Single crystal X-ray diffraction analysis was performed on crystals with composition Ba1 − δSrδMgF4 (δ ≤ 0.55). The complete structure was analyzed for single domain crystals with nominal (refined) values of δ = 0.25 (0.27(2)) and 0.5 (0.55(2)). Interatomic distances vary in a characteristic manner, when smaller strontium ions replace the barium ions. Optical studies of Sm(II) doped samples show significant inhomogeneous line broadening and confirm the disorder On the Ba Site.