A Mössbauer-effect Study of the Electronic and Magnetic Properties of Voltaite, a Mixed-valence Mineral

The Mössbauer-effect spectrum of voltaite, K2FeII 5FeIII 3Al(SO 4)12·18H2O, exhibits four well-resolved lines between room temperature and 4.2 K. Two of these quadrupole-split lines are assigned to the iron(II) ion, which occupies the M(2) site having the FeO4(H2O)2 pseudooctahedral coordination geometry. The remaining two lines are assigned to the iron(IIl) ions, which occupy both the M(2) site and the M(1) site which has the FeO6 octahedral coordination geometry. The relative intensities of these lines are in excellent agreement with earlier site occupancy factors established by single-crystal X-ray determinations and have a ratio of Fe(II),M(2) to Fe(III),M(1) to Fe(III),M(2) of 5:2:1. At 1.3 K, the intensity of the two iron(III) lines decreases significantly. This decrease is probably associated with the onset of ferrimagnetic ordering in the voltaite, an ordering that is reported to occur below 1 K. The Mössbauer-effect spectra of the solid-state solutions of cadmium in voltaite, K2FeII xCdII 5-xFe III 3Al(SO4)12·18H 2O, in which x varies from 0.19 to 3.10, also exhibit four lines, but the iron(II) lines exhibit reduced intensity. An analysis of these spectra indicates that the quadrupole interaction at the iron(II) site decreases slightly as the cadmium content increases. The cubic lattice parameter for these compounds increases linearly as the cadmium content increases. The all-cadmium voltaite, K2Cd5FeIII 3Al(SO4) 12·18H2O, exhibits two iron(III) Mössbauer absorption lines and remains paramagnetic down to 1.3 K. In an external applied magnetic field this compound exhibits different internal hyperfine fields on the two crystallographically different iron(III) sites. The relative intensities of the magnetic components are consistent with paramagnetic behavior, but the components are considerably broadened. At 4.2 K the internal hyperfine field increases from 72 to 333 kOe for the M(2) site and from 207 to 489 kOe for the M(1) site in external fields ranging from 2 to 6 T. At 1.7 K the Fe(III),M(1) site is very close to saturation and exhibits an internal hyperfine field of 555 kOe in a 6-T applied field. The Fe(III),M(2) site displays a distribution of hyperfine fields. It is suggested that the line broadening and the behavior of the spectra as a function of applied field are due to a combination of relaxation effects and to the presence of a range of hyperfine fields at Fe(III),M(2) sites due to the random occupation of M(2) sites by either Cd(II) or Fe(III). © 1980 American Chemical Society