Crystallographical and chemical hypothesis for the formation process of CuO(s) and Cu(OH)(2(s)) from Na2Cu(OH)4((s)).

Addition of water into systems containing the solid Na2Cu(OH)(4(s)), and it s saturated solution allows the formation of two different solids: CuO(s) a nd Cu(OH)(4(s)). Cooper oxide is obtained by a slow addition and corrrespon ds to the equilibrium state: copper hydroxide is obtained by the fast addit ion of a large amount of water and is a metastable phase. In order to expla in these different behaviours we propose a hypothesis involving two differe nt reaction mechanisms. When systems containing Na2Cu(OH)(4(s)) are softly diluted, Na+ ions leave the crystal structure towards the solution. In para llel, the two longest Cu-O bonds of the octahedral surrounding of copper br eak down to give rise to free Cu(OH)(4)(2-) comples ions, stable in solutio n, which constitute elementary bricks for the formation of CuO4(s). Synthes is of Cu(OH)(2(s)) is only possible when dilution of systems containing Na2 Cu(OH)(4(s)) is carried out in a large amount of water to make OH- ion conc entration quickly decrease in order to avoid the formation of Cu(OH)(42-(aq )) complex ions, precursors of CuO(s). In these conditions, Na2Cu(OH)(4(s)) gives rise to Cu(OH)(2(s)) by a topotactic reaction. (C) 2000 academie des sciences/Editions scientifiques et medicales Elsevier SAS.