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, Delta phi, 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 Delta phi 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. (C) 1997 Elsevier Science Ltd. All rights reserved.