Influence of the degree of crystal perfection and deviation from the stoichiometric composition on the diffusion processes in samarium sulfide

The processes of diffusion of samarium and europium in nonstoichiometric samarium sulfide (SmS) at temperatures in the range 950–1600°C have been investigated by the radioactive isotope method and the method of weight loss upon evaporation of excess samarium. It has been found that there is a correlation between the diffusion coefficient D and the size of coherent X-ray scattering regions in SmS samples, as well as between the diffusion coefficient and the degree of deviation from the stoichiometric composition: the diffusion coefficient of impurities decreases as the size of coherent X-ray scattering regions increases and the stoichiometric composition is approached. The calculation of the diffusion coefficient of electrons in samarium sulfide at T = 77–300 K has demonstrated that the value of D increases with increasing temperature and increasing size of coherent scattering regions.     

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.     

Influence of the mechanical effect and impurity doping on the endothermic effect in SmS

The effect of heat absorption has been found for powder samples of samarium sulfide (SmS) in a temperature range of 235–250 K. The comparison with a similar effect in single-crystal SmS has been performed and thermodynamic parameters of the process have been determined. It has been shown that the collective injection of electrons into the conduction band from impurity donor levels is responsible for the appearance of this effect in both cases. The shift of the endothermic peak upon powdering toward lower temperatures and an increase in the enthalpy are associated with a decrease in the characteristic size of the X-ray coherent scattering regions and, as a consequence, with an increase in the concentration of impurity donor levels.     

Specific features of the structure of semiconducting sms polycrystals in the homogeneity region

The structure and thermal properties of polycrystalline samples of samarium monosulfide have been investigated in the homogeneity region (Sm_{1 + x} S at 0 < x < 0.17). The X-ray structural parameters and porosity of the samples have been measured, and their correlation with the thermodynamic parameters of the first-order phase transition occurring in polycrystalline samarium monosulfide at temperatures in the range from 240 to 260 K in Sm_{1 + x} S has been established. It has been assumed that the measured nanopores correspond to voids between misoriented coherent X-ray scattering regions. It has been shown that the maximum of the absorbed heat energy is achieved in the case where the volume of the pore becomes comparable to the volume of the nucleus of a new phase of samarium sulfide, which is calculated from thermodynamic relationships.     

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 Sm_1+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.     

Europium diffusion in SmS

Europium diffusion in samarium sulfide was studied in the temperature range from 780 to 1100°C. Data on the diffusion coefficient and activation energies for the diffusion of europium in single-crystal and polycrystalline SmS samples were obtained. In single-crystal samarium sulfide, europium was shown to migrate predominantly over lattice sites (D ? 10^?12?10^?9 cm²/s). In SmS polycrystals, diffusion was found to exhibit a complex pattern and have both a slow (D ? 10^?10?10^?9 cm²/s) and a fast (D ? 10^?8?10^?7 cm²/s) component. Europium diffusion in a polycrystal is primarily due to europium migration over the boundaries of single-crystal grains in the polycrystal, whose characteristic size is assumed to be that of x-ray coherent-scattering regions.     

SmS phase transition in thin films prepared by reactive evaporation

Preparation and analysis of thin films containing samarium sulfide (SmS) is presented along with a review of relevant SmS properties. Films were deposited onto unheated substrates by the reactive evaporation of samarium in a backpressure of hydrogen sulfide. This technique yields films that contain significant quantities of impurities; however high-quality SmS crystals are also formed. A phase transition in the SmS crystals was observed both by spectrophotometry and x-ray diffraction. The film optical properties can be modeled with an effective medium calculation. The predicted spectra successfully reproduce the observed qualitative features over a wide range of wavelengths.     

Influence of grain size distribution on the lattice thermal conductivity of Bi2Te3-Sb2Te3-based nanostructured materials

The change of the lattice thermal conductivity of bulk nanostructured materials based on Bi₂Te₃-Sb₂Te₃ solid solutions with grain size distribution has been studied. These materials have a polycrystalline structure with grain sizes ranging from a few tens of a nanometer to a few micrometers. Large grains may contain inclusions or consist of several smaller parts which can be identified with coherent scattering regions seen in X-ray diffraction. The change of the lattice thermal conductivity mediated by additional scattering by inclusions and grain boundaries has been calculated. This calculation allows for the effect of nanoparticle size distribution. The calculated estimates are compared with the available experimental data.     

Emf induced by a change in the samarium ion valence as a result of a phase transition in SmS single crystals

The mechanism of the emf appearing in a semiconductor under heating in the absence of external temperature gradients, an effect revealed by the present authors, is considered. The experiments were performed on samarium sulfide (SmS) single crystals. It is shown that the emf is generated by an abrupt change in the samarium ion valence, which results from the ion screening by the electrons activated into the conduction band. We succeeded in obtaining emf pulses 1.3 s long with an amplitude of up to 2.5 V at T ～ 460 K, as well as CW emf generation within the 375-to 405-K temperature interval with a maximum value of ～50 mV.     

