A MOSSBAUER-EFFECT STUDY OF THE ELECTRONIC AND STRUCTURAL-PROPERTIES OF FE9(PO4)O8

Fe9(PO4)O8 is a mixed valence compound with both layers of (FeO)6, which are similar to those in stoichiometric wüstite, FeO, and layers of Fe3PO6, which are similar to those found in anhydrous iron(III) phosphate, FePO4. A detailed Mössbauer effect study between 232 and 850 K of the electronic and structural properties of Fe9(PO4)O8 has been undertaken for comparison purposes and to study any valence averaging electron delocalization or exchange that may be present. An earlier single crystal X-ray study has revealed that Fe9(PO4)O8 crystallizes with five distinct iron sites in the ratio of 1:1:2:4:1. The differently distorted octahedral Fe(1), Fe(3), and Fe(4) sites contain divalent iron, the tetrahedral Fe(5) site contains divalent iron, and the octahedral Fe(2) site contains trivalent iron. Because of the variety of iron sites, the paramagnetic Mössbauer spectra of Fe9(PO4)O8 are complex and exhibit many partially resolved lines. The logarithm of the Mössbauer spectral absorption area and the median isomer shift vary linearly with temperature and yield an effective Mössbauer temperature of 300 K for Fe9(PO4)O8. The temperature dependence of the median isomer shift indicates electron delocalization into an unspecified conduction band above 630 K. The differing site degeneracies, site symmetries, and site valencies make it possible to fit the Mössbauer spectra of Fe9(PO4)O8 with two different models, both of which yield a realistic temperature dependence of the hyperfine parameters, but which lead to different conclusions about the presence of valence averaging electron exchange. Hence, the Mössbauer spectra can not, unequivocally, demonstrate the presence of valence averaging in Fe9(PO4)O8. However, the spectra do indicate the presence of structural changes, both above and below 295 K, which are consistent with a monoclinic space group as suggested by the presence of the weak superlattice reflections reported earlier. The relative component spectral areas indicate, in agreement with the relative Wigner-Seitz cell volumes, that the iron(III) on the Fe(2) site has a relatively low recoil-free fraction, whereas the six-coordinate iron(II) on the Fe(1) site has a relatively high recoil-free fraction. © 1990 J.C. Baltzer A.G. Scientific Publishing Company.