Structural Modification of the Cation-Ordered Ruddlesden-Popper Phase YSr₂Mn₂O₇ by Cation Exchange and Anion...

Calcium-for-strontium cation substitution of the a^-b⁰c⁰/b⁰a^-c⁰-distorted, cation-ordered, n = 2 Ruddlesden-Popper phase, YSr₂Mn₂O₇, leads to separation into two phases, which both retain an a^-b⁰c⁰/b⁰a^-c⁰-distorted framework and have the same stoichiometry but exhibit different degrees of Y/Sr/Ca cation order. Increasing the calcium concentration to form YSr_0.5Ca_1.5Mn₂O₇ leads to a change in the cooperative tilting on the MnO₆ units to a novel a^-b^-c^-/b^-a^-c^- arrangement described in space group P2₁/n11. Low-temperature, topochemical fluorination of YSr₂Mn₂O₇ yields YSr₂Mn₂O_5.5F_3.5. In contrast to many other fluorinated n = 2 Ruddlesden-Popper oxide phases, YSr₂Mn₂O_5.5F_3.5 retains the a^-b⁰c⁰/b⁰a^-c⁰ lattice distortion and P4₂/mnm space group symmetry of the parent oxide phase. The resilience of the a^-b⁰c⁰/b⁰a^-c⁰-distorted framework of YSr₂Mn₂O₇ to resist symmetry-changing deformations upon both cation substitution and anion insertion/exchange is discussed on the basis the A-site cation order of the lattice and the large change in the ionic radius of manganese upon oxidation from Mn³⁺ to Mn⁴⁺. The structure property relations observed in the Y-Sr-Ca-Mn-O-F system provide insight into assisting in the synthesis of n = 2 Ruddlesden-Popper phases, which adopt cooperative structural distortions that break the inversion symmetry of the extended lattice and therefore act as a route for the preparation of ferroelectric and multiferroic materials.