Sulfur Defects Research Articles

An ENDOR analysis of a diatomic sulphur defect in RbI

In this article, an ENDOR analysis of the S2- defect in RbI is presented. A complete angular variation of one set of 85Rb and 87Rb ENDOR transitions is measured. The corresponding hyperfine and nuclear quadrupole coupling tensors are determined. The orientation of the principal axes of these tensors is in good agreement with the overall D2h symmetry of the S2- ion in the lattice. The ENDOR results clearly lead to a monovacancy model, wherein the S2- ion is replacing one I- ion on a lattice site. The linewidth of the EPR signals can be explained using the ENDOR data. A comparison with previous ENDOR studies of O2- and S2- defects in rubidium halides is made.

Structure and Physical Properties of the Barium Niobium Sulfides BaNbS 3 and BaNb 0.8S 3-δ

BaNb 0.8S 3-δ and BaNbS 3 were prepared by heating desired amounts of BaCO 3 and Nb 2O 5 under CS 2/N 2 flow. We found that nominal composition of BaNb 0.8S has sulfur defect, and therefore should be written as BaNb 0.8S 3-δ. XANES experiments of Nb K-edges and XPS experiments proved that the oxidation state of Nb is the same in BaNb 0.8S 3-δ and BaNbS 3. To understand their structures, the electronic and physical properties of the Ba MX 3 ( M = V, Nb, X = S; M = Ta, X = S, Se) phases are compared. In spite of their structural similarity, they exhibit rather different physical properties. It has been known that BaVS 3 is a metal and undergoes a metal to semiconductor transition at 130 K, and this phase shows the ferro antiferromagnetic transition at a lower temperature. However, BaNbS 3 and BaTaS 3 are diamagnetic semiconductors at room temperature. The major factor stabilizing the semiconducting state for BaTa X 3 and BaNbS 3 may be a large spin-orbit coupling rather than a structural distortion.

Preparation and characterization of iron-doped RuS 2 single crystals

Preparation of single crystals of Ru xFe 1-xS 2 is attempted by chemical vapor transport and flux methods. The difficulties encountered in the formation of the solid solutions Ru xFe 1-xS 2 are reported. Only a small amount of iron can replace some of the ruthenium in RuS 2 and its distribution is inhomogeneous. The defect nature of the crystal is studied by electron paramagnetic resonance (EPR). A sulfur point defect is established to account for the spin S = 1/2 paramagnetic species. Electrical resistivity and Hall effect measurements have revealed n-type semiconducting behavior. The results of optical absorption measurements indicate that the presence of small quantities of iron in RuS 2 shifts the absorption edge of the compound into the longer wavelength region.

Effects of the Cations on the Raman Spectra of Sulphur Defect Chalcopyrites

We report the Raman spectrum of defect chalcopyrite compounds CdAl2S4, CdGa2S4, ZnGa2S4 and of the isostructural mixed compounds Zn0.7Cd0.3Al2S4, ZnGa0.2Al1.8S4 and ZnGa0.8Al1.2S4. The role of the cations on the vibrational dynamics is discussed in terms of simple models.

Sputtering of thin pyrite films

The high production costs of conventional solar cells make it necessary to investigate alternative production methods and materials with respect to their suitability for solar energy conversion. Semiconductors with high absorption coefficients are favorite candidates because of their potential to reduce material costs by using a thin film as active layer. One of these materials is pyrite, which is in the ideal case of composition FeS 2 and has an extraordinary high optical absorption coefficient (a>~6x 105 cm -1 for hv> 1.3 eV) [1]. A pyrite film of 0.1 ~m thickness absorbs more than 99% of the sunlight (AM 1.5) and makes the material very promising for solar cell applications. The band gap energy of synthetic crystals was measured [1,2] to be 0.92 to 0.95 eV and, at room temperature, electron mobilities in the range from 100 to 360 cm2/V s were found [3]. Problems with the material arise in connection with the open circuit voltage Uoc measured in photoelectrochemical cells [1] which are limited to 0.2 V until now. In principle it should be possible to yield half the band gap energy as open circuit voltage [4], i.e. ca. 0.45-0.5 eV for pyrite is expected. Since the efficiency of a solar cell is proportional to Uo~ this quantity must be enhanced to make the material valuable for technical applications. The small values measured for Uoc are perhaps due to the small difference of Fermi levels between pyrite and the electrolyte used (iodide-iodine). This assumption is supported by recent electro-reflectance measurements [5]. Another explanation concerns the strong non-stoichiometry in pyrite, that can lead to a sulfur deficiency in the percentage range as shown in recent works [6,7]. The sulfur defects were shown to be simple point defects for which ligand field theory predicts the development of electronic defect states in the

Two Charge States of Sulphur in GaSb

Such anomalous effects as metastability and persistent photoconductivity at temperatures below 80 K were found for sulphur in GaSb already in the early seventies [1]. Dmowski et al. [2] gave the explanation for this peculiar behavior based on a large lattice relaxation model. Experimental data in the present work allow to reexamine the previous models and consider the sulphur defect as belonging to the socalled DX centres which, according to recent experimental [3] and theoretical [4] indications, appear to have two charge states and form a negative-U system [5] .

