To study the mixed compositions (Cs 2 Ti 1-x Sn x X 6 ; X = I, Br, Cl), the special quasi-random structure (SQS) approach was employed, whereby supercells are generated with the Sn/Ti cations arranged 'quasi-randomly', such that the atomic pair correlation functions give as close a match as possible to that of an ideal infinite random distribution. The use of this approach and assumption of random cation mixing is validated by the near-negligible mixing enthalpies observed (Figure ), demonstrating minimal inter-octahedral interactions and thus negligible short-range ordering effects. The icet package was used to generate 72-atom SQS supercells via a Monte Carlo simulated annealing algorithm, which gave a best objective function score (closest match to random cation distribution) with a nondiagonal expansion matrix of the 9-atom primitive cell: