Ni-Fe pyroaurite-type hydroxychlorides were prepared by aerial oxidation of Ni(II)-Fe(II) hydroxides precipitated in aqueous solution with various P = Fe/Ni ratios. When P< or =1/3, Ni(II)-Fe(II)-Fe(III) hydroxychlorides characterized by a specific Fe(III)/[Fe(II)+Ni(II)] ratio of 1/3, corresponding to the idealized formula of Ni II (sub 3-x) Fe IIx Fe III (OH) 8 Cl.nH 2 O) (with 0< or =x< or =3), were obtained at the end of the first stage of oxidation. In a second reaction stage, these hydroxychlorides oxidize with deprotonation of hydroxyl ions into O (super 2-) ions, i.e. the remaining Fe II (OH) 2 groups are transformed into Fe III OOH groups. Along with the Ni(II)-Fe(III) hydroxychloride which contains a part of the Fe III OOH groups a second phase is obtained. It is an amorphous Fe(III) or Ni(II)-Fe(III) oxyhydroxide when 1/3<P< or =3/2, and a ferric oxyhydroxide identified as gamma -FeOOH (lepidocrocite) when P>3/2. On the other side of the domain, when P<1/3, the Fe(III)/[Fe(II)+Ni(II)] ratio cannot reach the specific value of 1/3; this gives rise to a pyroaurite-type Ni(II)-Fe(III) hydroxychloride with a lower chloride content, that is with an average composition of Ni II (sub 3+gamma ) Fe III (sub 1-gamma ) (OH) 8 Cl (sub 1-gamma ) .nH 2 O where gamma = {[4/(1+P)]-3}, down to minimum Fe(III) and Cl contents corresponding to gamma = 1/3 (P = 1/5). The in situ mechanisms of oxidation of Ni(II)-Fe(II) hydroxides into Ni(II)-Fe(II)-Fe(III) hydroxychlorides are discussed.

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