The structure of the chromium dihalides $\mathrm{Cr}{X}_2$ ($X$ = Br, I) resembles that of many other transition-metal dihalides and dichalcogenides, but the Jahn-Teller distortion arising from the ${\mathrm{Cr}}^{2+}$ ions makes the $\mathrm{Cr}{X}_2$ structures distinct from most of these other compounds. Nevertheless, the $\mathrm{Cr}{X}_2$ compounds have received little study, with the helimagnetic order of ${\mathrm{CrBr}}_2$ and ${\mathrm{CrI}}_2$ being reported only recently. We used neutron powder diffraction to study the structure of ${\mathrm{CrBr}}_2$ and ${\mathrm{CrI}}_2$ at high temperature and found interpolytypic transitions in both of these compounds. Monoclinic ${\mathrm{CrBr}}_2$ ($C2/m, \beta =94.5^{\circ }$) and orthorhombic ${\mathrm{CrI}}_2$ ($Cmc{2}_1$) transition to the same polytype at high temperature, a distorted version of the ${\mathrm{CdCl}}_2$ structure with a $C2/m$ space group and a monoclinic $\beta$ angle of ${117}^{\circ }$. The phase transition occurs by layer sliding for ${\mathrm{CrBr}}_2$ and an additional change of layer orientation for ${\mathrm{CrI}}_2$. The intralayer structure is largely unchanged, with a possible small enhancement of the Jahn-Teller distortion on warming.

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• Jahn-Teller effect
• Specific phase transitions
• Neutron diffraction