Changes in magnetic couplings after chimie douce reactions: Magnetic structures of LiMnXO4(OD) (X = P, As)

The structural and magnetic changes that take place after the lithium exchange reactions of MnXO4 . D2O to give LiMnXO4(OD) (X=P, As) have been studied by magnetic susceptibility and variable-temperature neutron diffraction techniques. The crystal structures of the lithium derivatives have been refined at room temperature and 10 K by time-of-light neutron ponder diffraction data using the Rietveld method. These compounds behave as Curie-Weiss paramagnets at high temperatures and order antiferromagnetically below Neel temperatures of 28 and 30 K for X=P and As, respectively. The magnetic structures have been determined from the low-temperature neutron data. Although the four magnetic structures are antiferromagnetic, the magnetic couplings in the Mn-O-Mn infinite chains are ferromagnetic in the lithium derivatives and antiferromagnetic in the parent materials. These different magnetic behaviors are explained on the basis of subtle differences in the crystal structures after the lithium exchange reaction. (C) 1997 Academic Press.