Effect of hydrostatic pressure on the elastic moduli of As2S3 and As2Se3 glasses at 195 and 296 K

Velocities of 30 MHz longitudinal and shear ultrasonic waves have been measured in As₂S₃ and As₂Se₃ glasses as a function of hydrostatic pressure up to 1.5 kbar at 195 K and 3 kbar at 296 K. The elastic stiffness moduli are found to have relatively large, positive, pressure dependences which are about the same at both temperatures for both glasses. This behavior is attributed to the weakness of bonding between layers comprised of AsS₃ and AsS₃ pyramids.Inspection of data for a variety of glasses reveals a correlation between the value of C_L/3C_T and whether the elastic moduli are increased or decreased by pressure. (C_L is the longitudinal modulus and C_T the shear modulus.)Using the pressure dependences of the elastic moduli obtained in the present work, it is found that volume change is responsible for most of the temperature dependences of the moduli. In addition elastic gammas are obtained which are consistent with thermal Grüneisen gammas at 12 K. The pressure dependence of the volume of As₂S₃ glass at 296 K is calculated using the present results in the Murnagham equation. Agreement with volumetric data of Weir is obtained.     

Short, intermediate and mesoscopic range order in sulfur-rich binary glasses

Pulsed neutron and high-energy X-ray diffraction, small-angle neutron scattering, Raman spectroscopy and DSC were used to study structural changes on the short, intermediate and mesoscopic range scale for sulfur-rich AsS_x (x ? 1.5) and GeS_x (x ? 2) glasses. Two structural regions were found in the both systems. (1) Between stoichiometric (As₂S₃ and GeS₂) and ‘saturated’ (AsS_2.3 and GeS_2.7) compositions, excessive sulfur atoms form sulfur dimers and/or short chains, replacing bridging sulfur in corner-sharing AsS_3/2 and GeS_4/2 units. (2) Above the ‘saturated’ compositions at [As] < 30.5 at.% and [Ge] < 27 at.%, sulfur rings and longer sulfur chains (especially in the AsS_x system) appear in the glass network. The glasses become phase separated with the domains of 20-50 Å, presumably enriched with sulfur rings. The longer chains S_n are not stable and crystallize to c-S₈ on ageing of a few days to several months, depending on composition.     

Growth and Raman spectroscopic characterization of As4S4 (II) single crystals

As described by Kutoglu (1976 [16]), single crystals of As₄S₄ (II) phase have been grown using a new two-step synthesis that drastically increases the reproducibility that is attainable in synthetic experiments. First, through photo-induced phase transformation, pararealgar powder is prepared as a precursor instead of AsS melt. Then it is dissolved and recrystallized from CS₂ solvent. Results show that single crystals of the As₄S₄ (II) phase were obtained reproducibly through the dissolution–recrystallization process. Single crystals of As₄S₄ (II) obtained using this method were translucent and showed a uniform yellow-orange color. The crystal exhibits a platelet-like shape as a thin film with well-developed faces (0 1 0) and (0 1¯ 0). The grown crystals are as large as 0.50×0.50×0.01 mm. They were characterized using powder and single crystal X-ray diffraction techniques to confirm the phase identification and the lattice parameters. The As₄S₄ (II) phase crystallizes in monoclinic system with cell parameters a=11.202(4) Å, b=9.954(4) Å, c=7.142(4) Å, β=92.81(4)°, V=795.4(6) Å³, which shows good agreement with the former value. Raman spectroscopic studies elucidated the behavior of the substance and the relation among phases of tetra-arsenic tetrasulfide.     

Properties and structure of glasses in the system AsS

In obtaining the glass, at first, the raw materials were mixed up and melted by using porcelain crucible, but in this case it was found by infra-red absorption method that the glass contains some oxygen impurities. Then a range of glasses was prepared by melting elementary pure As and S in definite proportions in a sealed vacuum tube. The infra-red absorption, molecular volume, molecular refraction, hardness, thermal expansion and viscosity of sample glasses with various compositions were investigated and, additionally, solubility of these glasses into CS₂ was measured. The structure of glasses in the system AsS was studied by X-ray diffraction and a structural model was set up. From the measurements the following conclusions were made. The structure of As₂S₃ glass is a distorted form of the crystalline orpiment structure. With increasing S content above As₂S₃, S is likely to exist in the chain-like form, but when the S content is greater than that in As₂S_8–10, both chain-like and ring type forms co-exist. With decreasing S content below As₂S₃, S between As and As is eliminated and AsAs bonds are formed. Consequently a deformation of layer occurs and an expansion of the layer distance was observed.     

