C. Gleitzer

Etude du système SbMoO à 500°C: Mise en évidence de deux nouveaux oxydes de molybdène-antimoine

Abstract The ternary system SbMoO has been investigated by reduction using H 2 H 2 O gas mixtures and direct synthesis under vacuum. The products were studied by X-ray diffraction, electron diffraction, and high-resolution electron microscopy. Two new compounds were characterized: Sb 0.2 MoO 3.1 , which is orthorhombic, with an average oxidation state for molybdenum of 5,5; and Sb 0.4 MoO 3.1 , which is hexagonal and in which the average oxidation state of molybdenum is 5. These two phases can be considered in some ways as molybdenum bronzes, but without possessing all the related bronze characteristics.

Perturbation de l'échange électronique rapide par les lacunes cationiques dans Fe3−xO4(x≤0,09)

Abstract As most previous work on non-stoichiometric magnetite Fe 3− x O 4 has been carried out with fine powders prepared in the metastable domain, with the inherent uncertainties, we have prepared small crystals by quenching from the high temperature stability range. With increasing x , the cell parameter decreases, the Verwey temperature falls below 4.2 K for x = 0.09, the thermal stability decreases (disproportionation occurs at ~550K for x = 0.09), the saturation magnetization at 125 K varies as (4.1 – 2 x ) suggesting that the vacancies are mainly octahedral; this is confirmed by the Mossbauer effect at 150 K and 5 Teslas. At 4.2 K with 8 T , the Mossbauer spectrum is very complex but displays a mixed-valence signal involving a fraction (~ 10%) of the octahedral sites. This signal is evidence for electron tunnelling. The conductivity, at 295 K, decreases more rapidly than (1 − 3 x ); the conductivity of stoichiometric magnetite is described by the dynamic correlation model applied to short-range ordering; for x = 0.09 an activation energy of 0.077 eV is deduced between 39 and 120K. These results are interpreted as evidence that the tunnel effect at low temperature is limited to domains separated by vacancies, associated with 5 times more electron holes, and by regions where short-range order prevails.

Unusual valences and fast electron exchange in MoFe2O4

Abstract The distribution of d electrons over the cations in MoFe 2 O 4 , which is represented by the formal valence assignment, is shown to be complicated by the equilibrium reactions Fe 2+ B + Fe 3+ A + Mo 3+ ⇌ Fe 3+ B + Fe 2+ A + Mo 4+ We have used thermal treatment to confirm that the Mo are primarily on octahedral sites; Fe A [Mo B Fe B ]O 4 . K -shell absorption and Mossbauer data at T = 423 K > T c demonstrate that the iron has an average valence near 2.5+ with fast electron transfer ( τ h −8 sec) on both octahedral and tetrahedral sites. Paramagnetic susceptibility data give a Curie constant C M = 7.95 ± 0.2 emu/mole and a Weiss constant θ p = −445 K; magnetometer measurements confirm a compensation point near 160 K. Transport data give a surprisingly high electronic conductivity, but also give an activated mobility similar to that found in AlFe 2 O 4 and CrFe 2 O 4 where mixed Fe 3+ 2+ valences on both A and B sites have been demonstrated. However, a positive Seebeck coefficient and a preexponential factor one order of magnitude higher in MoFe 2 O 4 point to involvement of a fraction of the Mo atoms in electronic transport, which would be consistent with the observation of a τ h −8 sec on the A sites of a spinel. An energy diagram consistent with these data and other information about the relative redox potentials of these ions in oxides are proposed for this system.

Some remarkable features in the reduction of iron oxides

Abstract The reduction of iron oxides, from hematite to iron, is a complex series of interrelated steps, with many new observations and still some unexplained questions. The present review describes a few remarkable features, observed and analyzed recently in the authors laboratory, mainly based upon the prominent role of the reaction product texture, and concerning the hematite-magnetite and the wustite-iron steps. Emphasis is put on two very different and uncommon features observed in the reduction of hematite into magnetite: cracking of the matrix and growth of lamellae which are tentatively considered as inner whiskers. In each case the mechanism is discussed, and a semi-quantitative treatment is proposed. In the wustite-iron step, the existence of a rate minimum in the temperature range, and the catalytic role of potassium are described and briefly discussed.

Stability, structure refinement, and magnetic properties of β-Fe2(PO4)O

Abstract We previously reported the existence and properties of a low-temperature modification of metastable β-Fe2(PO4)O. Its structure was proposed by analogy with NiCr(PO4)O, but the magnetic measurements were hampered by traces of Fe3O4. We have now obtained a purer sample and a single crystal, allowing precise structure refinement, detailed magnetic characterization, and an investigation of the temperature stability range. The single crystal X-ray study confirms the structure as previously proposed: tetragonal (Z = 4), SG I4 1 amd , with a single iron site in face-sharing octahedra, and isolated PO4 tetrahedra; the reliability factor is R = 0.0345 (Rw = 0.0363). The magnetic susceptibility has been measured from 4 to 850 K. The magnetization at zero applied field is around 0.01 emu/g at 300 and 88 K, and 0.04 emu/g at 3.5 K. The χ = f(T) curve displays several unusual features: above TN (408 K) the Curie constant continuously decreases as a consequence of short-range magnetic order; below 100 K the susceptibility displays a small second maximum at 12 K. The magnetic structure has been investigated by low temperature powder neutron diffraction methods. At 17 K the Fe (II/III) spins are ordered ferromagnetically within the chains of face-sharing FeO6 octahedra, with the spin direction along (001). The chains are ordered antiferromagnetically with respect to each other. The phase stability has been investigated by high temperature X-ray diffraction. The nonreversible β → α transition takes place at ∼800°C and is completed at ∼840°C; however, the β phase stability is slightly lower in the presence of FeCl2 traces or under high pressure.

