High-temperature structural phase transition in multiferroic LiCu_2O_2
  LiCu_2O_2 single crystals were studied in the temperature range 300-1100 K by means of heating-cooling curves of differential thermal analysis (DTA), thermogravimetry (TG), X-ray powder diffraction and electrical measurements. A reversible first-order phase transition between orthorhombic and tetragonal phases was found to take place at 993 K. At the transition, a peak is observed in the DTA curves, as well as jumps of the unit cell parameters and electrical resistivity. Considering the crystal structure of LiCu_2O_2 and the entropy change associated with the phase transition, it is concluded that the phase transition is related to processes of order-disorder of the Cu2+ and Li+ cations onto their crystallographic positions. 
S. A. Ivanov, A. A. Bush, K. E. Kamentsev, E. A. Tishchenko, M.   Ottosson, R.Mathieu, P. Nordblad
cond-mat.mtrl-sci