CRYSTAL-STRUCTURE OF MONOCLINIC MNSE2O5 AND COMPARATIVE MAGNETIC STUDY OF MONOCLINIC AND ORTHORHOMBIC VARIETIES

Manganese diselenite, MnSe2O5, exhibits two allotropic forms. The structure of the monoclinic m variety has been determined (a = 7.960(3), b = 10.938(1), c = 5.850(2)Å, β = 118.27(2)°, Z = 4, space group = C2/c) and compared with the structure orthorhombic one, o. Both structures present similarities: (1) zigzagging chains of octahedra [MnO6], running along the [001] axis; (2) each octahedron shares opposite edges with the two neighbor ones; (3) Mn-Mn intrachain distances are similar, 3.4422(8) Å for the monoclinic vs 3.394(2) Å for the orthorhombic; (4) each chain is interconnected with six other chains via Se2O2-5-anions. Differences appear between the m and o forms are also apparent: (1) the zigzagging angle is sharper in the former; (2) the bridge angle Se-O-Se is higher in the m form in comparison to other diselenites including the o form; (3) the interchain distances range from 5.169 Å to 7.965 Å for m, and between 6.190 Å to 6.822 Å for the o form. Differences are also noticed by infrared spectra. The phase transformation m ⇔ o does not occur by heating, even up to decomposition. A magnetic study of both varieties shows a weak antiferromagnetic interaction between manganese ions along a chain and reveals the occurence of a long range antiferromagnetic ordering at significantly different temperatures: TN = 9 and 5.2 K for m and o species, respectively. A magnetostructural correlation is proposed. © 1993 Academic Press, Inc.