Single crystals of K 3 Cu 2 O 4 were prepared by the azide/nitrate route from respective stoichiometric mixtures of KN 3 , KNO 3 and CuO, at 923 K, whereas powder samples were synthesised by solid state reaction of K 2 O, KCuO 2 and CuO, sealed in gold ampoules and treated at 723 K. According to the single crystal structure analysis (Cmcm, Z = 4, a = 6.1234(1), b = 8.9826(2), c = 10.8620(2) Å, R 1 = 0.044, R 2 = 0.107), the main structural feature are undulating 1 ∞ CuO 2 chains built up from planar, edge sharing CuO 4 square units. From an