Structural and Magnetic Phase Transitions in the A_nB_nO_3n–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_nB_nO_3n–2 homologous series with n = 5 and 6, respectively, with a unit cell related to the perovskite subcell a_p as a_p√2 × a_p × na_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 (n – 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 T ≈ 750 K for Pb₂Ba₂BiFe₅O₁₃ and T ≈ 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 n = 4, 5, 6 A_nFe_nO_3n–2 (A = Pb, Bi, Ba) feature robust antiferromagnetism with similar Néel temperatures of 623–632 K.