The crystal structure of Mn3As2(I)

Abstract

AbstractA whole series of closely related structures exists for compositions close to Mn3As2. One basic structure, designated Mn3As2(I) is described here. It is monoclinic, space groupC2/m,a= 1385.6(6) pm,b= 377.7(2) pm,c= 1362.2(8) pm,β= 107.79(3)°,V= 0.6788 nm3withZ= 8 formula units per cell. The refinement of the occupancy parameters showed that one Mn position is occupied to only 79.1(3)% in the crystal used for the structure determination. Thus the composition can be formulated as Mn3 – xAs2withx= 0.104(1). The residual for this refinement isR= 0.034 for 65 variable parameters and 1584 structure factors. Twinning of Mn3As2(I) wrongly suggests aFcentered orthorhombic cell with a ten times larger cell volume. The structure is intermediate between the NiAs and Ni2In type structures. The As atoms form close packed planes with hexagonal stacking, which is interrupted by one cubic close packed layer alternatingly after six and eight hexagonal layers. The As environments of the seven independent Mn atoms vary from tetrahedral over square pyramidal and trigonal bipyramidal to octahedral, augmented by additional Mn atoms. The four independent As atoms are situated in trigonal prisms formed by the Mn atoms with one ore two more Mn neighbours outside the rectangular faces of the prisms. The compound is a metallic conductor. This is rationalized from the high coordination numbers of all atoms and the many weak Mn – Mn bonds with bond lengths all greater than 278 pm.