Magnetic Properties of the Novel Low-Dimensional Cuprate   Na5RbCu4(AsO4)4Cl2
  The magnetic properties of a new compound, Na5RbCu4(AsO4)4Cl2 are reported. The material has a layered structure comprised of square Cu4O4 tetramers. The Cu ions are divalent and the system behaves as a low-dimensional S=1/2 antiferromagnet. Spin exchange in Na5RbCu4(AsO4)4Cl2 appears to be quasi-two-dimensional and non-frustrated. Measurements of the bulk magnetic susceptibility and heat capacity are consistent with low-dimensional magnetism. The compound has an interesting, low-entropy, magnetic transition at T = 17 K. 
J. A. Clayhold, M. Ulutagay-Kartin, S.-J. Hwu, H.-J. Koo, M.-H.   Whangbo, A. Voigt, and K. Eaiprasertsak