title: Structural and magnetic phase transitions in the A(n)B(n)O(3n-2) anion-deficient perovskites Pb2Ba2BiFe5O13 and Pb(1.5)Ba(2.5)Bi2Fe6O16.
abstract: Novel anion-deficient perovskite-based ferrites Pb2Ba2BiFe5O13 and Pb(1.5)Ba(2.5)Bi2Fe6O16 were synthesized by solid-state reaction in air. Pb2Ba2BiFe5O13 and Pb(1.5)Ba(2.5)Bi2Fe6O16 belong to the perovskite-based A(n)B(n)O(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(101)p crystallographic shear (CS) planes. The CS operation results in (101)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(1.5)Ba(2.5)Bi2Fe6O16 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(1.5)Ba(2.5)Bi2Fe6O16 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(n)Fe(n)O(3n-2) (A = Pb, Bi, Ba) feature robust antiferromagnetism with similar Néel temperatures of 623-632 K.
authors: Electron Microscopy for Materials Research, University of Antwerp, Groenenborgerlaan 171, B-2020, Antwerp, Belgium. artem.abakumov@ua.ac.be
authors: A M
authors: AM
authors: Abakumov
authors: M
authors: M
authors: Batuk
authors: A A
authors: AA
authors: Tsirlin
authors: O A
authors: OA
authors: Tyablikov
authors: D V
authors: DV
authors: Sheptyakov
authors: D S
authors: DS
authors: Filimonov
authors: K V
authors: KV
authors: Pokholok
authors: V S
authors: VS
authors: Zhidal
authors: M G
authors: MG
authors: Rozova
authors: E V
authors: EV
authors: Antipov
authors: J
authors: J
authors: Hadermann
authors: G
authors: G
authors: Van Tendeloo