P.B. Fabritchnyi

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.

Iron-intercalated molybdenum disulfide obtained from single-layer dispersion

MoS[sub 2] intercalation compounds with transition metals could be of great interest for catalysis of petrochemical processes, and also from the magnetic point of view since, for two-dimensional metallic layers included into a diamagnetic matrix, unusual magnetic properties can be expected. Iron-intercalated molybdenum disulfide, obtained by flocculation of a MoS[sub 2] single-layer dispersion in the presence of ferrous sulfate, was studied by electron microscopy and Moessbauer spectroscopy. Iron was found to occur at both II and III oxidation states between the MoS[sub 2] layers. The formation mechanism of this intercalated compound and its composition are discussed.

Interactions hyperfines magnetiques et mecanisme de compensation de la charge pour les ions d'impurete 119Sn4+ dans Cr2O3

Abstract Mossbauer spectra of Cr2O3 sesquioxide doped with 119Sn4+ ions lead to the hypothesis of three non-equivalent magnetic sites for Sn4+. The identification of these sites has allowed to estimate the relative importance of exchange interactions between Cr3+ ions along [111] axis and in (111) plane.

Local environment and vibrational characteristics of 119Sn(II) Mössbauer probe atoms located on the surface of Cr2O3 microcrystals

Abstract Two different sites have been evidenced for SN(II) dopant atoms located at the surface of the Cr 2 O 3 substrate. Their inequivalence appears to be essentially due to the number of neighboring Cr(III) cations which are responsible for the magnetic hyperfine fields H transferred at the 119 Sn nuclei. While a large difference is observed in the saturation values of H, the isomer shift and quadrupole splitting—which are predominantly determined by tin-oxygen bonding—differ for the two sites to a lesser extent. Moreover, the main component V zz of the electric field gradient at 119 Sn nuclei is found, in both cases, to be negative and oriented at the same angle with respect to the c-axis of the Cr 2 O 3 lattice. The magnetic ordering temperature detected by the surface-located 119 Sn(II) atoms appears to be in good agreement with that previously determined from antiferromagnetic resonance experiments for undoped Cr 2 O 3 . The Goldanskii-Karyagin effect, which is observed for Sn(II) dopant atoms in the paramagnetic region, indicates that the mean square vibration amplitudes are smaller along the V zz axis than perpendicular to this direction.

Caracterisation par spectrometrie mössbauer de la repartition des ions etain dans le sesquioxyde de chrome utilise en tant que catalyseur pour oxydation de l'oxyde de carbone

Abstract Mossbauer spectra of four Cr 2 O 3 samples corresponding to the same chemical composition (Cr 1.9934 Sn 0.0066 O 3 ) but with different tin contents in the grain surface layer have been investigated, the concentration of surface located Sn 4+ ions being determined from ESCA data. For all samples 119 Sn Mossbauer measurements reveal at low temperature magnetic hyperfine interactions induced by the spin density transfer from neighboring Cr 3+ ions. The spectra have been analyzed by the Heese and Rubartsch method. The procedure permitting to determine the distributions of the magnetic hyperfine fields acting on Mossbauer nuclei is shown to be sensitive enough to account for the behavior of Sn 4+ ions during calcination of Cr 2 O 3 in various experimental conditions. The samples characterized by Mossbauer spectroscopy have been tested as catalysts of the oxidation of CO by O 2 . The presence of Sn 4+ ions on the surface is found to be responsible for a significant increase of the apparent activation energy of the reaction. This effect is interpreted as a result of the reactivity decrease of the surface oxygen species located on the O 2− vacancies surrounded by both Cr 3+ and Sn 4+ ions, as compared with the reactivity of those only related to coordinately unsaturated Cr 3+ ions.

Etude du comportement dynamique des ions dopants a la surface et dans le volume des grains de Cr2O3 par spectrometrie Mössbauer de la sonde diamagnetique 119Sn

Abstract In order to compare the dynamical properties of surface- and bulk-located dopant ions, Mossbauer spectra of several Cr 2 O 3 samples corresponding to the same chemical composition (Cr 1.9934 Sn 0.0066 O 3 ) but with different 119 Sn 4+ ion distributions have been investigated over the temperature range 294 K ≤ T ≤ 615 K. The Mossbauer lattice temperature Θ M probed by surface-located 119 Sn 4+ is shown to be considerably lowered ( Θ M = 314 K) if compared to that for bulk positions ( Θ M = 400 K). The greater thermal vibration amplitude of surface-located ions is related to the reduced effective coordination sphere of the surface sites. The Neel temperature probed by 119 Sn 4+ (T N = 305 ± 2 K), in surprisingly good agreement with that of undoped Cr 2 O 3 (308 K), does not depend upon the 119 Sn 4+ distribution with respect to the surface, which shows that the ordering temperature is the same for surface Cr 3+ as for bulk ones.

Interactions hyperfines pour les sondes atomiques 119Sn dans le volume et la surface de l'oxyde V2O3 de part et d'autre de la temperature de transition

Resume The first results concerning the Mossbauer investigation of hyperfine interactions for 119 Sn probe atoms both above and below the temperature of rhombohedral to monoclinic structure transition, T t of V 2 O 3 matrix are presented. In the samples obtained by annealing in hydrogen flow tin atoms are shown to be stabilized in comparable proportion in two valence states, Sn 4+ ions being located in the bulk and Sn 2+ on the surface of V 2 O 3 microcrystallites. In the rhombohedral phase (T > T t ) surface Sn 2+ ions ( δ /BaSnO 3 = + 2.80 ± 0.04 mm/s) are characterized by a strong quadrupole interaction (Δ = 1.98 ± 0.04mm/s) resulting from the lone pair of Sn 2+ . The bulk Sn 4+ ions (Δ/BaSnO 3 = + 0.24 ± 0.03 mm/s) does not display any significant quadropole interaction in accordance with a high symmetry of O h site in such a phase. The surface localization of Sn 2+ ions, confirmed by ESCA analysis, is chemically demonstrated by a rapid oxidation to Sn 4+ observed in Mossbauer spectra after a brief contact with air at ambient temperature. The chemical bonding of the resulting Sn 4+ s surface ions is shown to be more ionic as compared with that of Sn 4+ v ions located in the bulk. In monoclinic phase (T t ) the 119 Sn spectra reveal magnetic hyperfine interaction due to antiferromagnetic ordering of V 3+ moments. For surface Sn 2+ ions the magnetic interaction is weak as compared with dominant quadrupole interaction. For bulk Sn 4+ v ions only a sextet pattern corresponding to an internal magnetic field of 70 ± 1 kOe at 4.2K has been observed. On the contrary, at the same temperature, Mossbauer measurements suggest very weak (no larger than few kiloersted) or zero magnetic field at Sn 4+ s surface ions. The comparison of the results obtained with those concerning 119 Sn hyperfine interactions in Cr 2 O 3 matrix suggests a different charge compensation mechanism for tin impurity in these oxides.

Synthesis and structure of Fe4(CO)10(μ2-CO)(μ4-Te)2 and 57Fe Mössbauer spectra of Fe3(CO)9(μ3-Te)2 and Fe4(CO)10(μ2-CO)(μ4-Te)2

Abstract The compound Fe 4 (CO) 10 ( μ 2 -CO)( μ 4 -Te) 2 ( 1 ) has been prepared by the reaction of Fe 3 (CO) 9 Te 2 ( 2 ) and Fe 2 (CO) 9 in toluene. Compound 1 is also formed by UV light irradiation of 2 and Fe(CO) 5 in THF. The structure of 1 was established by single-crystal analysis. Crystal data: orthorhombic, Pccn , a =6.843(1), b =15.814(1), c =17.436(1) A; V =1887 A 3 ; Z =4; T =293 K, R 1 =0.040. 1 consists of a planar array of four iron atoms with a quadruply bridging telluro ligand on each side of the plane. The shortest metalmetal bond contains a bridging carbonyl ligand, semi-bridging carbonyl ligands bridge the two adjacent metalmetal bonds. 125 Te-NMR investigations show a conversion from 1 to 2 within several hours. 57 Fe Mossbauer spectra show two doublets for 1 and only one broadened doublet for 2 . The ratio of the intensities of the doublets of 1 is found close to unity and confirms the existence of two equipopulated crystallographic sites of iron. On the contrary, in the case of 2 , the analysis of the spectra does not allow the expected correlation with the results of the crystal structure determination. The same difficulty was previously encountered in the case of the isostructural selenide Fe 3 (CO) 9 Se 2 .

Local environment and dynamic characteristics of the 119Sn(II) and 119Sn(IV) Mössbauer probes on the surface of Cr2O3 exposed to hydrogen sulfide atmosphere

Abstract Tin dopant located on the surface of Cr 2 O 3 crystallites interacts easily, at room temperature, with adsorbed H 2 S molecules. Contrary to the initial oxygen-surrounded tin species, the resulting Sn(II) and Sn(IV) sulfur-surrounded entities exhibit no spin polarization, at least down to 4.6 K, which shows that they are more distant from the magnetically ordered oxide substrate. Additionally, their thermal vibration amplitudes are greater than in oxygen-surrounding, which denotes a weaker bonding of the tin within the H 2 S adsorbed layer. Moreover, for Sn(II), sufficiently large quadrupole splitting allows the Goldanski-Karyagin effect to be seen. Thermal vibration amplitudes of Sn(II) are then found to be greater in the plane of the three neighbouring sulfur atoms than in the perpendicular direction.

FeIV disproportionation (2FeIV→FeIII+ FeV) in AFeO3(ACa, Sr) perovskite lattices studied by 119Sn diamagnetic probe Mössbauer spectroscopy

Using 119SnIV as a diamagnetic probe in the perovskite lattice of AFeO3 oxides (ACa, Sr), the Fe disproportionation (2FeIV⇄ FeIII+ FeV) is discussed. An electronic phenomenon previously described for CaFeO3 is detected in SrFeO3 due to the local breakdown of the electronic delocalisation. Steric interactions seem to be the predominant factor for explaining the oxidation state of iron in the SnIV environment.