Mössbauer diagnosis of the electronic structure and local environment of <Superscript>119</Superscript>Sn in MgTiO<Subscript>3</Subscript>

Annealing of a hydroxide precursor containing equimolar amounts of Mg²⁺ and Ti⁴⁺ and small additions of Sn⁴⁺ (0.1 at %) in air at 900°C leads to titanate MgTiO₃ with an ilmenite structure. The ¹¹⁹Sn 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 MgTiO₃ containing Sn²⁺ ions (a doublet at δ = 2.82 ± 0.01 mm/s and Δ = 1.66 ± 0.03 mm/s) (the Sn²⁺/MgTiO₃ sample). According to the spectral parameters, the ¹¹⁹Sn²⁺ 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 ¹¹⁹Sn²⁺ were previously observed during the Mössbauer study of the samples containing tin on the surface of Cr₂O₃, α-Al₂O₃, and MgO crystallites. The conclusion drawn from analysis of the ¹¹⁹Sn²⁺ Mössbauer parameters that tin in the Sn²⁺/MgTiO₃ sample has surface localization was supported by X-ray photoelectron spectroscopy. Mössbauer measurements show that the tin of Sn²⁺/MgTiO₃ when in contact with air is oxidized much more slowly than on the surface of Cr₂O₃, α-Al₂O₃, or MgO crystallites. The inhibition of the oxidation reaction is explained to be due to passivation of adsorbed O₂ molecules caused by their interaction with mobile t _2g electrons of Ti³⁺ forming in titanate during high-temperature annealing in H₂. In addition to the Sn²⁺ doublet, the ¹¹⁹Sn 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 MgTiO₃. This component is explained by persistence in titanate of some Sn⁴⁺ ions immobilizing the mobile t _2g electron at one of their neighboring Ti⁴⁺ cations.     

IR spectroscopic study of the structural transformations of cellulose during its nitration

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 54, No. 2, pp. 201–206, February, 1991.     

Hydrogen adsorption on the silicon (001) surface and on a step on the (111) surface

Adsorption of atomic hydrogen on an ideal (001) silicon surface is investigated in the present paper. Saturation of one of the two dangling bonds of a silicon atom on this surface by hydrogen removes the interaction (hybridization) between them, resulting in the appearance of a bonding and an antibonding chemisorption state associated with the attacked dangling bond, and in the shift of the peak of the remaining unsaturated dangling bond to the energy typical of a surface state of the (111) surface. Further saturation leads to the disappearance of this peak from the energy spectrum. An analogous situation occurs for the silicon atom with two dangling bonds on a step on the (111) surface, when hydrogen is chemisorbed. Both examples testify to the local chemical nature of Shockley surface states in silicon.The authors thank A. N. Sorokin for useful discussions.     

Mössbauer effect at <Superscript>119</Superscript>Sn<Superscript>4+</Superscript> nuclei in fine crystalline MnO

The hyperfine structure of the low-temperature Mössbauer spectra of dopant ¹¹⁹Sn⁴⁺ ions (0.4 at. %) in a fine crystalline MnO sample, characterized by X-ray diffraction and magnetic measurements, has been studied. The ¹¹⁹Sn spectra show that tin is distributed over positions with an inhomogeneous cationic environment. The appearance of such positions is explained by the local compensation for the excess charge of Sn⁴⁺ by manganese vacancies V _Mn and segregation of defects resulting in precipitation of MnSnO₃ clusters. The non-uniform distribution of Sn⁴⁺ and the related dependence of the number of broken magnetic bonds on the local tin concentration are responsible for the fluctuation of the T _N values, which is “perceived” by ¹¹⁹Sn in different MnO crystallites. The ¹¹⁹Sn spectra exclude the possibility that tin enters into the composition of superpara-magnetic particles.     

The Golden Mean and self-similar structures in optics

Based on numerical modeling it has been found that if self-similarity (which often connects with the Golden proportions) is present in structures of Fibonacci gratings and multilayer systems, it appears in their optical characteristics. Properties of Fibonacci optical structures have been compared with those of other aperiodic systems. It has been shown that structures based on the Golden and Silver means have highly related optical properties.     

Electronic spectrum of small silicon clusters in silicon dioxide

Institute of Theoretical and Applied Mechanics, Siberian Branch, Academy of Sciences of the USSR. Translated from Zhurnal Strukturnoi Khimii, Vol. 30, No. 4, pp. 23–26, July–August, 1989.     

Characteristics of the Analytical Resonator of a Weak Absorption Spectrometer

Journal of Applied Spectroscopy - The characteristics of a weak absorption spectrometer consisting of a frequency-tunable diode laser and an external analytical resonator are considered. It is...