Temperature-induced structural changes inCaCl2,CaBr2, andCrCl2: A synchrotron x-ray powder diffraction study

Abstract

The halides ${\mathrm{CaCl}}_2,{\mathrm{CaBr}}_2$, and ${\mathrm{CrCl}}_2$ all adopt, at room temperature, the same distorted rutile structure, in orthorhombic space group $\mathit{Pnnm}$, known as the calcium chloride structure. Upon heating, ${\mathrm{CaCl}}_2$ and ${\mathrm{CaBr}}_2$ each undergoes a continuous transformation to the true tetragonal rutile structure, in space group $P{4}_2∕\mathit{mnm}$, the transition temperatures being 235 and 553 °C, respectively. By contrast, the structural change in ${\mathrm{CrCl}}_2$ 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 ${\mathrm{CrCl}}_6$ octahedron.