MANGANESE(II,III) OXYBORATE, MN2OBO3 - A DISTORTED HOMOMETALLIC WARWICKITE-SYNTHESIS, CRYSTAL-STRUCTURE, BAND CALCULATIONS, AND MAGNETIC-SUSCEPTIBILITY

The manganese(II,III) oxyborate with the composition Mn2OBO3 has been synthesized by high-temperature techniques. X-ray studies show that cr ystals of the specimen, grown with borax as Aux, are monoclinic, with space group P2(1)/n, = 9.2866(7), b = 9.5333(10), c = 3.2438(3) Angstr om, and beta = 90.757(7)degrees. A model of the crystal structure has been refined with the 2064 most significant (l greater than or equal t o 5 . sigma(1)) X-ray reflections with sin(theta)/lambda less than or equal to 1.08 Angstrom(-1) to R = 0.40. The structure of Mn2OBO3 can b e considered to be a distorted modification of the orthorhombic warwic kite structure. The distortions, apparently caused by Jahn-Teller effe cts 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, P2(1)/n. The structural re sults 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 -13 2(1) K indicates an antiferromagnetic ordering at low temperature. The bond distances and calculated band valence sums indicate that the tri valent manganese ions are located in the two inner columns of the four -octahedra-wide walls. This metal charge distribution is supported by extended Huckel band calculations on some homometallic warwickites. Th e difference in metal coordination around one of the berate oxygen ato ms is reflected by a significant deviation of the berate group geometr y from the ideal trigonal symmetry. (C) 1995 Academic Press, Inc.