A Mössbauer 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

Diffractomètre θ-θ avec monochromateur, obtenu par transformation d'un diffractomètre θ, 2 θ

Thanks to a — 0 rotation of the table bearing the θ, 2 θ goniometer with horizontal axis, we obtain an apparatus équivalent to θ-θ, while keeping the commercial goniometer and the monochromator. The realization includes a motor, a transmission with a gear-box, a driving friction and an adjustable stand for the tube.En faisant tourner de — 0 la table support d'un goniomètre θ, 2 θ à axe horizontal, on obtient un montage équivalent à θ-θ, tout en conservant le goniomètre commercial et le monochromateur. La réalisation comprend un moteur, une transmission par boîte de vitesses et friction et un support réglable pour le tube

A NOVEL MAGNETIC PHASE-TRANSITION IN ANHYDROUS IRON(III) PHOSPHATE, FEPO4

Magnetic susceptibility, Mossbauer spectroscopy and powder neutron diffraction experiments have shown that iron(III) phosphate, FePO4, undergoes a novel spin reorientation transition at 17K between two antiferromagnetic phases. The ordered magnetic moments lie in the basal plane of the trigonal unit cell between the Neel temperature (24K) and 17K, below which temperature the spins align along the unique (c) axis

Propriétés diélectriques de quelques oxydes de composition Ba

L’étude de la constante diélectrique, en fonction de la température et de la fréquence, de deux pérovskites quadratiques, oxydes semiconducteurs contenant du fer III et du fer IV : Ba1-ySryFeO3-x, montre des constantes diélectriques considérables et des angles de pertes élevés. Au contraire, deux oxydes isolants Ba1-ySryFe1-zMzO2,5 (M = Ga ou In), pérovskites cubiques exemptes de fer IV, ont des constantes diélectriques beaucoup plus faibles avec un angle de pertes qui peut être relativement petit. Toutes les pérovskites étudiées sont transparentes dans l’infrarouge très lointain de 10 à 40 cm-1 où débute une large bande d'absorption qui s’étend vers les fréquences élevées. Des présomptions de pyroélectricité sont aussi décrites