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.     

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.     

Destruction of SmS single crystals during the semiconductor-metal phase transition under hydrostatic compression

The behavior of internal microstresses, the size of the X-ray coherent scattering region, and the residual amount of the metal phase during the cyclic loading of SmS single crystals at hydrostatic pressure above the critical pressure of the semiconductor-metal phase transition has been investigated. It has been shown that the samples are destroyed as the microstresses reach the values corresponding to the ultimate stress of SmS single crystals. As the number of loading cycles increases, the coherent scattering region gradually decreases, which is accompanied by a decrease in the amount of the metal phase in the samples.     

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 (P _c ～ 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 P _c . 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.     

Heating-induced endothermal effect in semiconducting samarium sulfide

The effect of heat absorption is revealed in the temperature range where the thermovoltaic effect occurs in single-crystal samarium sulfide (SmS). It is shown that the heat absorption is caused by the collective injection of electrons from donor levels into the conduction band.     

Sizes of X-ray radiation coherent domains in thin SmS films and their visualization

Technical Physics - The size of X-ray radiation coherent domains (250 ± 20 Å) is determined in a thin polycrystalline SmS film using X-ray diffraction patterns (θ–2θ...     

Investigation of impurity levels in thin polycrystalline SmS films

Data obtained in the study of the behavior with temperature of the electrical resistance of thin polycrystalline SmS films (thickness ～0.5–0.8 μm) performed in the temperature region 4.2–440 K have been used to correct the band structure model of this material. It has been shown that the main impurity levels in thin polycrystalline SmS films are levels corresponding to localized states close to the conduction band bottom, as well as the impurity donor levels E _i which belong to Sm ions filling vacancies in the S sublattice. The tail of localized states has been found to extend up to the energy of impurity donor levels.     

The influence of structural features on the thermal conductivity of polycrystalline zinc sulfide

The thermal conductivity of single-crystal zinc sulfide and optically transparent zinc sulfide polycrystals differing in crystal grain size and density is experimentally investigated in the temperature range 80–400 K. It is shown that the thermal conductivity of polycrystalline samples substantially depends on the crystal grain size and the defect concentration in the grain-boundary layers. In zinc sulfide samples with a grain size of 1 μm, excess thermal resistance due to phonon scattering by grain boundaries is observed at temperatures T<130 K. It is demonstrated that, at higher temperatures (T>210 K), the heat transfer is associated not only with transverse phonon modes but also with longitudinal phonon modes and the role of the latter modes increases with an increase in temperature. __________ Translated from Fizika Tverdogo Tela, Vol. 44, No. 2, 2002, pp. 251–256. Original Russian Text Copyright ? 2002 by Lugueva, Luguev.     

Low temperature thermal conductivity of pure and doped single crystals of semiconducting SmS

We measured the thermal conductivity of pure SmS and of SmS doped with P, As and Se between 1.5 and 350 K. The lattice thermal conductivity of pure samples and of SmSSe obeys a T^{?$\text{3}\text{2}$} law for temperatures T greater than 150 K, and is very sensitive to point defects in the lattice. The highest values are measured on samples close to the stoichiometric composition. P and As dopants act as phonon scattering centers and considerably reduce the low temperature lattice conductivity.     

Structure and the magnetic and magneto-optical properties of Co-Sm-O nanogranular films

The magnetic properties and magneto-optical effects in nanocomposites based on Co-Sm-O films prepared through pulsed plasma sputtering of a SmCo₅ target are investigated. It is shown that, depending on the technological conditions and regimes of subsequent annealing, the films can have different structures from cobalt nanoparticles distributed in the dielectric samarium oxide matrix with a magnetic phase volume of more than 60% to a continuous polycrystalline cobalt film with embedded samarium oxide nanoparticles. The evolution of the spectra of the magneto-optical Kerr effect and the field dependences of the magnetization is studied as a function of the film structure.     

The contribution of grain boundary and defects to the resistivity in the ferromagnetic state of polycrystalline manganites

In the present study we report the precise resistivity measurements for the polycrystalline bulk sample as well as highly oriented thin-films of La_0.8Ca_0.2MnO₃. The poly crystalline sample was prepared by standard solid-state reaction route and the oriented thin film was prepared by pulsed laser deposition (PLD). The phase purity of these samples was confirmed by X-ray diffraction and the back-scattered electron imaging using scanning electron microscopy (SEM). The oxygen stoichiometry analysis was done by iodimetry titration. The resistivities of these samples were carried out with four-probe resistivity measurement setup. The observed temperature dependence of resistivity data for both the samples was fitted using the polaron model. We have found that polaronic model fits well with the experimental data of both polycrystalline and single crystal samples. A new phenomenological model is proposed and used to estimate contribution to the resistivity due to grain boundary in the ferromagnetic state of polycrystalline manganites and it has been shown that the scattering of electrons from the grain boundary (grain surface) is a function of temperature and controlled by the effective grain resistance at that temperature.