Charge-stripe order in the electronic ferroelectric LuFe2O4

The structural features of the charge ordering states in LuFe2O4 are characterized by in situ cooling transmission electron microscopy observations from 300 K down to 20 K. Two distinctive structural modulations, a major q(1)=(1/3,1/3,2) and a weak q(2)=q(1)/10+(0,0,3/2), have been well determined at the temperature of similar to 20 K. Systematic analysis demonstrates that the charges at low temperatures are well crystallized in a charge-stripe phase, in which the charge-density wave behavior in a nonsinusoidal fashion results in elemental electric dipoles for ferroelectricity. It is also noted that the charge ordering and ferroelectric domains often change markedly with lowering temperatures and yield a rich variety of structural phenomena.