  We report a neutron diffraction study of the multiferroic mechanism in
(ND4)2FeCl5D2O, a molecular compound that exhibits magnetically induced
ferroelectricity. This material exhibits two successive magnetic transitions on
cooling: a long-range order transition to an incommensurate (IC) collinear
sinusoidal spin state at TN=7.3 K, followed by a second transition to an IC
cycloidal spin state at TFE=6.8 K, the later of which is accompanied by
spontaneous ferroelectric polarization. The cycloid structure is strongly
distorted by spin-lattice coupling as evidenced by the observations of both odd
and even higher-order harmonics associated with the cycloid wave vector, and a
weak commensurate phase that coexists with the IC phase. The appearance of the
2nd-order harmonic coincides with the onset of the electric polarization,
thereby providing unambiguous evidence that the induced electric polarization
is mediated by the spin-lattice interaction. Our results for this system, in
which the orbital angular momentum is expected to be quenched, are remarkably
similar to those of the prototypical TbMnO3, in which the magnetoelectric
effect is attributed to spin-orbit coupling.
