Ordering in FeII-III hydroxysalt green rusts from XRD and Mossbauer analysis (chloride, carbonate, sulphate, oxalate ...);: about the structure of hydrotalcite-like compounds

The domain of existence of FeII-III hydroxysalt green rusts (GR) is characterised by the ratio x = [Fe-III]/[Fe-total] lying in [1/4, 1/3] as it is for any layered double hydroxide (LDH). Since Fe-III cations balance the charges of intercalated anions, cation and anion distributions are correlated. However, the distribution of anions depends on their charge, shape and size. For Cl- and CO32-, x belongs to the complete range whereas for C2O42- and SO42- it is limited to x = 1/4 and 1/3, respectively. Mossbauer spectra present in all cases two ferrous and one ferric doublets, which characterise each GR by their relative abundance. Intermediate values of x are in fact obtained by intrincate domains of two types, one where the cation periodicity is 2xa (0) and the other one root 3xa (0) if a (0) is the lattice parameter.