  We report a detailed examination the magnetic structure of anhydrous cupric
chloride CuCl2 carried out by powder neutron diffraction, magnetic
susceptibility and specific heat measurements on polycrystalline and single
crystal samples as well as an evaluation of the spin exchange interactions by
first principles density functional theory (DFT) calculations. Anhydrous CuCl2
shows one dimensional antiferromagnetic behavior and long range
antiferromagnetic ordering below a Neel temperature of 23.9 K. Neutron powder
and single crystal diffraction reveal that, below 23.9K, CuCl2 undergoes a
phase transition into an incommensurate magnetic structure (propagation vector
(1,0.2257,0.5) with a spin-spiral propagating along b and the moments confined
in the bc crystallographic plane. Our DFT calculations show that the
spin-spiral results from competing ferromagnetic nearest neighbor and
antiferromagnetic next-nearest neighbor spin-exchange interaction along the
spin chains. Implications for possible multiferroic behavior of CuCl2 are
discussed.
