Quantum Materials - 02

Abstract

We investigated the structural stability of Ruddlesden-Popper (RP) phase oxychlorides by examining the synthesis conditions and crystal structures of (Ba_1-xSr_x)₃Fe₂O₅Cl₂. In this system, Fe3+ (3d5) is Jahn-Teller inactive. Sr₃Fe₂O₅Cl₂ adopts the ideal RP structure with tetragonal I4/mmm symmetry [1], while Ba₃Fe₂O₅Cl₂ crystallizes in a cubic I2₁3 structure containing FeO₄ tetrahedra [2] (Figure 1). A solid-solution series was synthesized by firing mixtures of Sr₂Fe₂O₅, Ba₂Fe₂O₅, SrCl₂, BaCl₂, and Fe₂O₃ in Ar at 800–1100 ℃. The tetragonal RP phase appeared only near x = 1, whereas the cubic phase was stable across a broad compositional range (Figure 2), suggesting that a slight increase in A-site ionic radius destabilizes the RP structure.

In addition, we discovered a new tetragonal RP-type phase by heating cubic Ba₃Fe₂O₅Cl₂ at 1000 ℃ under 6 GPa. High-pressure conditions likely stabilize this layered structure by increasing Fe coordination from 4 (ambient) to 5. These results demonstrate how pressure and composition influence the stability of RP-type oxychlorides.