Fautes d'empilement dans les oxalates dihydratés des métaux divalents de la série mangésienne (Mg,Fe,Co,Ni,Zn,Mn)

The dihydrated oxalates of the magnesium series (Mg, Mn, Fe, Co, Ni, Zn) may exist either as a three-dimensional ordered phase, isostructural with humboldtine, or as a disordered phase with stacking faults parallel to (001) planes. It is possible to describe the well ordered structure as a layer succession, ABCDABCD... . Then the Stacking faults correspond to permutations between A and C on the one hand and between B and D on the other. These stacking faults give, in X-ray single-crystal photographs, a continuous diffuse intensity along the rows [hk] when h and k are odd. The analysis of this phenomena allows one to give the intensity distribution I(l) along these rows as a function of the rate of faults α. For α= 1 a new ordered structure ABABAB... is obtained which is known in the particular case of the cobalt oxalates. For α = ½ a uniform diffuse intensity is observed. For α < ½ the diffuse intensity has maxima for l_o = 2n + 1, while for α > ½ the maxima are at l_o= 2n (I_o is related to a double pseudo-orthorhombic cell).