All Ba10-xSrx(MnO4)(PO4)5Cl2 (0≤x≤5) samples investigated were shown to form a single phase apatite material (see supplementary info: Fig SI ). The structures (space group of P 63 / m) were refined using the Rietveld refinement method. The unit cell parameters (Figure ), showed the expected reduction in unit cell size on increasing Sr content, as expected due to smaller size of Sr 2+ compared to Ba 2+ . The refined lattice parameters and the Rietveld refinement goodness of fit parameters are presented in Table . In the Rietveld refinement, the manganese and phosphorous were all placed in the MO4 tetrahedral sites using the expected stoichiometric amounts. In line with previous literature studies on cation partitioning within apatite structures which have suggested large ions preferentially favour the A2 sites, with small cations the A1 sites , the refinements here confirmed this preference with Sr favouring the A1 site and Ba the A2 site. Figure shows a representative Rietveld refinement profile fit for the x=4 sample, Ba6Sr4(MnO4)(PO4)5Cl2 structure. For this Ba6Sr4(MnO4)(PO4)5Cl2 composition, there is complete ordering with Sr and Ba fully occupying the A1 and A2 sites respectively (the structural parameters for this phase are shown in Table ). In addition to the collection of X-ray diffraction data, IR spectra were recorded for the samples (figure ). The data were comparable to previous literature reports for apatite systems . Peaks between 700-1100 cm -1 are attributed to the MO4 tetrahedra, with the peaks around 1000 cm -1 associated with the PO4 3-group vibrational modes (v1 -symmetrical and v3modes). All samples also have v3 vibrational peaks associated with MnO4 3- around 720 to 780 cm -1 consistent with literature values for such a moiety. There is an observed shift of all vibrational modes to higher wavenumber on increasing the strontium