X-Ray Diffraction and Nuclear Quadrupole Resonance Studies of Chromium Trichloride

The infinite‐layer compound CrCl₃ is found to undergo a first‐order phase transformation near 240°K. The crystal structures of the high‐ and low‐temperature phases have been elucidated by detailed x‐ray diffraction and ³⁵Cl nuclear quadrupole resonance studies. The previously reported structure for CrCl₃ is shown to be incorrect. The present single‐crystal diffraction results at 298° and 225°K give monoclinic (C2/m: a₀=5.959 Å, b₀=10.321, c₀=6.114 Å, β=108.49°) and rhombohedral ($\text{R3̄}$ : a₀=5.942 Å, c₀=17.333 Å) structures, respectively. The ³⁵Cl quadrupole resonance frequencies, measured in polycrystalline samples, have been compared with results of field‐gradient calculations based on the ionic point‐charge model. The ionic contribution accounts for a large fraction of the total gradient making it impossible to evaluate the covalency parameters of the Cr–Cl bonds from the measured quadrupole coupling constants.