Structural and Magnetic Phase Transitions in the A_<i>n</i>B_<i>n</i>O_{3<i>n</i>–2} Anion-Deficient Perovskites Pb₂Ba₂BiFe₅O₁₃ and Pb_1.5Ba_2.5Bi₂Fe₆O₁₆

Novel anion-deficient perovskite-based ferrites Pb₂Ba₂BiFe₅O₁₃ and Pb_1.5Ba_2.5Bi₂Fe₆O₁₆ were synthesized by solid-state reaction in air. Pb₂Ba₂BiFe₅O₁₃ and Pb_1.5Ba_2.5Bi₂Fe₆O₁₆ belong to the perovskite-based A_<i>n</i>B_<i>n</i>O_{3<i>n</i>–2} homologous series with <i>n</i> = 5 and 6, respectively, with a unit cell related to the perovskite subcell <i>a</i>_p as <i>a</i>_p√2 × <i>a</i>_p × <i>na</i>_p√2. Their structures are derived from the perovskite one by slicing it with 1/2[110]_p(1̅01)_p crystallographic shear (CS) planes. The CS operation results in (1̅01)_p-shaped perovskite blocks with a thickness of (<i>n</i> – 2) FeO₆ octahedra connected to each other through double chains of edge-sharing FeO₅ distorted tetragonal pyramids which can adopt two distinct mirror-related configurations. Ordering of chains with a different configuration provides an extra level of structure complexity. Above <i>T</i> ≈ 750 K for Pb₂Ba₂BiFe₅O₁₃ and <i>T</i> ≈ 400 K for Pb_1.5Ba_2.5Bi₂Fe₆O₁₆ the chains have a disordered arrangement. On cooling, a second-order structural phase transition to the ordered state occurs in both compounds. Symmetry changes upon phase transition are analyzed using a combination of superspace crystallography and group theory approach. Correlations between the chain ordering pattern and octahedral tilting in the perovskite blocks are discussed. Pb₂Ba₂BiFe₅O₁₃ and Pb_1.5Ba_2.5Bi₂Fe₆O₁₆ undergo a transition into an antiferromagnetically (AFM) ordered state, which is characterized by a G-type AFM ordering of the Fe magnetic moments within the perovskite blocks. The AFM perovskite blocks are stacked along the CS planes producing alternating FM and AFM-aligned Fe–Fe pairs. In spite of the apparent frustration of the magnetic coupling between the perovskite blocks, all <i>n</i> = 4, 5, 6 A_<i>n</i>Fe_<i>n</i>O_{3<i>n</i>–2} (A = Pb, Bi, Ba) feature robust antiferromagnetism with similar Néel temperatures of 623–632 K