Synthesis and Crystal Structure of Na₇Cu₃O₈, Containing a New Type of Oxocuprate(III) Oligoanion

Abstract

Na 7 Cu 3 O 8 was prepared through oxidation of a Na 3 CuO 2 /NaCuO mixture (2:1) in dried oxygen at 450°C. Single crystals have been grown by annealing of Na 7 Cu 3 O 8 , in the presence of Na 2 O 2 , at 450°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 structure (P1, Z ϭ 1, a ϭ 5.5891(2), b ϭ 6.0945(2), c ϭ 7.8890(3) Å , α ϭ 110.059(2), β ϭ 108.669(2), γ ϭ 90.237(2)°) a new Cu 3 O 8 7Ϫ oxocuprate anion, consisting of three edge sharing CuO 4 squares, is the prominent structural feature.The discovery of high temperature conductivity in cuprates has stimulated very intensive research activity in this area. A huge number of new cuprates with different compositions were synthesised and investigated [1]. The most common partners of copper in these complex oxides are rare earth and earth alkaline metals. Alkali metal cuprates have attracted much less research activity, probably because of peculiarities of the solid state synthesis from the binary component oxides, normally used for obtaining such compounds.According to the structural and physical properties, all known alkali oxocuprates can be divided into two groups: cuprates(I), and cuprates(II) or (III). Compounds from the first group are transparent and diamagnetic (d 10 configuration of copper). The prominent structural feature here is a linearly coordinated copper(I) ion. The linear CuO 2 dumb-bells are condensed to various cuprate anions, depending on the Cu/O ratio and the alkali metal [2]. In the second group, cuprates(II) and (III), copper prefers a square planar coordination by oxygen. Although the oxidation state ϩ2 for copper in the solid state is, in general, rather common, Li 2 CuO 2 [3Ϫ5] is the only alkali oxocuprate(II) known so far. Here the CuO 4 squares, sharing common edges, are connected to infinite CuO 2 chains. Similar chains were also found in cuprates(III) with the formula ACuO 2 (A ϭ Li [6,7], Na [8Ϫ11], K [10,12], Rb [8,13], Cs [8,13]). Two edge sharing CuO 4 squares build up Cu 2 O 6 units in Li 3 CuO 3 [14], and Na 6 Cu 2 O 6 [15]. Additionally, a mixed-valent lithium cuprate(II,III) Li 3 Cu 2 O 4 [16, 17] is known which contains CuO 2 chains, too. On the boundary among the two classes mentioned here are cuprates(I,II): LiCu 2 O 2 [18Ϫ20], NaCu 2 O 2 [21] and LiCu 3 O 3 [18Ϫ20]. Here again, one can clearly distinguish between copper (I) Ϫ linear coordination, and (II) Ϫ square planar coordination. In this work we present a new cuprate(III), Na 7 Cu 3 O 8 , containing a new type of an oxocuprate anion.