L. N. Vasil’ev

Defect Samarium Ions and Electromotive-Force Generation in SmS

A model explaining electromotive-force generation in SmS under heating in the absence of external temperature gradients is considered. An analysis of data on the density of SmS single crystals with compositional deviations from stoichiometry in the homogeneity range suggests that excess samarium ions occupy vacancies on the sulfur sublattice. Possible concentrations of defect samarium ions are determined (up to 2.8×1021 cm−3). The temperature interval within which electromotive force appears in various SmS samples (440–640 K) and the critical conduction-electron concentrations at which the generation sets in [(6.0–8.5)×1019 cm−3] are calculated. An expression permitting estimation of the magnitude of the observed effect is proosed.

Effect of the size of x-ray coherent-scattering regions on the electrical parameters of semiconducting SmS

The relation of the electrical properties of semiconducting samarium sulfide (SmS) to the size of coherent scattering regions for x-rays is considered. The dependence of the carrier concentration on the size of coherent scattering regions in single-crystal and polycrystalline SmS samples, as well as in thin samarium sulfide films, is established experimentally. A satisfactory agreement of the calculated curves with the experiment suggests that the size of coherent scattering regions has a dominant effect on the concentrations of charge carriers and defect samarium ions in SmS. The effect of thermal shocks on the size of coherent scattering regions in SmS single crystals and polycrystals, as well as in polycrystalline Sm0.4Eu0.6S samples, is investigated.

The structure of a metallic-phase film produced by mechanical polishing of polycrystalline SmS

X-ray diffraction is used to study the structure of a metallic-phase film that forms during controlled polishing of homogeneous polycrystalline semiconducting Sm1+xS samples. Structural changes that appear in the semiconducting phase under these conditions were studied. The x dependence of the thickness of the metallic layer forming on the sample surface is analyzed to explain the effect of excess samarium ions on the transformation parameters. The cause of the stabilization of the metallic modification of SmS after polishing is terminated is explained using estimates based on the measured sizes of coherent domains in samples of different compositions. The appearance and stabilization of the metallic phase are related to a decrease in and subsequent conservation of the coherent-domain size, respectively.

Temperature dependence of the SmS lattice parameter

The behavior of the lattice parameter of single-crystal SmS with temperature was studied by x-ray diffractometry in the range 100–700 K. The observed features are assigned to a temperature-induced variation in the filling of the Sm2+f-shell ground-state multiplet levels. The temperature dependence of the thermal expansion coefficient of SmS was measured. It is shown that the lattice constant behavior in samples that exhibit a pronounced emf generation effect under heating is influenced by the transition of defect samarium ions from the divalent to trivalent state and that the effect itself derives from phase transitions in SmS.

The Mechanism of the Appearance of an Electromotive Force on Heating of SmS Single Crystals

We analyze the experimental variation of the concentration of conduction electrons in semiconducting SmS single crystals with increasing temperature within a shallow-impurity model. It is shown that the appearance of an electromotive force is due to accumulation of the critical concentration of free electrons, which results in screening of the Coulomb potential of Sm2+ impurity ions that are responsible for the creation of donor levels with an activation energy of 0.045 eV in the band gap of SmS single crystals.

Defect structure in SmS

The experimental dependence of the concentration of conduction electrons on the size of x-ray coherent scattering regions is obtained. The dependence is analyzed within the previously developed concentration model of the energy spectrum of samarium sulfide SmS. It is shown that the impurity donor levels in the SmS compound correspond to the samarium defect ions located at the boundaries of coherent scattering regions.

Concentration model of semiconductor-metal phase transitions in SmS

Model calculations explaining the mechanism of the semiconductor-metal phase transition in SmS are carried out. The model, slightly modified, draws upon methods employed earlier to account for the concentration mechanism of piezoelectric resistance and thermovoltaic effect in SmS. The stable results are obtained from calculations for the phase transition pressure under hydrostatic compression (Pc ∼ 700 MPa at T = 300 K). On this basis, it is concluded that the 4f levels of samarium ions and their excited states determine the value of Pc. The proposed model is universal in character and can be applied to calculations of other effects in SmS, which are associated with Mott transitions and are accompanied by collective carrier delocalization.

Changes in the mass balance of the Antarctic Ice Sheet over 50 years

The mass balance of the Antarctic Ice Sheet has been calculated based on instrumental estimates of the grounded ice discharge and snow accumulation data. The boundaries and sectional areas of the main ice catchment basins in West and East Antarctica have been determined, and the data on the grounded ice discharge and snow accumulation in these basins have been systematized. The intensity of accumulation and ablation processes in Antarctica has noticeably increased over the last 50 years. The mass balance of the ice sheet in East Antarctica has been and remains positive, while in West Antarctica it was positive in the middle of the last century and has become negative by now. The mass balance of the entire Antarctic Ice Sheet has been and remains positive, while the mass growth has noticeably decreased over the last 50 years.

Temperature dependence of the coefficient of linear thermal expansion of single-crystal SmS

The coefficient of linear thermal expansion of single-crystal SmS has been measured in the temperature range 300–850 K by dilatometry and X-ray diffraction. It is shown that the difference in the results obtained by these two methods is due to the heating-induced formation of SmS phases with small lattice parameters (5.62–5.8 Å) close to that for the metallic SmS phase.

Deformation mechanism of a phase transition during polishing of SmS samples

The behavior of a semiconductor samarium monosulfide contact with iron under the action of pressure has been investigated. It is shown that the jump observed in the conductivity of the contact corresponds to a phase transition of SmS into a metallic state. A conclusion about why a metallic phase appears on the surface of the SmS sample during polishing is drawn from a calculation of the deformations produced by a spherical indentor at the moment of the phase transition.