Mössbauer study of behavior of oriented single crystals of riebeckite at low temperatures and their magnetic properties

A series of powdered and oriented single crystals of riebeckite-arfvedsonite have been examined with the Mössbauer technique above and below the magnetic ordering temperatures. Using fitted spectra that result in essentially similar quadrupole pairs, site occupancies were determined for 5 samples. Fitted spectra of oriented single crystals are associated with inequal pairs. The quadrupole splitting and isomer shifts are indistinguishable from those of the powdered samples. For the purpose of determining site occupancy paramagnetic spectra at L.N. temperatures appear to have little advantage over those obtained at room temperatures. The use of cryogenics to obtain magnetic hyperfine splitting increases experimental times several fold; however we conclude that in most cases the additional information and greater accuracy of Fe⁺²/Fe⁺³ warrants the additional effort. Magnetic order detected at cryogenic temperatures has an overall antiferromagnetic character. The orientation of the magnetic moment for Fe⁺³ is not parallel to a principal crystallographic axis, and its orientation near 〈316〉 suggests that there is magnetic coupling between the cation ribbons containing the octahedrally coordinated iron. The magnetic behavior can be interpreted satisfactorily using the Ising model developed for two dimensional lattices and extended to loosely coupled chains.