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)°, V = 269.05(5) (angstrom)3, and Z = 2, as determined by single-crystal X-ray diffraction. The structure is composed of ^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/CuII and vc = -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 J2/J1 ratio within the alternating chain refined to 0.10(1), and the spin gap is estimated to be 127 K.