Manganese(II,III) Oxyborate, Mn2OBO3: A Distorted Homometallic Warwickite—Synthesis, Crystal Structure, Band Calculations, and Magnetic Susceptibility

Abstract

The manganese(II,III) oxyborate with the composition Mn2OBO3 has been synthesized by high-temperature techniques. X-ray studies show that crystals of the specimen, grown with borax as flux, are monoclinic, with space group P21/n, = 9.2866(7), b = 9.5333(10), c = 3.2438(3) Å, and β = 90.757(7)°. A model of the crystal structure has been refined with the 2064 most significant (l ≥ 5 · σ1) X-ray reflections with sin(θ)/λ ≤ 1.08 Å-1 to R = 0.40. The structure of Mn2OBO3 can be considered to be a distorted modification of the orthorhombic warwickite structure. The distortions, apparently caused by Jahn-Teller effects induced by the Mn3+ ions, remove the mirror symmetry of the parent undistorted warwickite. As a consequence, the space group symmetry is lowered from Pnam to one of its subgroups, P21/n. The structural results as well as the measured magnetic susceptibilities indicate high-spin manganese ions. The magnetic susceptibilities in the temperature region 110-300 K follow the Curie-Weiss law. The Weiss constant of -132(1) K indicates an antiferromagnetic ordering at low temperature. The bond distances and calculated bond valence sums indicate that the trivalent manganese ions are located in the two inner columns of the four-octahedra-wide walls. This metal charge distribution is supported by extended Hückel band calculations on some homometallic warwickites. The difference in metal coordination around one of the borate oxygen atoms is reflected by a significant deviation of the borate group geometry from the ideal trigonal symmetry.