  Microstructure properties, phase transitions, and charge ordering in the
LuFe2O4 materials have been extensively investigated by means of transmission
electron microscopy (TEM). The (001) twins as a common defect frequently appear
in the LuFe2O4 crystals along the c-axis direction, and the crystals across
each boundary are rotated by 180 degree with respect to one another. The
in-situ cooling TEM observations reveal remarkable temperature dependence of
the superstructures in correlation with charger ordering (CO). The Fe2+, Fe3+,
and Fe+2.5 ions are found to be crystallized in ordered stripes at the
frustrated ground state characterized by a modulation with the wave vector of
q1 =(1/3 1/3 2). In-situ heating TEM observations on LuFe2O4 clearly
demonstrate that this modulation becomes evidently invisible above a critical
temperature of about Tc=530K. These facts suggest that the CO should be the
essential driving force for the structural transitions and ferroelectricity
observed in present system.
