Neutron powder diffraction on monoclinic Li2CrCl4

The crystal structures of the room-temperature polymorphs of Li2CrCl4 and Li-deficient L(1.5)Cr(1.25)Cl(4), and of CrCl2 were determined by neutron powder diffraction studies using Rietveld's method. The refinements converged to final R(I) = 7.8%, 5.1% and 7.5%, respectively. Li2CrCl4 (and Li1.5Cr1.25Cl4) crystallize in a hitherto unknown deficient rocksalt superstructure with the space group C2/m (Z = 2). The vibrational modes of the Raman allowed lattice vibrations are given. The monoclinic distortion of the chromium compound is caused by the Jahn-Teller behavior of Cr2+. Li2CrCl4 mC14, which is metastable at ambient temperature as shown from cohesive-energy (MAPLE) calculations, decomposes below 585 K to LiCl and CrCl2 via Li deficient Li2-2xCr1+xCl4.