Structure, charge ordering and physical properties of LuFe2O4

Microstructural properties, phase transitions, and charge ordering of LuFe2O4 have been extensively investigated by means of transmission electron microscopy (TEM) in a large temperature range from 20 to 550 K. The experimental results demonstrate that the LuFe2O4 crystal is commonly modulated by charge ordering (CO), which is often recognizable by superstructure reflections. The (001) twinning domains as a common defect often appear in the LuFe2O4 crystals along the c-axis direction, with the crystals across each (001) boundary rotated by 180 degrees with respect to one another. The in situ cooling TEM observations from 300 K down to 20 K reveal remarkable alternations of the superstructures, suggesting a complex CO process in the present system. Careful analysis shows that the CO in the frustrated ground state is characterized by a modulation with a wave vector of q(1)=(1/3 1/3 2). In situ heating TEM observations from 300 to 550 K clearly reveal that the CO modulation in LuFe2O4 becomes invisible above a critical temperature of about T-C=530 K. These facts suggest that the CO should be the essential driving force for the structural transitions and ferroelectricity observed in this kind of layered material. Experimental measurements on the ferroelectricity show that the LuFe2O4 material, in general, has a large dielectric constant of about 10 000 at room temperature. In order to understand the properties of low-temperature phase transitions, the magnetization and specific heat from 300 to 4 K have been briefly discussed.