The associated total energy calculations were performed primarily using Density Functional Theory (DFT) within the Vienna Ab Initio Simulation Package (VASP). Scalar-relativistic pseudopotentials were employed to describe the interaction between core and valence electrons, via the projector-augmented wave (PAW) method. The Cs_sv (5s 2 5p 6 6s 1 ), Sn_d (4d 10 5s 2 5p 2 ), Ti_pv (3p 6 4s 2 3d 2 ), I (5s 2 5p 5 ), Br (4s 2 4p 5 ), and Cl (3s 2 3p 5 ) PAW potentials were used, with the parentheses listing the valence electron configurations of each. These choices were tested and validated in our previous work on the endpoint compositions (Cs 2 SnX 6 & Cs 2 TiX 6 ). The structural properties of these materials are well described with the HSE06 range-separated hybrid DFT functional including dispersion corrections (Grimme's D3 with the recommended PBE0+BJ parametrizations) to account for van der Waal's interactions between the isolated octahedra. As such, the same computational setup was used for the DFT calculations in this study, with a plane-wave energy cutoff of 300 eV and Γ-centered k-point sampling density of 0.27 Å -1 in reciprocal space (corresponding to a k-point mesh of 3 × 3 × 3 for the 9-atom primitive unit cell), previously obtained using vaspup2.0 47 to give energies converged to <0.1 meV/atom. A convergence criterion of 0.01 eV/Å was imposed on the forces on each atom during structural relaxation, with relaxations continually iterated until cell volumes were unchanged, to avoid Pulay stress effects.