Nucleation and early growth of magnetite on synthetic and natural hematite crystals

Few data have been reported on the nucleation of magnetite in the reduction of hematite, and previous workers have used either transmission electron microscopy or hotstage optical microscopy. In this work a scanning electron microscope was used, which allows easy observation of the surface, and a series of samples were prepared with reaction times ranging from 0.5 to 20 min in the reduction with CO-CO2 or H2-H2O at 650°C. Synthetic crystals, grown in the range 150–200 μm using chemical transport, display nuclei with a spherical cap shape, which grow and then become more polyhedral, with finally some open porosity when the whole surface is covered. The average radial growth rate is around 2.1·10−9 m s−1, in reasonable agreement with the value derived from kinetic macroscopic measurements (TGA) (3.8·10−9 m s−1). The natural crystals undergo much more sudden nucleation after a higher incubation period; the surface is then completely covered, within around 1 min, with nuclei in the range 40–300 nm. Elimination of traces of silica makes the phenomenon less abrupt. From a discussion based on the hematite defect structure it is concluded that the prevailing point defects are interstitial iron ions, Fei.., which develop dense nuclei partly outside the matrix. Rists model helps in understanding the transition from the dense to the porous state. The nucleation frequency is also compared for the different samples and shows a trend towards saturation of active sites at 700°C where the frequency reaches 5·108 m−2s−1

Etude du system Fe-P-O (pour FeP ⩾1) et d'une famille: les oxyphosphates de fer

Abstract In this system, for which no phases diagram was available, the investigation has been made by progressive reduction and by synthesis in sealed tubes under vacuum at 900°C. Four new oxyphosphates have been evidenced: Fe 4 (PO 4 ) 2 O, Fe 2 (PO 4 )O, Fe 9 (PO 4 )O 8 and Fe 5 (PO 4 ) 3 O, and the structures of the three first ones determined and published elsewhere recently, and the Fe 3 (PO 4 )O 3 one as well, a compound which have been reported previously, whereas for Fe 5 (PO 4 ) 3 O we give hereafter a triclinic cell. Beside these plentiful oxyphosphates, the structures of which do not derive from a common type, we emphasize the high frequence of the 5 coordination of iron (in trigonal bipyramids), the variable character of the distances and angles in the PO 4 3− ions, and the weak stability of these oxyphosphates compared to the orthophosphates as derived from the phases relations in the Fe-P-O system. Among these oxyphosphates, three are of the mixed-valence type: Fe 2 (PO 4 )O, Fe 9 (PO 4 )O 8 and Fe 5 (PO 4 ) 3 O; the d electrons are strongly localized in the first one, whereas the second shows mixed-valence sites.

Le systeme SbWO3 et les bronzes de tungstene — Antimoine

Abstract Evidence is obtained for a series of tungsten-antimony bronzes, through reaction of antimony with tungsten trioxide at 900°C. Five phases are characterized: the four first derive from WO 3 with the classical increase of symetry: monoclinic, orthorhombic, tetragonal and cubic; the last one is probably a superstructure of the perovskite.

Diffusion of Cr in CoO

Abstract Diffusion of Cr in CoO single crystals was studied by using the X-ray microprobe technique. The diffusion coefficient of Cr thus determined exhibits the following temperature dependence in the range 1273–1673 K: D cr = 7.3 × 10 −4 +6.0 × 10 4 −3.3×10 4 × exp{[− 204( ± 6.7) kJ mol − 1 ]/ RT } . The diffusion data obtained are about one order of magnitude higher than those determined for a polycrystalline CoO. The difference is considered in terms of the effect of a segregation-induced diffusive resistance on diffusion data determined for a polycrystalline specimen.

Decalcification of stabilized zirconia by silica and some other oxides

Abstract Calcia-stabilized cubic zirconia may react with different oxides, susceptible to combining with calcia, and be transformed into the monoclinic modification with undesirable consequences. The reaction with silica has been investigated, and the kinetics recorded by X-ray quantitative diffraction; the data have been fitted to the Jander equation, yielding the rate constant and the activation energy; comparison has been made with the action of silicon and some current oxides, allowing the following classification from the most to the least active: B 2 O 3 .Si>TiO 2 >SiO 2 >Al 2 O 3 >Fe 2 O 3 These data have been rationalized by the Polanyi relationship, which correlates the kinetics to the formation enthalpy of ternary calcium oxides like CaSiO 3 . The reaction interface is complex with three different layers: an ill-organized one in contact with cubic zirconia, a second layer with monoclinic calcium-free zirconia particles, and a third with silicates: CaSiO 3 , ZrSiO 4 and Ca 2 ZrSi 4 O 12 . The diffusion and nucleation are discussed; it seems that silicon plays a preponderant role in these processes.