Broadband dielectric spectroscopy on transition metal oxides: Is LuFe2O4 ferroelectric?

The mixed valence system LuFe2O4 was proposed to show a novel type of ferroelectricity based on charge order within triangular Fe-O-double layers at 330 K. However, an unambiguous evidence for ferroelectricity by means of dielectric polarization measurement, e.g. hysteretical P-E-loops, is difficult to give due to the relatively high residual conductivity in this material. We show results of temperature-dependent dielectric measurements on the potentially multiferroic material LuFe2O4 in a broad frequency range from mHz to GHz. Our measurements on a single-crystalline sample show very high magnitudes of both real and imaginary part of complex permittivity, decreasing towards low temperatures and high frequencies according to those reported in literature. However in the temperature range from 200-400 K this dispersive behaviour is well-described by a standard equivalent-circuit model considering Maxwell-Wagner-type contacts and variable range hopping AC-conductivity, without a pronounced contribution of intrinsic dipolar polarization and thus the ferroelectric character of the charge order in LuFe2O4 has to be questioned.