LAUSR

Abstract Single crystals of manganese pyro-arsenite [systematic name: dimanganese(II) pentaoxidodiarsenate(III)], Mn2As2O5, were grown under hydrothermal conditions. Mn2As2O5 adopts a novel structure type as revealed by single crystal X-ray diffraction from a crystal twinned by pseudo-merohedry. The crystal structure comprises of five independent MnII sites (two located on inversion centres, three in general sites) and two pairs of [As2O5]4- pyro-anions made up from corner-sharing trigonal-pyramidal [AsO3] units in general positions. Four of the five MnII sites exhibit distorted octahedral [MnO6] coordination polyhedra whereas one has five oxygen ligands in a distorted square-pyramidal environment. The manganese-oxygen polyhedra and [As2O5]4- pyro-anions share corners and edges under formation of a three-dimensional network with channels extending along [001]. The 4s2 electron lone pairs at the AsIII atoms are stereochemically active and point into the centre of the channels. Results of Raman spectroscopic studies are in agreement with the structural features of the pyro-arsenite anion. Magnetic susceptibility measurements on a powder sample confirm all Mn atoms to be in oxidation state ​+ ​II. An antiferromagnetic ordering below 14.3 ​K occurs that most likely points to a magnetic structure supporting a weak ferromagnetic component (‘weak ferromagnetism’).