Ferrimagnetic properties in FeII–III (oxy)hydroxycarbonate green rusts

A ferrimagnetic behaviour is observed in green rust GR(CO32−), the layered double hydroxide FeII–III hydroxycarbonate FeII4FeIII2(OH)12CO3, by using Mössbauer spectroscopy and susceptibility measurements. Moments lying within two-dimensional cation layers are parallel for same species and antiparallel between FeII and FeIII that lead to two magnetic ordering temperatures, 5.2 and 7 K, respectively. Mössbauer spectrum shows at 1.4 K a sextet with field distribution from 350 to 580 kOe for ferric ions, and an octet reflecting a mixture of states with field of 130 kOe and quadrupole splitting of −3.0 mm s−1 for ferrous ions. Ferric oxyhydroxycarbonate GR(CO32−)∗, FeIII6O12H8CO3, displays at 4 K a sextet with field distribution between 400 and 500 kOe. Transition is at around 80 K. Spectra of deprotonated GR(CO32−)∗ at a ferric molar fraction x = 0.88 display a ferric sextet behaving similarly with transition at around 70 K and two paramagnetic doublets, one ferrous and one ferric that remain at 4.2 K. This suggests that the deprotonation of GR(CO32−) gives magnetic domains of definite compositions at x = 1/3, 2/3 and 1 and long range order of FeIII ions along a translation a × √3 if a is the parameter of the cation hexagonal pavement is confirmed.

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