title: Synthesis, structure and magnetic ordering of the mullite-type Bi2Fe(4-x)CrxO9 solid solutions with a frustrated pentagonal Cairo lattice.
abstract: Highly homogeneous mullite-type solid solutions Bi2Fe(4-x)CrxO9 (x = 0.5, 1, 1.2) were synthesized using a soft chemistry technique followed by a solid-state reaction in Ar. The crystal structure of Bi2Fe3CrO9 was investigated using X-ray and neutron powder diffraction, transmission electron microscopy and (57)Fe Mössbauer spectroscopy (S.G. Pbam, a = 7.95579(9) Å, b = 8.39145(9) Å, c = 5.98242(7) Å, RF(X-ray) = 0.022, RF(neutron) = 0.057). The ab planes in the structure are tessellated with distorted pentagonal loops built up by three tetrahedrally coordinated Fe sites and two octahedrally coordinated Fe/Cr sites, linked together in the ab plane by corner-sharing forming a pentagonal Cairo lattice. Magnetic susceptibility measurements and powder neutron diffraction show that the compounds order antiferromagnetically (AFM) with the Néel temperatures decreasing upon increasing the Cr content from TN ∼ 250 K for x = 0 to TN ∼ 155 K for x = 1.2. The magnetic structure of Bi2Fe3CrO9 at T = 30 K is characterized by a propagation vector k = (1/2,1/2,1/2). The tetrahedrally coordinated Fe cations form singlet pairs within dimers of corner-sharing tetrahedra, but spins on the neighboring dimers are nearly orthogonal. The octahedrally coordinated (Fe,Cr) cations form antiferromagnetic up-up-down-down chains along c, while the spin arrangement in the ab plane is nearly orthogonal between nearest neighbors and collinear between second neighbors. The resulting magnetic structure is remarkably different from the one in pure Bi2Fe4O9 and features several types of spin correlations even on crystallographically equivalent exchange that may be caused by the simultaneous presence of Fe and Cr on the octahedral site.
authors.affiliation: Department of Chemistry, Moscow State University, 119991 Moscow, Russia. a.abakumov@skoltech.ru.
authors.forename: M G
authors.initials: MG
authors.lastname: Rozova
authors.affiliation: Department of Chemistry, Moscow State University, 119991 Moscow, Russia. a.abakumov@skoltech.ru.
authors.forename: V V
authors.initials: VV
authors.lastname: Grigoriev
authors.affiliation: Frank Laboratory of Neutron Physics, Joint Institute for Nuclear Research, 6 Joliot-Curie Street, 141980 Dubna, Russia.
authors.forename: I A
authors.initials: IA
authors.lastname: Bobrikov
authors.affiliation: Department of Chemistry, Moscow State University, 119991 Moscow, Russia. a.abakumov@skoltech.ru.
authors.forename: D S
authors.initials: DS
authors.lastname: Filimonov
authors.affiliation: Low Temperature Physics and Superconductivity Department, Department of Physics, Moscow State University, 119991 Moscow, Russia.
authors.forename: K V
authors.initials: KV
authors.lastname: Zakharov
authors.affiliation: Low Temperature Physics and Superconductivity Department, Department of Physics, Moscow State University, 119991 Moscow, Russia and Theoretical Physics and Applied Mathematics Department, .ral Federal University, 620002 Ekaterinburg, Russia.
authors.forename: O S
authors.initials: OS
authors.lastname: Volkova
authors.affiliation: Low Temperature Physics and Superconductivity Department, Department of Physics, Moscow State University, 119991 Moscow, Russia and Theoretical Physics and Applied Mathematics Department, .ral Federal University, 620002 Ekaterinburg, Russia.
authors.forename: A N
authors.initials: AN
authors.lastname: Vasiliev
authors.affiliation: Department of Chemistry, Moscow State University, 119991 Moscow, Russia. a.abakumov@skoltech.ru.
authors.forename: E V
authors.initials: EV
authors.lastname: Antipov
authors.affiliation: Experimental Physics VI, Center for Electronic Correlations and Magnetism, Institute of Physics, University of Augsburg, 86159 Augsburg, Germany.
authors.forename: A A
authors.initials: AA
authors.lastname: Tsirlin
authors.affiliation: Department of Chemistry, Moscow State University, 119991 Moscow, Russia. a.abakumov@skoltech.ru and EMAT, University of Antwerp, Groenenborgerlaan 171, B-2020, Antwerp, Belgium.
authors.forename: A M
authors.initials: AM
authors.lastname: Abakumov