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 b een studied by magnetic susceptibility and variable-temperature neutro n diffraction techniques. The crystal structures of the lithium deriva tives have been refined at room temperature and 10 K by time-of-light neutron ponder diffraction data using the Rietveld method. These compo unds behave as Curie-Weiss paramagnets at high temperatures and order antiferromagnetically below Neel temperatures of 28 and 30 K for X=P a nd As, respectively. The magnetic structures have been determined from the low-temperature neutron data. Although the four magnetic structur es are antiferromagnetic, the magnetic couplings in the Mn-O-Mn infini te chains are ferromagnetic in the lithium derivatives and antiferroma gnetic in the parent materials. These different magnetic behaviors are explained on the basis of subtle differences in the crystal structure s after the lithium exchange reaction. (C) 1997 Academic Press.