Raman scattering of the mixed chalcogenide glass system As2SxSe3-x

Polarized room temperature Raman spectra of glassy As₂S_xSe_3-x for 0≦×≦3 have been measured. Spectra for crystalline As₂S₁Se₂ are reported. The polarization and intensity dependence upon composition are consistent with mixed pyramids of composition As₂S_nSe_3-n and preclude phase separation in the glassy system.     

Structural study of AsS2–Ag glasses over a wide concentration range

(As_0.33S_0.67)_100-xAg_x (0 ≤ x ≤ 28) bulk glasses showing micro-phase separation in a wide concentration range have been studied by X-ray diffraction, neutron diffraction and extended X-ray absorption fine structure measurements. The AsAgS₂ composition, which forms a homogeneous glass, is modeled with the reverse Monte-Carlo simulation technique. It is established that Ag prefers the environment of S; Ag―As bonding cannot be observed. Similarly to the AsAgS₂ crystalline modifications smithite and trechmannite, the main structural units of the glass are AsS₃ pyramids. The covalent network of As and S atoms becomes fragmented in the glassy AsAgS₂ unlike in the glassy AsS₂. The environment of Ag atoms in the AsAgS₂ glass differs from that in the crystalline state. In average, each Ag atom has four nearest neighbors, three of them being S and one being Ag.     

The structure of vapour-deposited arsenic sulphides prepared using a cooled (78 K) substrate

Amorphous molecular As₄S₄ and non-stoichiometric “As₂S₃” evaporated films have been prepared by condensing onto a substrate at liquid nitrogen temperature in situ in the X-ray diffractometer. The structures of these films are shown to be similar, thus confirming the importance of As₄S₄ (realgar-type) molecules in amorphous films prepared by evaporation of “As₂S₃” glass. The local molecular packing in the as-deposited amorphous As₄S₄ film shows a preference for a packing arrangement similar to that found in the α-crystalline phase. Both as-deposited films contain gross structural defects such as molecular clusters and/or voids which irreversibly reduce in importance as the film structures relax on warming to ambient temperatures. Also, the As₄S₄ film partially devitrifies to the high-temperature β-crystalline phase on warming to ambient, but no devitrification occurs in the non-stoichiometeric film due to the presence of more than one molecular type.     

Raman and infrared spectra of As2Sx chalcogenide glasses with x ⩽ 3

Raman laser and far infrared spectra of As₂S_x glasses with x ? 3 are given and discussed. The purpose of the work is to bring a vibrational spectroscopic contribution to the study of these glasses and to the hypothesis, still under discussion in the literature, that they might be constituted either by a homogeneous vitreous phase or by a mixture of As₂S₃ and As₄S₄.Our results confirm a phase separation, with formation of ß-As₄S₄, below a certain value of x, which depends not only on the preparation method of the samples but also on other factors such as melting time. Laser irradiation of ß-As₄S₄ modifies its Raman spectrum. Such a phenomenon is attributable to two principal factors, either a partial polymerization or formation of a species richer in arsenic. A structural and formation model of the As₂S_x glasses is given, starting from a more generalized structural model of vitreous As₂S₃ which is an accord with the vibrational results and those by the diffraction method in the literature.     

Nanocrystallites in Bi–As–S system

Bi₂S₃–As₂S₃ composite formation was performed by two methods: by the direct insertion of Bi₂S₃ nanocrystals into a molten As₂S₃ glass which was further solidified and by the crystallization of a rapidly quenched (As₂S₃)_1−x (Bi₂S₃)_x glasses with x = 0.005, 0.01, 0.02 and 0.04 at different conditions. Fine tuning of the annealing of the quenched glass as well as the mixing of nanocrystals in to the molten glass resulted glass-crystalline composites with different amounts and distribution of 20–50 nm large Bi₂S₃ nanocrystals as well as larger, up to few micrometer long, needle-like crystals. Structural and optical investigations support the presence of the Bi₂S₃ crystalline phase in all composites. Optical absorption and the photoconductivity of bulk composite samples follow the structural changes of the structure in the amorphous and amorphous-crystalline phase. In addition, the 290 cm⁻¹ characteristic band in Raman spectra may be used for tracing the formation of the nanocomposites.     

