  The layered FeTe2O5Cl compound was studied by specific-heat, muon spin
relaxation, nuclear magnetic resonance, dielectric, as well as neutron and
synchrotron x-ray diffraction measurements, and the results were compared to
isostructural FeTe2O5Br. We find that the low-temperature ordered state,
similarly as in FeTe2O5Br, is multiferroic - the elliptical amplitude-modulated
magnetic cycloid and the electric polarization simultaneously develop below 11
K. However, compared to FeTe2O5Br, the magnetic elliptical envelop rotates by
75(4) deg and the orientation of the electric polarization is much more
sensitive to the applied electric field. We propose that the observed
differences between the two isostructural compounds arise from geometric
frustration, which enhances the effects of otherwise subtle Fe3+ (S = 5/2)
magnetic anisotropies. Finally, x-ray diffraction results imply that, on the
microscopic scale, the magnetoelectric coupling is driven by shifts of the O1
atoms, as a response to the polarization of the Te4+ lone-pair electrons
involved in the Fe-O-Te-O-Fe exchange bridges.
