We report a detailed examination of the magnetic structure of anhydrous cupric chloride ${\text{CuCl}}_2$ 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 ${\text{CuCl}}_2$ shows one-dimensional antiferromagnetic behavior and long-range antiferromagnetic ordering below a Néel temperature of 23.9 K. Neutron powder and single-crystal diffraction reveal that, below 23.9 K, ${\text{CuCl}}_2$ 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 interactions along the spin chains. Implications for possible multiferroic behavior of ${\text{CuCl}}_2$ are discussed.

• Received 16 April 2009

DOI:https://doi.org/10.1103/PhysRevB.80.024404