Synthesis, structure, and magnetic properties of the layered copper(II) oxide Na2Cu2TeO6

A new quaternary layered transition-metal oxide, Na2Cu2TeO6, has been synthesized under air using stoichiometric (with respect to the cationic elements) mixtures of Na2CO3, CuO, and TeO2. Na2Cu2TeO6 crystallizes in the monoclinic space group C2/m with a = 5.7059(6) angstrom, b = 8.6751(9) angstrom, c = 5.9380(6) angstrom, beta = 113.740(2)degrees, V= 269.05(5) angstrom(3), and Z = 2, as determined by single-crystal X-ray diffraction. The structure is composed (2)(infinity)[Cu2TeO6] layers with the Na atoms located in the octahedral voids between the layers. Na2Cu2TeO6 is a green nonmetallic compound, in agreement with the electronic structure calculation and electrical resistance measurement. The magnetic susceptibility shows Curie-Weiss behavior between 300 and 600 K with an effective moment of 1.85(2) mu(B)/Cu-II and Theta(c) = -87(6) K. A broad maximum at 160 K is interpreted as arising from short-range one-dimensional antiferromagnetic correlations. With the aid of the technique of magnetic dimers, the short-range order was analyzed in terms of an alternating chain model, with the surprising result that the stronger intrachain coupling involves a super-superexchange pathway with a Cu-Cu separation of > 5 angstrom. The J(2)/J(1) ratio within the alternating chain refined to 0.10(1), and the spin gap is estimated to be 127 K.