A. Lecerf

Etude physico-chimique et structurale de l'hydroxyde de cadmium μ Cd(OH)2

Abstract Pure γ Cd(OH) 2 has been synthesized by reaction of CdO with aqueous solutions of sodium hydroxide. D.T.A. and T.G.A. measurements have been performed. The enthalpy of the reaction: γCd(OH) 2 → CdO + H 2 O changes with the crystal sizes of the samples. the I.R. spectrum of γ Cd(OH) 2 is characterized by 3 absorption bands at 3580; 3530 and 3240 cm −1 . The crystal structure has been revisited, the atomic positions has been refined from X ray powder pattern. The crystal data are: monoclinic a = 5.688 (8) b = 10.28 (2) c = 3.420 (5) A β = 91.4 (1)°.

Synthese et etude de l'hydroxy-cadmiate de sodium Na2Cd(OH)4: Intermediaire reactionnel pour la preparation de Cd(OH)2 γ

Abstract Na2Cd(OH)4 is an intermediate product of great interest in the formation of Cd(OH)2 γ which is composing the active anodic material of discharged nickel-cadmium batteries. Synthesis an study of Na2Cd(OH)4 were carried out. Unit-cell parameters have been determined from the X-ray diffraction powder pattern. The structure is deriving from the NaCl type. The symmetry is orthorhombic: a ( A ) = 10.339(8) ; b ( A ) = 10.572(9) ; c ( A ) = 9.440(8) ; z = 8; ϱ calc = 2.91 g.cm−3.

The ternary diagram of system Na2O–CdO–H2O at room temperature

The ternary diagram of system Na2O–CdO–H2O at room temperature has been investigated. CdO and NaOH solids were used to prepare starting reactional systems. They were slowly hydrated by water vapor. Cadmium and sodium were analyzed in saturated solutions, at each step of the hydration process, using an ICP spectrometer. The solubility curves of γ-Cd(OH)2 and Na2Cd(OH)2 were drawn, and a value of the solubility product of γ-Cd(OH)2 is proposed: Ksp = 4 × 10−14 at room temperature.

Étude du type structural de γ-FeO(OH)(s) et comparaison avec la structure de Cu(OH)2(s)

Abstract Comparison of the crystal structures of Cu(OH) 2(s) and γ-FeO(OH) (s) shows a structural analogy between both solid phases, despite the difference of formulas. This analogy is related to similar connection modes of the polyhedra, which give rise to two-dimensional structures composed of wavy sheets. Lepidocrocite, γ-FeO(OH) (s) , is obtained from divalent iron hydroxide Fe(OH) 2(s) , which exhibits also a two-dimensional framework of brucite type. We propose a mechanism for this reaction, which is probably topotactic. The resulting lepidocrocite structural type allows strong deformations of the octahedral cationic coordination, contrarily to the brucite type. Deformations can be due to the loss of hydrogen atoms in γ-FeO(OH) (s) or to the strong Jahn–Teller effect that affects divalent copper in Cu(OH) 2(s) . If copper hydroxide cannot be isostructural with brucite as it is for Fe(OH) 2(s) , an analogy exists with lepidocrocite, even if copper has a pentahedral surrounding, different of that of trivalent iron, which is octahedral. In both crystal structures, one oxygen atom has a peculiar surrounding composed of only two cations and one hydrogen atom. This feature explains probably the low stability of this structural type.

Hypothèse cristallochimique des mécanismes de formation de CuO(s) et de Cu(OH)2(s) à partir de Na2Cu(OH)4(s)

Abstract 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 its saturated solution allows the formation of two different solids: CuO(s) and Cu(OH)2(s). Copper oxide is obtained by a slow addition and corresponds to the equilibrium state; copper hydroxide is obtained by the fast addition of a large amount of water and is a metastable phase. In order to explain these different behaviours, we propose a hypothesis involving two different reaction mechanisms. When systems containing Na2Cu(OH)4(s) are softly diluted, Na+ ions leave the crystal structure towards the solution. In parallel, the two longest Cu O bonds of the octahedral surrounding of copper break down to give rise to free Cu(OH)4 2– (aq) complex ions, stable in solution, which constitute elementary bricks for the formation of CuO(s). Synthesis of Cu(OH)2(s) is only possible when dilution of systems containing Na2Cu(OH)4(s) is carried out in a large amount of water to make OH– ion concentration quickly decrease, in order to avoid the formation of Cu(OH)4 2– (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.

Synthèse et structure cristalline de β-Cd2(OH)3Cl

Resume La structure cristalline de l’hydroxychlorure de cadmium β-Cd 2 (OH) 3 Cl a ete determinee sur monocristal. La maille est orthorhombique de groupe spatial Pnma. Les parametres cristallins sont les suivants: a (A) = 6,790 (4), b (A) = 7,408 (4), c (A) = 9,901 (4), Z = 4, ρ calc. = 4,15 g.cm −3 . La structure a ete affinee jusqu’a un facteur residuel final R = 0,028 ( R w = 0,055), en utilisant 476 reflexions independantes [ I > 3 σ( I )].La structure est isotype du compose correspondant du cuivre, l’atacamite Cu 2 (OH) 3 Cl. Elle est tridimensionnelle et peut etre decrite comme un enchevetrement croise de chaines d’octaedres de cadmium [CdO 4 Cl 2 ] et [CdO 5 Cl], reliees entre elles.