M. I. Afanasov

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.

5s5p Element Dopant Cations: Møssbauer Probes for Studying Chemical Reactions at the Solid-Gas Interface

Annealing in reducing atmosphere allows stabilization of the Sn4+, Sb5+, or Te6+ dopant cations in a lower oxidation state. Due to its stereochemical activity, the lone electron pair of the resulting Sn2+, Sb3+, or Te4+ favors the location of these species in low-coordination sites, immediately on the surface of the substrate-compound crystallites. This allows 119Sn, 121Sb, or 125Te Møssbauer spectroscopy to be applied for studying the processes occurring at the solid-gas interface. Results of such studies, mainly devoted to the Cr2O3 antiferromagnetic substrate, were discussed, along with the prospects of searching appropriate substrate-compounds of other types.

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.

Oxidation-induced change in spin polarization of the antimony Mössbauer dopant on the surface of Cr2O3 crystallites

Abstract 121Sb Mossbauer measurements show that oxidation of Sb(III) dopant ions located on the surface of Cr2O3 increases the value of the transferred magnetic hyperfine field H(121Sb) by a factor of about 2. This change is analogous to that previously reported for the 119Sn Mossbauer dopant located on the surface of the same matrix. In both cases, the increase in H values, which can be related to removal of the Sb(III) or Sn(II) lone-pair electrons, indicates that spin polarization of both Sb(V) and Sn(IV) dopants within magnetically ordered insulators is governed by 3d-electron density transfer from neighboring magnetic cations into 5s orbitals of the diamagnetic ones. Further comparison of relevant H(121Sb) and H(119Sn) values points to location of Sb(III) dopant ions mainly on sites with only one neighboring Cr(III) while isoelectronic Sn(II) dopant was shown to occupy surface sites of two types with one and three Cr(III) neighbors, respectively. Lower values of H characterizing both Sb(III) and Sn(II) are probably due to another spin polarization mechanism which involves participation of weakly magnetized 2s and 2p-electrons of neighboring O2−.

Mössbauer evidence for fast electron hopping between Fe(II) and Fe(III) in iron hydroxide/molybdenum disulfide lamellar nanocomposite

Abstract Nanocomposite compounds with iron hydroxide and molybdenum disulfide alternating layers have been studied using 57Fe Mossbauer spectroscopy. Evidence is given of the presence of two oxidation states for iron (Fe(II)/Fe(III)≈1.9). In the temperature range 60–160 K thermal evolution of the absorption areas obeys Debye’s model, with the lattice temperature ΘM equal to 275 and 320 K for Fe(II) and Fe(III), respectively. At higher temperature (T≥180 K), a new iron state is observed, which can be related to fast electron hopping between certain heterovalent iron ions. Tin-doping enhances this process which can already be detected at 140 K.

Mössbauer study of reorganization processes on the surface of Cr2O3 crystallites after reaction of the Sn2+ dopant with halogen vapor

Abstract Upon room temperature exposure to Cl 2 (or Br 2 , or I 2 ), Sn 2+ Mossbauer dopant located on the surface of antiferromagnetic Cr 2 O 3 microcrystals is readily oxidized. In the case of Cl 2 at least, primary local environment of resulting Sn 4+ , with mixed anionic surrounding (O + Cl), is unstable at this temperature. After stabilization, part of the Sn 4+ ions remain in the topmost cationic layer, with pure oxygen coordination and magnetically active cationic surrounding. The other Sn 4+ are found completely isolated from the magnetic substrate by their anionic environment which uniquely involves chloride ions.

119Sn Mössbauer study of the influence of a gas atmosphere on the valence state and distribution of tin atoms in the MgO structure

Analysis of the Mössbauer spectra of dopant 119Sn in cubic MgO has demonstrated that the Sn2+ ions can be stabilized on the surface of crystallites of an oxide with a structure differing from the corundum structure. The Mössbauer parameters (at 100 K, the isomer shift is δ = 2.50 ± 0.01 mm/s and the quadrupole splitting is Δ = 2.30 ± 0.02 mm/s) point to the stereochemical activity of the lone pair of Sn2+. Being in contact with oxygen at 295 K, tin is rapidly converted to the tetravalent state (δ = 0.08 ± 0.01 mms, Δ = 0.58 ± 0.01 mm/s). The lack of formation of Sn2+ on the surface of another cubic oxide (MnO) can be explained by rapid segregation of tin from the bulk of crystallites as stannate clusters.

The one-step synthesis of polymer-based magnetic γ-Fe2O3/carboxymethyl cellulose nanocomposites

A novel one-step procedure is described for synthesizing water soluble biocompatible nanocomposites from maghemite nanoparticles and carboxymethyl cellulose (CMC). The procedure allows the magneto-sensitive nanocomposites with a controlled content of the inorganic phase. The maghemite formation has been proved by X-ray diffraction analysis and Mossbauer spectroscopy. An average diameter of the maghemite nanoparticles is equal to 11nm according to transmission electron microscopy. As shown by FTIR spectroscopy, the nanoparticles bind to the polymer matrix via electrostatic and coordination interactions. The diameter of the nanocomposites in dilute aqueous solutions vary from 50nm at the iron content of 2-4.3wt.% to 140nm at the iron content of 5.2-8.6wt.%. The study of specific magnetization of the nanocomposites vs. applied magnetic field indicates their ferromagnetic properties. The saturation magnetization and coercive force of the sample with the maximum maghemite content (8.6wt.%) are 11.5emu/g and 30.08Oe, respectively. The nanocomposite motion has been shown to be controlled by an external magnetic field. The biocompartible maghemite-CMC nanocomposites seem to be promising for encapsulation and delivery of biologically active compounds.