Effects of Sr substitution and high pressure on the phase stability of Ba₃Fe₂O₅Cl₂

Ba₃Fe₂O₅Cl₂ exhibits two crystal structures: the ambient pressure α phase and the high pressure β phase. The former contains FeO₄ tetrahedra, whereas the latter adopts a Ruddlesden–Popper (RP)–type structure with Fe ions octahedrally coordinated by the anions, as in Sr₃Fe₂O₅Cl₂. By preparing (Ba1-xSrx)₃Fe₂O₅Cl₂, we established the x–temperature phase diagram and determined the precise upper x limit of the α phase. Near this limit, the FeO₄ tetrahedra become highly compressed, which may drive the phase transformation to the RP structure. In contrast, the pressure-induced transformation appears to proceed indirectly, as suggested by the emergence of unknown phase(s) between the α and β phases under applied pressure. The magnetic properties of the β phase were examined for the first time, suggesting an antiferromagnetic transition above 400 K, comparable to that observed in Sr₃Fe₂O₅Cl₂.