O decomposition) which dictate the aqueous decomposition energies. In agreement with the experimental observations, we find that stability increases as we move up the halogen periodic group from iodine to bromine, and the Sn VODPs are more stable than the Ti-based compounds, for both oxygen and aqueous decomposition. The greater instability of Cs 2 TiX 6 compared to Cs 2 SnX 6 is attributed to the stronger thermodynamic driving force (more exothermic formation energy) for TiO 2 compared to SnO 2 . Given the large (~1.5 eV) difference in free energies of decomposition for Sn and Ti vacancy-ordered perovskites compared to the configurational entropy contribution for cation mixing (~20 meV), the thermodynamic stabilities of the alloys are predicted to follow the linear combination of the pure tin and titanium phases, weighted by the mixing fraction x, such that: