The electronic state and local surrounding of both iron ions and tin dopant ions in Lu 1 – x Ca x Fe 0.997 Sn 0.003 O 3 ( x = 0.003; 0.1; 0.2) compounds were studied by 57 Fe and 119 Sn Mössbauer spectroscopy. The analysis of 57 Fe spectra showed that in the Lu 0.8 Ca 0.2 Fe 0.997 Sn 0.003 O 3 sample, obtained by air annealing, the charge deficit created by the substitution of Lu 3+ by Ca 2+ was compensated by the partial transition of Fe 3+ to the +5 oxidation state. With a further increase in the value of x (in the Lu 0.8 Ca 0.2 Fe 0.997 Sn 0.003 O 3 compound) a mixed compensation mechanism appeared, including the formation of oxygen vacancies VO in addition to Fe 5+ ions. Annealing the sample with x = 0.1 in H 2 at 400°C led to the reduction of Fe 5+ to Fe 3+ and, accordingly, to the pure vacancy mechanism of the Ca 2+ charge deficit compensation. The analysis of 119 Sn spectra showed that the substitution of Lu 3+ by Са 2+ in the Lu 0.8 Ca 0.2 Fe 0.997 Sn 0.003 O 3 structure led to the increase in the value of the magnetic field H , probed by some Sn 4+ ions, as compared to the value of H > observed in the case of Lu 0.997 Ca 0.003 Fe 0.997 Sn 0.003 O 3 . This change can be explained by a local increase in the angle of the indirect exchange interaction in the chain Fe 3+ –О 2 ––Sn 4+ , reflecting the presence of Ca 2+ cation, larger than that of Lu 3+ , in the vicinity of Sn 4+ ion.

The influence of Ca2+ doped into the holmium sublattice on the magnetically active surrounding of Sn4+ ions located in the chromium sublattice of Ho1–x Ca x Cr0.997Sn0.003O3 (x = 0, 0.003, and 0.1) compounds was studied by 119Sn Mössbauer spectroscopy. At concentrations [Ca] = [Sn] = 0.3 mol %, an increase was observed in the spectral contribution of Sn4+ sites, having the full number of nearest-neighbor Cr3+cations (n = 6), where they perceived a magnetic field H(Sn)4.2 K = 82 kOe, compared to the contribution of the relevant sites in the undoped chromite (x = 0). This observation was interpreted as resulting from a reduced probability of appearance of Cr3+ vacancies in the nearest surrounding of heterovalent Sn4+ ions. For x = 0.1, on the contrary, the 119Sn spectrum revealed a reduced contribution from the Sn4+ sites with n = 6. This evolution is shown not to be due neither to the appearance of Cr4+ nor Cr6+ ions in the nearest neighborhood of Sn4+ in the chromium sublattice to balance the charge deficiency of the Ca2+ ions doped into the holmium sublattice. This allowed us to suggest that the observed effect was due to the onset of Sn4+ segregations in the structure of Ho0.9Ca0.1Cr0.997Sn0.003O3, which contained a far greater amount of Ca2+ ions whose charge deficiency was balanced mostly by Cr4+ formation. Studies of samples that were prepared under a hydrogen atmosphere revealed the reduction of Sn4+ to the oxidation state +2, with the concomitant stabilization of the formed Sn2+ ions on crystallite surfaces on sites having low coordination numbers.

A specific feature of the Mössbauer spectra of 119 Sn in Y 1– x Ca x CrO 3 samples containing 0.3 at % Sn 4+ in the bulk of crystallites is an extra sextet component corresponding to hyperfine field H ( 119 Sn) = 65 kOe at 4.2 K. This effect is explained by the formation of Sn 4+ impurity associates in the chromium sublattice. A drastic weakening in spin polarization is observed for the Sn 4+ ions in surface sites, but is found not to be related to the presence of heterovalent Ca 2+ ions.

The irradiation with UV light of anatase powders containing additives of Sb3+ on the surfaces of crystallites, submerged in water, changes the parameters of 121Sb Mössbauer spectrum indicating the transition of antimony to the pentavalent state. It is shown that this transformation is due to the emergence of holes in the valence band of TiO2.

Composite electrode materials based on LiFeBO3 are synthesized under different conditions and studied as the cathodic materials for lithiumion batteries. Composites with different degrees of iron oxidation are synthesized by annealing in a closed system with the use of metal-oxide getters. Based on the results of cyclic voltammetry and galvanostatic cycling of samples with different Fe(II) contents, it is concluded that the surface composition is the determining factor for applicability of materials to reversible processes of inter-calation-deintercalation.

The 119 Sn Mössbauer spectra of polycrystalline NiTiO 3 samples impregnated with a solution containing 0.3 at % Sn 4+ are evidence that annealing in H 2 converts tin into the state with the electron density |Ψ(0)| 2 on 119 Sn nuclei corresponding to “Sn 3+ ” ions. The stabilization of tin atoms in such an untypical formal oxidation state occurs at a depth of no more than 2–3 nm from the surface of titanate crystallites. It was revealed that the Sn 3+ ions are not subjected to spin polarization even at temperatures considerably lower than the Néel temperature of NiTiO 3 , which can be explained by their location in the Ni 2+ positions. The formation of Sn3+ prevents the further reduction of tin to the divalent state and, hence, precludes localization of 119 Sn probe cations in positions at the interface.

Annealing of a hydroxide precursor containing equimolar amounts of Mg 2+ and Ti 4+ and small additions of Sn 4+ (0.1 at %) in air at 900°C leads to titanate MgTiO 3 with an ilmenite structure. The 119 Sn Mössbauer spectrum of the sample (unresolved doublet with the isomer shift δ = 0.10 ± 0.01 mm/s and the quadrupole splitting Δ = 0.49 ± 0.02 mm/s) is evidence that the tin atoms are still in the oxidation state +4. Annealing of the precursor at the same temperature in a hydrogen atmosphere yields MgTiO 3 containing Sn 2+ ions (a doublet at δ = 2.82 ± 0.01 mm/s and Δ = 1.66 ± 0.03 mm/s) (the Sn 2+ /MgTiO 3 sample). According to the spectral parameters, the 119 Sn 2+ ions have a low coordination number (CN ≪ 6) and are abnormally resistant to reduction to the metal. Analogous features of the crystal-chemical behavior of 119 Sn 2+ were previously observed during the Mössbauer study of the samples containing tin on the surface of Cr 2 O 3 , α-Al 2 O 3 , and MgO crystallites. The conclusion drawn from analysis of the 119 Sn 2+ Mössbauer parameters that tin in the Sn 2+ /MgTiO 3 sample has surface localization was supported by X-ray photoelectron spectroscopy. Mössbauer measurements show that the tin of Sn 2+ /MgTiO 3 when in contact with air is oxidized much more slowly than on the surface of Cr 2 O 3 , α-Al 2 O 3 , or MgO crystallites. The inhibition of the oxidation reaction is explained to be due to passivation of adsorbed O 2 molecules caused by their interaction with mobile t 2 g electrons of Ti 3+ forming in titanate during high-temperature annealing in H 2 . In addition to the Sn 2+ doublet, the 119 Sn spectrum shows a spectral component with parameters (δ ∼ 1.6 mm/s, Δ ≤ 0.2 mm/s) not fitting the known tin species that can form in MgTiO 3 . This component is explained by persistence in titanate of some Sn 4+ ions immobilizing the mobile t 2 g electron at one of their neighboring Ti 4+ cations.

The results of recent studies involving the application of diamagnetic Mossbauer dopant cations for the investigation of heterogeneous catalytic reactions are described. Possibilities for using the information contained in Mossbauer spectra to clarify the nature of active catalytic centers are discussed. It is shown that, in some cases, the catalytic tests allow the correctness of the chemical interpretation of spectral parameters of the dopant atoms under study to be verified.

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.