High-temperature sulfidation of Fe-Nb alloys

The sulfidation behavior of Fe-Nb alloys containing up to 30 w/o Nb was studied over the range of 600–900°C in 0.01 aim. S2 vapor. All alloys were two-phase, consisting of an Fe-rich solid solution and Fe2Nb, and followed the parabolic rate law at all temperatures. Scales consisted of two layers-an outer layer of FeS and an inner, complex layer which contained some FeS, FeNb2S4 (possibly some FeNb3S6), NbS2, and intermetallic particles which were either completely or only partially sulfidized. Platinum markers were located always at the interface between the two layers, which corresponded to the original metal surface. Activation energies were 18±3 kcal/mol in close agreement with the 19.8 reported for pure iron. The sulfidation rate decreased markedly with increasing Nb content of the alloys. The decrease is attributed to increasing amounts of Fe2Nb with increasing Nb, the net effect being that the diffusion path for outward iron diffusion through the inner layer is reduced as the Nb content increases. An analysis of the structure of NbS2 reveals that it is easily intercalated with Fe between loosely bonded layers of S-Nb-S. The S-Nb-S layers are covalently bonded which results in very low diffusivities of either S or Nb in pure NbS2. Although intercalated Fe tends to change the Van der Waal's type bonding between layers to more ionic or covalent, Fe diffuses readily between the layers in NbS2. Intercalation of Fe also increases the concentration of sulfur defects in NbS2, which in turn increases the diffusivity of sulfur. Nb was observed to be immobile. Thus, it is thought that either outward iron diffusion or inward sulfur diffusion in the inner layer is the rate-controlling step, in spite of the close agreement of activation energies with that of the sulfidation of pure iron.

Hydrogen passivation of a substitutional sulfur defect in silicon.

Hydrogen passivation of a substitutional sulfur defect in silicon has been studied using the modified intermediate neglect of differential overlap molecular orbital method. We find that the sulfur center can be passivated using one or two hydrogen atoms. The calculations indicate that the most stable positions of the hydrogen atoms are between the sulfur and its silicon neighbors. The hydrogens bond to the nearest silicon atoms and only interact weakly with the sulfur. Thermochemistry considerations predict that a single hydrogen passivates the sulfur center, providing these centers are abundant in the silicon.

On the sulfur defect model for the lead chevrel compound — The density measurement

Density measurements were performed on almost single-phased, polycrystalline PbMo 6S 8−y specimens which were prepared with a sintering technique. The measured densities suggest that the Mo substitutional and/or interstitial atoms appear to compensate for the sulfur defects in the samples with large sulfur deficiency. The results of magnetic susceptibility measurements are also described.

The phase field of the Chevrel phase PbMo 6S 8 at 900°C and some superconducting and structural properties

The phase field of PbMo 6S 8 at 900°C was investigated by X-ray diffractometry. At 900°C the PbMo 6S 8 phase is surrounded by three two-phase regions — PbMo 6S 8 + Pb(1.), PbMo 6S 8 + Mo and PbMo 6S 8 + MoS 2. This synthesis temperature is rather low, and the grain size of the samples is considerably smaller (0.5–1.5 μm). The superconducting transition temperatures of various Pb xMo 6S 8−y samples were measured inductively, and a clear correlation between Tc and the composition was observed. The critical temperatures got higher when the sulfur content of the PbMo 6S 8 phase increased. The samples were characterized with X-ray diffraction, scanning electron microscopy and energy dispersive X-ray microanalysis. The variation of Tc is discussed in relation to the sulfur defects.

