All compounds in the series SrxBi4-xO4Se1-xCl2+x for x = 0, 0.25, 0.5, 0.75, 0.95, 0.99 and 1.0 were identified from X-ray diffraction as having the same I4/mmm unit cell reported for the x = 0 and 1.0 end members (Bi4O4SeCl2 and SrBi3O4Cl3). SEM-EDX was used to check the actual composition versus the nominal value of x. All samples showed small plate-like crystals, as expected from the van der Waals gap containing structure. One key difference is that for x = 0, platelike crystals with approximate in plane dimensions of 10 µm were observed (Figure ), whereas for all other compositions, crystallites with in-plane dimensions of one order of magnitude smaller were observed (Figure ). This indicates that that a small change in the components involved in synthesis affects the nucleation and crystal growth kinetics in a way that favours smaller particle sizes in the same experimental conditions. This could be an important consideration when either designing these materials for photovoltaic or photocatalytic applications. The experimental value for x was calculated both from the Se/Cl ratio and the Sr/Bi ratio averaged from 5 measurements for each composition and compared to the nominal value. Figure shows good agreement between the experimental and nominal values, with the values calculated by Sr/Bi and Se/Cl falling within error of each other and within error of the nominal composition. Along with the preservation of the unit cell, this suggests that the composition was successfully tuned by aliovalent substitution for the series SrxBi4-xO4Se1-xCl2+x. A large uncertainty is observed for x = 0.25 likely due to the difficulty of finding sufficiently flat surfaces. It is worth noting that the value for x calculated by the Sr/Bi ratio cannot be less than zero due to zero being the minimum content of Sr, while the value of x calculated by the Se/Cl ratio can go below zero as x = 0 corresponds to a ratio of 0.5 Se/Cl. The composition wasn't measured for x = 0.99 due to the amount of Se being below the detection limit of the instrument. an example Rietveld refinement for x = 0.75. Full Rietveld refinements for the whole series are shown in the supporting information. The diffraction patterns show the same I4/mmm pattern for every x, with small shifts in the lattice parameters observable. The peaks are also broader for any composition with alkali earth on Bismuth substitution than for the x = 0 phase, due to the addition of Sr/Bi disorder into the system. Figure shows the full modelled structure for the series as determined by Rietveld refinements of powder XRD data. Compositions were constrained to the nominal composition due to the agreement of the nominal and experimental compositions shown in Figure . The structure of the series overall is defined by Bi2O2 fluorite type layers separated alternatively by a ionic bridging layer (Se1/Cl1) and a van der Waals layer (Se2/Cl2). Refining the Sr occupancy on both the Bi1 (adjacent to the ionic bridging layer) and Bi2 (adjacent to the van der Waals layer) sites in the Bi2O2 demonstrated that within error, all Sr occupies the Bi2 position at every composition, consistent with the reported structure for SrBi3O4Cl3. For x = 0, the Se is distributed both across the ionic bridging layer and van der Waals layer, with a statistical site preference for the ionic bridging layer. Upon Sr substitution, this site preference gets more marked; at compositions of x = 0.75 and larger, Se only occupies the ionic bridging layer to within error of the model, refining to a slightly negative value when freely refined, though a small amount of Se on the Se2 site cannot be ruled out. Lattice parameter trends are shown in Figure . The in-plane lattice parameter a varies linearly with x across the series, consistent with Vegard's law and the larger size of Sr 2+ compared to Bi 3+ . By contrast, the lattice parameter c varies non-monotonically with x. The c lattice parameter depends on the thickness of the ionic bridging layer, the van der Waals layer and the Bi2O2 fluorite type layer. These layers are being affected simultaneously in different ways, with the total amount of Se being less with x, the site preference of Se for the Se1 site increasing with x and the amount of Sr increasing on the Bi2 site. The site preference can be seen when the ratio of the occupancy of Se in the Se2 site versus the Se1 site is plotted as a function of x in Figure (b). A quick drop off is observed around x = 0.5, after which, within uncertainty of modelling the diffraction data, all the Se occupies the Se1 site. Therefore, the amount of Se in the Se1 site is not directly proportional to x. The effect of this on the lattice parameter c can be seen when plotting the thickness of the ionic bridging layer, as defined by the cross-plane Bi1-Bi1 distance (Figure ). This value changes monotonically but not linearly with x. By instead plotting it is a function of the occupancy of Se in the Se1 site, xSe1, a linear trend is observed (Figure ). Indeed, this layer thickness for CaBi3O4Cl3 fits on this linear trend as well, despite the smaller overall c lattice parameter of CaBi3O4Cl3 compared to SrBi3O4Cl3 (26.6398(5) Å and 26.9997(4) Å respectively). This demonstrates that the thickness of the ionic bridging layer is dependent only on the anion composition at the bridging site, a consequence of the tendency of the alkali earth to only sit at the Bi2 site. The smaller c axis of CaBi3O4Cl3 compared to SrBi3O4Cl3 is therefore related to a contraction of the Bi2O2 and van der Waals layers due to the smaller Ca 2+ cation. The ionic bridging layer can therefore be tuned by the anion composition at the bridging site, and the other layers tuned by choice of alkali earth, allowing for different compositional degrees of freedom for modifying the layer thicknesses at different points in the structure. Given the importance of these layers for e.g. thermal conductivity, the ability to tune the bonding at each layer in this type of natural heterostructure could be another method of introducing phonon engineering.