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 YSr0.5Ca1.5Mn2O7leads to a change in the cooperative tilting on the MnO6units to a novel a–b–c–/b–a–c–arrangement described in space group P21/n11. Low-temperature, topochemical fluorination of YSr2Mn2O7yields YSr2Mn2O5.5F3.5. In contrast to many other fluorinated n= 2 Ruddlesden–Popper oxide phases, YSr2Mn2O5.5F3.5retains the a–b0c0/b0a–c0lattice distortion and P42/mnmspace group symmetry of the parent oxide phase. The resilience of the a–b0c0/b0a–c0-distorted framework of YSr2Mn2O7to 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.