Temperature-induced structural changes in CaCl2, CaBr2, and CrCl2:: A synchrotron x-ray powder diffraction study -: art. no. 214114

The halides CaCl2,CaBr2, and CrCl2 all adopt, at room temperature, the same distorted rutile structure, in orthorhombic space group Pnnm, known as the calcium chloride structure. Upon heating, CaCl2 and CaBr2 each undergoes a continuous transformation to the true tetragonal rutile structure, in space group P4(2)/mnm, the transition temperatures being 235 and 553 degrees C, respectively. By contrast, the structural change in CrCl2 upon heating is just further elongation of octahedra already lengthened by Jahn-Teller effects, and no phase transition occurs. The orthorhombic structure is maintained by a strong and temperature-dependent geometrical coupling of the orthorhombic strain to the order parameter, represented by the tilt angle of the CrCl6 octahedron.