Symmetry-dependent strong reduction of the spin exchange interactions in Cs2CuCl4 by the 6p orbitals of Cs+ ions.

Despite a three-dimensional arrangement of its CuCl(4)(2-) ions, the magnetic properties of Cs(2)CuCl(4) are explained by a two-dimensional frustrated triangular antiferromagnetic spin-lattice. The origin of this low-dimensional magnetism was explored by evaluating the spin exchange interactions of A(2)CuCl(4) (A = Cs, Rb, K, Na) on the basis of first principles density functional calculations. The calculated spin exchange parameters agree with experiment only when the Cs(+) ions located between the CuCl(4)(2-) ions are not neglected. The antiferromagnetic spin exchange interaction between adjacent CuCl(4)(2-) ions is strongly reduced by the 6p orbitals of the intervening Cs(+) ions when the arrangement of the CuCl(4)(2-) and Cs(+) ions has either mirror-plane or inversion symmetry. The observed magnetism of Cs(2)CuCl(4) arises from this symmetry-dependent participation of the 6p orbitals of the Cs(+) ions in the spin exchange interactions between CuCl(4)(2-) ions.