Adsorption of sulfur dioxide and the interaction of coadsorbed oxygen and sulfur on Pt(111)

The adsorption of sulfur dioxide and the interaction of adsorbed oxygen and sulfur on Pt(111) have been studied using flash desorption mass spectrometry and LEED. The reactivity of adsorbed sulfur towards oxygen depends strongly on the sulfur surface concentration. At a sulfur concentration of 5 × 10 14 S atoms cm −2 ( ( 3 × 3 )R30° structure) oxygen exposures of 5 × 10 −5 Torr s do not result in the adsorption of oxygen nor in the formation of SO 2. At concentrations lower than 3.8 × 10 14 S stoms cm −2 ((2 × 2) structure) the thermal desorption following oxygen dosing at 320 K yields SO 2 and O 2. With decreasing sulfur concentration the amount of desorbing O 2 increases and that of SO 2 passes a maximum. This indicates that sulfur free surface regions, i.e. holes or defects in the (2 × 2) S structure, are required for the adsorption of oxygen and for the reaction of adsorbed sulfur with oxygen. SO 2 is adsorbed with high sticking probability and can be desorbed nearly completely as SO 2 with desorption maxima occurring at 400, 480 and 580 K. The adsorbed SO 2 is highly sensitive to hydrogen. Small H 2 doses remove most of the oxygen and leave adsorbed sulfur on the surface. After adsorption of SO 2 on an oxygen predosed surface small amounts of SO 3 were desorbed in addition to SO 2 and O 2 during heating. Preadsorbed oxygen produces variations of the SO 2 peak intensities which indicate stabilization of an adsorbed species by coadsorbed oxygen.

Interactions between sulphur and microstructural defects in a Ni-16 wt% Cr-7 wt% Fe alloy and microsulphide nucleation

The distribution of sulphur in the form of isolated atoms or microscopic sulphide particles (microsulphides) in the structure of two Inconel 600 alloys of different purity was analysed with the help of two new techniques: high-resolution sulphur autoradiography (with 35S*) for the segregation of sulphur and high-resolution sulphur microprint to observe large amounts of sulphur. Among the various places where sulphur may segregate (grain boundaries, microprecipitates, slip-planes, isolated dislocations) some are preferred sites for further nucleation and growth of microsulphides. As a result, preferential interactions may be observed between sulphur and chromium oxycarbides, especially in the case of industrial alloys, in as much as they have a higher carbon content. The results have been considered essentially in terms of: (i) a gain in energy during the chemical reaction, which leads to the formation of chromium sulphides from chromium carbides and (ii) a reduction in overcrowding through the formation of microsulphides. The critical free energy W* for the nucleation of microsulphide is considered with the help of the Burke equation.

Observation of unequal densities for sulfur defects in silicon predeposited by low fluence ion implantation*

The dynathermal current and capacitance responses of two isotopes of sulfur, 32s and 34s predeposited by low fluence ion implantation into siliconp +- n junction diodes are presented. As opposed to all previous studies, in this work unequal densities are seen for the shallow and deep sulfur defects, for both the 32s− and 34− related defects. Additionally, the dynathermal response of the deep 34s center is seen to exhibit a shift of 3 K from that of 32S at a fixed heating rate of 9.5 ±0.1 K/s, consistent with the isotope dependence of the thermal emission process observed earlier for these cen-ters by isothermal capacitance measurements.

Thulium as a monitor of lattice damage in cadmium sulphide

The Tm3+ sharp line emission spectrum at 10 K in cadmium sulphide has been shown to be very sensitive to electron radiation damage at sample temperatures above 320 K and at incident electron energies above 114 keV (identical with 8·7 eV sulphur displacement threshold energy). Changes in the rare earth emission spectra result from the production of complex centres involving thulium ions and sulphur defects. The radiation-induced defects can be removed completely by warming the sample to temperatures greater than 420 K, and activation energies for annealing have been determined.

Radiation-Induced Changes in the Photoluminescence Spectra of Cadmium Sulfide

Radiation-induced changes in the photoluminescence spectra of cadmium sulfide have been measured over a wavelength range from 4800 to 5400 A. Gamma-ray induced changes in the 4.2°K photoluminescence spectra show a reduction in intensity of bound exciton transitions I1 and I5 and edge emission in contrast to an enhancement of a spectral line at 4867.2 A (which we designate as IR). These changes were measured for irradiation temperatures of 300°, 78°, and 4.2°K. The 78°K photoluminescence shows that exciton emission is more sensitive to radiation-induced changes than is edge emission. Thermal annealing takes place below room temperature, particularly in the 140° to 260°K region. The sensitivity of the luminescence intensity to radiation exposure only at cryogenic temperature anid the thermal annealing of the radiation-induced changes are attributed to the mobility of defects at and below room temperature. Studies of polarized luminescence and the temperature dependence of the luminescence show that IR is like the I2; exciton lines. Optical quenching at 4.2°K indicates that the defect responsible for IR is sensitive to its charge state. The results of electron exposures at various energies at approximately 90°K show that IR is associated with a sulfur defect, possibly SI