L. Fournes

Electrochemical Oxygen Intercalation into Oxide Networks

The electrochemical oxidation of Sr 2 Fe 2 O 5 brownmillerite-type ferrite and La 2 M O 4 ( M = Ni, Cu) oxides is reviewed. The oxidation process appears as a very powerful method for preparing high oxidation states of transition-metal oxides. The characterization of the reaction products shows that they are well described as intercalation compounds of oxygenated anions. The fact that the intercalated species are the same as the host anions distinguishes these materials from most other intercalated compounds, in which guest species usually are chemically different from the host ions. The question of the nature and site of the intercalated species as well as that of their diffusion into the sample bulk is discussed on the basis of various characterizations and physical measurements as well as within the scope of recent considerations of the electronic distribution in oxides of transition elements in a high oxidation state.

Characterization of six-coordinated iron (V) in an oxide lattice

Abstract On the basis of a discussion of the correlations between the stability of the d 3 configuration and the symmetry and size of the cation site, six-coordinated iron (V) has been stabilized in the La 2 LiFeO 6 perovskite-type oxide and characterized by magnetic susceptibility, Mossbauer resonance and EPR measurements.

Mössbauer resonance studies on Sn substituted Fe4N

Preliminary results of Mossbauer resonance investigations (57Fe and 119Sn) on ferromagnetic Sn substituted Fe4N show: 1. (i) a preferential substitution in the Fe(I) sites of the lattice 2. (ii) a decrease of the particle size with increasing Sn content.

Influence of the substitution of manganese for iron in the Fe4N lattice on particle formation and magnetic properties

Abstract Mossbauer resonance studies on Mn-substituted Fe4N show that manganese occupies statistically both iron sites in the lattice. Replacement of Fe by Mn modifies the crystal growth process of Fe4N magnetic particles and leads to improvement of the extrinsic magnetic properties, i.e., Hc and σ r σ s , required for magnetic recording applications.

Preparation, properties and crystal structure of TlxV6S8

Abstract A new sulfide of vanadium and thallium TlxV6S8 (x = 0.52±0.2) has been prepared. Its unit cell has hexagonal symmetry, space group P63 and dimensions: a = 9.187(7) A , c = 3.298(4) A , z = 1. The structure is characterize by infinite triple rutile chains along the c axis containing short V-V (2.860 A) distances. The compound shows metallic behavior.

On an intermediate electronic configuration of cobalt (+III)

Abstract In a D 4h site with a strong elongation the 3 A 2g term of cobalt (+III) can be stabilized with an electronic configuration intermediate between low spin ( 1 A 1g ) and high spin ( 5 E g ): d 2 xz d 2 yz d 1 xy d 1 z 2d 0 x 2 −y 2. The La 2 Li 0.5 Co 0.5 O 4 phase, whose layer structure favours strongly such a distortion of the CoO 6 octahedra, illustrates very well this possibility for cobalt (+III). 1 A 1g → 3 A 2g and 1 A 1g → 5 E g transitions appear as the temperature increases.

Investigation of mixed valency ferrites La1−xCaxFeO3−y (0<x<0.50) with the perovskite structure

Abstract Investigation of mixed ferrites La1−xCaxFeIII1−τFeIVτO3−y (0 ≤ x ≤ 0.05, 0 ≤ y ≤ 0.07) has been carried out by magnetic, electrical and Mossbauer resonance measurements. Mossbauer spectra at low temperature can be interpreted assuming a disproportionation of tetravalent iron according to a scheme previously proposed for CaFeO3: 2 FeIV → FeIII + FeV. Electrical measurements show an increasing electronic localization when the temperature is decreased. This result corroborates the proposed model which moreover corresponds to a stabilization of the isotropic configurations t32ge2g of FeIII and t32ge0g of FeV in regular octahedral sites. The fit of the Mossbauer spectra at 4.2 K is also discussed in relation to the distribution of FeIII and FeV in the lattice.

A mössbauer resonance investigation of 57Fe doped Sr2Co2O5

Abstract Sr 2 Co 2 O 5 has two crystallographic forms (brownmillerite or 2H-related hexagonal perovskite) the existence of which depends on the spin configuration of cobalt. Over a wide range of temperature (200 ≲ T ≲ 700 K) magnetic measurements have given evidence of a medium spin configuration. A Mossbauer resonance study of 57 Fe doped Sr 2 Co 2 O 5 has firstly confirmed brownmillerite structure and high spin configuration of cobalt in the high temperature variety. Secondly in the low temperature form, it has shown the existence of two kinds of crystallographic sites, a very distorted octahedral site and two sites of lower coordination (i.e. a pyramidal site (C 4v symmetry) and a square planar site (D 4h symmetry)). The stabilization of the various electronic configurations is discussed in relation to a proposed structure implying vacancy ordering.

Mössbauer resonance studies on the Ca2Fe2O5-LaFeO3 system

Abstract Phases between Ca 2 Fe 2 O 5 and LaFeO 3 with La 1−2y Ca 2y Fe 3+ O 3−y formulation have been prepared in order to study the oxygen defects in perovskite-related ferrites. X-ray-diffraction analysis and magnetic measurements have confirmed the previous results i.e. the existence of a critical concentration of vacancies above which long range ordering appears. A detailed Mossbauer resonance study shows a continuous evolution of the iron environment, tetrahedra being formed even for low values of y. This result has been discussed and compared with previous ones for homologous CaTi 1−2y Fe 2y O −y phases.

Two new iron(IV) oxides with high-spin configuration: SrLaMg0.50Fe0.50O4 and SrLaZn0.50Fe0.50O4☆

Two new iron(IV) oxides have been prepared using high oxygen pressure: SrLaM/sub 0.50/Fe(IV)/sub 0.50/O/sub 4/ (M = Mg, Zn). In both phases with a K/sub 2/NiF/sub 4/-type-derived structure the elongation of the (FeO/sub 6/) octahedron is sufficient to stabilize a high-spin state (t/sub 2g//sup 3/d/sub z//sup 1/2). The electronic configuration has been characterized by structural, magnetic, and Moessbauer spectroscopic investigations.