A novel situation in which long-range magnetic order coexists with (super)paramagnetism occurs in a new graphite biintercalation compound: FeCl3–NiCl2–graphite. Several compounds have been prepared by sequential intercalation of the two intercalates and have been characterized by powder diffractometry, X-ray precession photographs, high-precision X-ray single-crystal measurements, chemical analysis, and high-temperature susceptibility measurements. All measurements are consistent with the stacking sequence –C–NiCl2–C–FeCl3–C– and corroborate the crystallographic integrity of separate NiCl2 and FeCl3 sublattices. Since the in-plane magnetic interactions of the NiCl2 and FeCl3 layers are, respectively, ferromagnetic and antiferromagnetic, the two types of chloride layers have been probed independently by combining low-temperature superconducting quantum interference device magnetometry and 57Fe Mössbauer-effect spectroscopy. The NiCl2 layers order magnetically at 20.5 K, whereas the FeCl3 layers do not order down to 4.2 K. The drop of 1.5 K in the NiCl2-related ordering temperature, induced by the intercalation of FeCl3 into stage-2 NiCl2–graphite, is attributed to the increased NiCl2–layer separation, suggesting that the transition in NiCl2–graphite is not two-dimensional. Some of the FeCl3–NiCl2–graphite compounds show NiCl2-related transitions at temperatures lower than 20.5 K, which is explained in terms of lower in-plane NiCl2 concentrations. Partial oxidation and hydrolysis of in-gallery FeCl3 has been demonstrated and is found to depend strongly on the mean host-graphite-flake size. A charge transfer of 0.2 electrons per iron atom in the FeCl3 layers is inferred from the Mössbauer spectra collected at 4.2 K, which is the same as in singly intercalated FeCl3–graphite compounds of all known stages. Time and temperature-hysteresis effects in the magnetization have been measured and show distinct "re-entrant spin-glass" behaviour. Relevant aspects of the magnetism and structure of the related singly intercalated materials, the pristine chlorides, and the new biintercalation compounds are compared in detail. The overall features are consistent with a recent model based on interacting island supermoments.
X-ray and237Np Mössbauer effect investigation on the NpB2C compound