Synthesis, structure and properties of new chain cuprates, Na3Cu2O4 and Na8Cu5O10

Na3Cu2O4 and Na8Cu5O10 were prepared via the azide/nitrate route from stoichiometric mixtures of the precursors CuO, NaN3 and NaNO3. Single crystals have been grown by subsequent annealing of the as prepared powders at 500 °C for 2000 h in silver crucibles, which were sealed in glass ampoules under dried Ar. According to the X-ray analysis of the crystal structures (Na3Cu2O4: P21/n, image, image, image, c=8.0261(3) Å, image, 2516 independent reflections, R1(all)=0.0813, wR2 (all)=0.1223; Na8Cu5O10: Cm, image, image, image, image, image, 2949 independent reflections, R1(all)=0.0349, wR2 (all)=0.0850), the main feature of both crystal structures are CuO2 chains built up from planar, edge-sharing CuO4 squares. From the analysis of the Cu–O bond lengths, the valence states of either +2 or +3 can be unambiguously assigned to each copper atom. In Na3Cu2O4 these ions alternate in the chains, in Na8Cu5O10 the periodically repeated part consists of five atoms according to CuII–CuII–CuIII–CuII–CuIII. The magnetic susceptibilities show the dominance of antiferromagnetic interactions. At high temperatures the compounds exhibit Curie–Weiss behaviour (Na3Cu2O4: image, image, Na8Cu5O10: image, image, magnetic moments per divalent copper ion). Antiferromagmetic ordering is observed to occur in these compounds below 13 K (Na3Cu2O4) and 24 K (Na8Cu5O10).