Phase stability of Ruddlesden–Popper-type Sr₃Fe₂O₅Cl₂ with Ba substitution and discovery of a new high-pressure polymorph of Ba₃Fe₂O₅Cl₂

Sr₃Fe₂O₅Cl₂ is a member of the Ruddlesden–Popper (RP) phase compounds with tetragonal I4/mmm symmetry, whereas Ba₃Fe₂O₅Cl₂ has a different crystal structure with cubic I2₁3 symmetry. To elucidate the phase relation between these compounds, we systematically investigated the solid-solution system (Ba_1−xSr_x)₃Fe₂O₅Cl₂ at various temperatures. The RP phase appears only above x=0.9, and the phase boundary is almost independent of temperature. Below x=0.5 at 800 °C, the cubic phase exists as a single phase, whereas a mixed phase of RP and cubic structures appears between the two single-phase regions. The compositional range of the mixed phase narrows as the temperature increases. By contrast, the RP phase becomes significantly more stable over a wider composition range under high pressure; even for x=0, the cubic structure transforms into the RP-type structure under 6 GPa. The RP-type Ba₃Fe₂O₅Cl₂ represents a new polymorph. We analyzed its crystal structure and discuss it in the context of the recently proposed criterion for the stability of the RP phase.