Neutron studies of an inorganic plastic glass

A neutron diffraction investigation has been performed of the structure of four chalcogenide glasses, using the D4c diffractometer on the high-flux reactor at the Institut Laue-Langevin (ILL). Vitreous Ge₃As₅₂S₄₅ is shown to have a structure based on As₄S₃ molecules, whereas that of its selenide analogue, vitreous Ge₃As₅₂Se₄₅, involves far fewer As₄Se₃ molecules. Two As₂X₃ reference glasses (X = S or Se) have purely network structures. As-X, and As-As bond lengths have been extracted, together with their associated co-ordination numbers, from peak fits to the real space correlation functions, T(r), and suggest that all four glasses are chemically ordered; i.e. that they contain the maximum possible number of As-X (and Ge-X) bonds. The molecular nature of vitreous Ge₃As₅₂S₄₅ is evident from the enhanced first diffraction peak relative to the As₂S₃ reference glass, and has been further investigated by comparing its interference function, Qi(Q), with that expected for an isolated As₄S₃ molecule. On the other hand, the second diffraction peak for vitreous Ge₃As₅₂S₄₅ is not reproduced by a random orientation molecular model, indicating that there is orientational correlation between adjacent molecules, and this is discussed with respect to the structure of the corresponding As₄S₃ crystalline phases. The neutron-weighted vibrational density of states (VDOS) for vitreous Ge₃As₅₂S₄₅, obtained with the ISIS MARI spectrometer, unambiguously confirms the existence of As₄S₃ molecules, as revealed by a peak at ~ 34 meV arising from the triangle of As atoms that form the base of the molecule. MARI data recorded at ambient temperature also reveal a strong quasi-elastic component in the scattered intensity, which has been further investigated using the IN5 quasi-elastic scattering spectrometer (ILL) and indicates the presence of rotational diffusion of the As₄S₃ molecules; i.e. that, at ambient temperature, vitreous Ge₃As₅₂S₄₅ behaves as a plastic glass. A possible structural model for vitreous Ge₃As₅₂S₄₅ comprises a two-dimensional tangled GeS₂ net with As₄S₃ molecules trapped in its folds whereas, for vitreous Ge₃As₅₂Se₄₅, a clathrate model appears more appropriate.     

The kinetics of photodissolution of Ag in amorphous As2S3 films

A new technique, measurement of the electrical resistance change of the Ag layer, is developed to study the kinetics of photodissolution of Ag in amorphous As₂S₃. It is shown that the photodissolution rate is proportional to the light intensity absorbed in the As₂S₃ at the As₂S₃Ag interface, but photoelectrons ejected from the Ag into the As₂S₃ also contribute. The process is shown to be two-stage. Firstly a critical “radiation damage” dose must be accumulated in the As₂S₃. Secondly, the Ag atom is photon-assisted across the As₂S₃Ag interface activation barrier.     

A photoelectric study on the interface between metallic silver and vitreous As2S3

The electrostatic potential at the AgAs₂S₃ interface was investigated. In the dark, cells of a structure, Ag/As₂S₃/Al behaved like and an electrochemical battery. When light fell onto the cells, short-circuit currents were observed, but their appearancesvaried much, depending on the excitation wavelength and the material of the illuminated electrode. At wavelengths longer than the absorption edge of As₂S₃ glass, photocurrents were characteristic of the polarization current and little influenced by the external field. A model for explaining these findings was proposed on an assumption of an interface reaation between silver and vitreous As₂S₃ in the dark. The interfacial reaction was supposed to accompany a charge separation leading to formation of a potential barrier at the interface.     

