Structural Dimerization In The Commensurate Magnetic Phases Of Nafe(Wo4)(2) And Mnwo4

The structural distortion and magnetoelastic coupling induced through commensurate magnetism has been investigated by neutron diffraction in structurally related MnWO4 and NaFe(WO4)(2). Both systems exhibit a competition of incommensurate spiral and commensurate spin up-up-down-down ordering along the magnetic chains. In the latter commensurate phases, the alternatingly parallel and antiparallel arrangement of Fe3+ or Mn2+ moments leads to sizable bond-angle modulation and thus to magnetic dimerization. For NaFe(WO4)(2) this structural distortion has been determined to be strongest for the low-field up-up-down-down arrangement, and the structural refinement yields a bond-angle modulation of +/- 1.15(16) degrees. In the commensurate phase of MnWO4, superstructure reflections signal a comparable structural dimerization and thus strong magneto-elastic coupling different to that driving the multiferroic order. Pronounced anharmonic second- and third-order reflections in the incommensurate and multiferroic phase of MnWO4 result from tiny commensurate fractions that can depin multiferroic domains.