For this reason, the amount of Sn and Ti in the NC solutions were measured using inductively-coupled plasma optical emission spectroscopy (ICP-OES). Table (a) and (b) lists the lattice parameters of each sample, extracted from Rietveld refinement (the refinement parameters are provided in Table ) along with the percentage concentration of tin, where the Sn% corresponds to x in Cs 2 Ti 1-x Sn x X 6 . For both halides (X = I, Br), the amount of tin that is incorporated to the NC (as measured by ICP-OES) is higher than the precursor ratio of tin loaded in the flask at the beginning of the synthesis (indicated under "Sample name" in the table), indicating that the formation of the Sn-based compound is more favorable than that of the Ti-based one. Comparisons of the experimental and computed trends in the lattice parameters as a function of mixing ratio x are shown in Figure (c) and (f). In the idealized bulk computational model for these compounds, a perfectlylinear trend in the lattice parameters is predicted, in accordance with Vegard's law. We attribute the nonlinearity in the experimental structure parameters to the nanocrystal strain effects described above. A small (~1%) systematic underestimation of the lattice parameter is witnessed with our static computational model, likely related to the neglect of thermal expansion in these calculations, and thus we apply rigid upshifts of +0.075 Å and +0.145 Å for the bromide and iodide respectively to allow direct comparison of the trends. A discussion on the performance of various DFT functionals in modelling the structural properties of Cs 2 (Sn/Ti)X 6 is provided in Ref.12. As shown in Figure , analysis of the optimized crystal structures for the mixed compositions reveals that while the volume increases linearly and monotonically with increased Sn content, the intra-octahedral M-X bond lengths remain essentially fixed, with d(Sn-X) & d(Ti-X) in Cs 2 Ti 1-x Sn x X 6 retaining the same values as in Cs 2 SnX 6 and Cs 2 TiX 6 . While d(Sn-X) is ~5% larger than d(Ti-X) for all X = I, Br, Cl, due to the larger ionic radius of Sn 4+ compared to Ti 4+ , the metal-halide bond lengths individually vary by less than 0.005 Å (~0.2%) within each (Sn/Ti)X 6 octahedron across the mixing range x. Instead, as depicted in the inset of Figure I and shown in Figure , it is the vacant inter-octahedral space which is found to vary as the Sn/Ti ratio changes, while the differently-sized MX 6 octahedra behave as rigid building blocks with minimal disruption of local bonding environments upon mixing.