Ordering in FeII–III Hydroxysalt green rusts from XRD and Mössbauer analysis (chloride, carbonate, sulphate, oxalate...); about the structure of hydrotalcite-like compounds
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The domain of existence of FeII–III hydroxysalt green rusts (GR) is characterised by the ratio x = [FeIII]/[Fetotal] lying in [1/4, 1/3] as it is for any layered double hydroxide (LDH). Since FeIII 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 CO3 2−, x belongs to the complete range whereas for C2O4 2− and SO4 2− it is limited to x = 1/4 and 1/3, respectively. Mössbauer 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 2×a 0 and the other one √3×a 0 if a 0 is the lattice parameter.
Key wordsgreen rust Mössbauer ordering chloride carbonate sulphate oxalate layered double hydroxide
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