Synthesis, structure and magnetic ordering of the mullite-type Bi2Fe4−xCrxO9 solid solutions with a frustrated pentagonal Cairo lattice

Abstract

Highly homogeneous mullite-type solid solutions Bi₂Fe_4−xCr_xO₉ (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 ⁵⁷Fe Mössbauer spectroscopy (S.G. Pbam, a = 7.95579(9) Å, b = 8.39145(9) Å, c = 5.98242(7) Å, R_F(X-ray) = 0.022, R_F(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 T_N ∼ 250 K for x = 0 to T_N ∼ 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.