X-ray photoelectron spectroscopic studies on Se/As2S3 and Sb/As2S3 nanomultilayered film

Photoinduced diffusion in Se/As₂S₃ and Sb/As₂S₃ nanomultilayered thin films are studied by X-ray photoelectron spectroscopy (XPS). The XPS measurements show the atomic movements during photoinduced diffusion in Se/As₂S₃ and Sb/As₂S₃ nanomultilayered film. The analysis of experimental data describes the nature of light induced changes in different structural units.     

Structure of pulsed-laser deposited arsenic-rich As–S amorphous thin films,and effect of light and temperature

Thin amorphous films from As–S system (∼As₅₀S₅₀) were prepared by the pulsed laser deposition technique. Light- and thermally-induced changes of structure of studied films have been investigated using Raman scattering spectroscopy results and interpreted in terms of chemical reactions and/or phase transitions between individual structural units. The irradiation of as-deposited thin films causes light-induced reactions, in which As₄S₃, α- and β-As₄S₄, and pararealgar/χ-As₄S₄ molecules are formed. Thermal-annealing of exposed thin films leads to the formation of β-As₄S₄ molecules.     

Kristallzüchtung von Arsentrisulfid

The solubility of As₂S₃ in organic amines was determined. As₂S₃ crystals were prepared under pressure from CS₂-solution and hydrothermally. The (α − β) transition in As₂S₃ could not be verified, but the color change from yellow to red is probably caused by lattice defects.     

Urbach absorption edge and disordering processes in As2S3 thin films

The temperature studies of transmission spectra for as-deposited and annealed amorphous As₂S₃ thin films are carried out. The optical absorption spectra in the range of their exponential behaviour are analysed, and the dispersion dependences of refractive index as well as their temperature variation are investigated. The Urbach behaviour of optical absorption edge is revealed, the Urbach absorption edge parameters are determined, and their temperature dependences are studied. The effect of different type of disordering on the optical absorption processes in As₂S₃ thin films is studied. A comparative analysis of Urbach absorption edge parameters of As₂S₃ bulk glasses and thin films is performed.     

Anomalous anisotropic deformations of As2S3 flakes induced by linearly-polarized bandgap illumination

Semi-free As₂S₃ flakes undergo visible-scale anisotropic deformations when exposed to linearly-polarized bandgap illumination. We investigate the behavior and also those in amorphous Se, GeS₂, AgAsS₂, and crystalline As₂S₃. These results suggest that the deformation occurs through photo-induced birefringence, photo-induced fluidity, and optical force.     

Atomic structure and short- and medium-range order parameters in amorphous chalcogenide films prepared by different methods

The atomic structure of amorphous As₂Se₃ and As₂S₃ films prepared by thermal evaporation in a vacuum and by RF ion-plasma sputtering has been studied by the methods of X-ray diffraction and Raman spectroscopy. The techniques of film preparation had different conditions of substance vaporization and atom condensation on a substrate. It has been established that films prepared by these methods have significant differences in the dimensions of the medium-range order and in the local atomic structure, which causes considerable differences in their electronic properties.     

Evolution of chemical structure during silver photodiffusion into chalcogenide glass thin films

The change of chemical structure resulting after X-ray and photo-induced silver diffusion into chalcogenide glass (ChG) thin films is monitored by high resolution X-ray photoelectron spectroscopy (XPS). As₄₀S₆₀ and Ge₃₀Se₇₀ thin films, which are based on pyramids and tetrahedral structural units, are investigated as model materials. Survey, core level (As 3d, S 2p, Ge 3d, Ge 2p, Se 3d, Ag 3d_5/2, O 1s, C 1s) and valence band spectra have been recorded and analyzed. Reference point for the binding energy is established by the subsequent deposition of thin gold film on top of the measured samples. The chemical structure gradually changes during diffusion of silver in all the samples. The mechanism of change depends on the chemical composition, thickness of the diffused silver layer and conditions of irradiation. It is revealed that surface oxygen can play important role in the Ag photodiffusion process, leading to phase separation on the surface of the films. Photodiffusion of Ag into As₄₀S₆₀ film leads to the formation of a uniform ternary phase and arsenic oxides on the surface. The formation of ethane-like Ge₂(S_1/2)₆ units together with germanium oxidation are the main outcomes of X-ray induced Ag diffusion into Ge₃₀Se₇₀ film.