Phase separation and ferroelectric ordering in charge-frustrated LuFe2O4−δ

The transmission electron microscopy observations of the charge ordering (CO) which governs the electronic polarization in LuFe2O4−δ clearly show the presence of a remarkable phase separation at low temperatures. Two CO ground states are found to adopt the charge modulations of Q1=(1/3,1/3,0) and Q2=(1/3+ε,1/3+ε,3/2), respectively. Our structural study demonstrates that the incommensurately Q2-modulated state is chiefly stable in samples with relatively lower oxygen contents. Data from theoretical simulations of the diffraction suggest that both Q1- and Q2-modulated phases have ferroelectric ordering. The effects of oxygen concentration on the phase separation and electric polarization in this layered system are discussed.