X-ray structure analyses of chloranil above and below phase transition temperature

The crystal structure of chloranil above and below the phase transition temperature, T_c (= 90 K), has been studied by single crystal X-ray diffraction at 20, 40, 60, 70, 80, 85, 90, 95, 100, 110, 130, and 150 K. Below T_c the difference, Δφ, between the angle of an a axis and an O...O vector in a chloranil molecule [projected to (010) plane] at any temperature and the angle at T_c increases with a power law relation with a decrease in temperature. It shows that the Δφ is an order parameter. It has been found that the mean-square-displacements of O and Cl atoms increase around T_c. The intermolecular nearest O...C distance decreases with a decrease in temperature above T_c. However, it splits into two types below T_c, and their mean value does not decrease with a decrease in temperature. We have concluded that two types of molecular rotations of clockwise and counterclockwise for neighboring molecules induce this phase transition to avoid too small O...C distance below 90 K.