where is the energy per atom of the mixed system, and are the energies per atom of 𝐸 𝐶𝑠 2 𝑆𝑛 𝑥 𝑇𝑖 1 -𝑥 𝑋 6 𝐸 𝐶𝑠 2 𝑆𝑛𝑋 6 𝐸 𝐶𝑠 2 𝑇𝑖𝑋 6 the pure-phase Sn and Ti endpoint compositions, and x is the Sn mixing fraction in Cs 2 Ti 1-x Sn x X 6 ; X = I, Br, Cl. The entropy of mixing was computed considering the configurational entropy contribution, given by the equation: