Article

Study of As―Se―Te glasses by neutron-, X-ray diffraction and optical spectroscopic methods

Article

Study of As―Se―Te glasses by neutron-, X-ray diffraction and optical spectroscopic methods

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Abstract

The atomic structures of amorphous As 40 Se (60−x) Te x (x= 10 and 15) and As 40 Se 60 glasses have been investigat-ed by neutron and high energy X-ray diffraction methods. The two datasets were modeled simultaneously by reverse Monte Carlo (RMC) simulation technique. The RMC simulations revealed a glassy network built-up from As(Se, Te) 3 pyramids in which Te atoms substitute Se atoms. The As―Se correlation function shows a strong and sharp first peak at 2.4 Å and two broad and much less intense peaks at 3.7 and 5.6 Å, related to 1st, 2nd and 3rd neighbor distances of the As―Se bonds, respectively. They are an evidence for existence of short and medium ordering in the studied glasses. The similarity of Θ Te―As―Te and Θ Se―As―Se bond distributions suggests that Te atoms have a similar role in the structure formation as Se atoms. The FTIR spectra analysis revealed impurity bonds of Se―H, As―O, Se―O, and Te―O in the glasses which contributed to enhanced ab-sorption in visible spectral range. From the ellipsometric data analysis the optical constants and the energetic parameters of the studied glasses were established. The compositional variation of these parameters is explained in terms of chemical bonds formation and change in the density of charged defects.

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... Hence, the recorded bands are vibrational bands due to extrinsic impurities being present in the studied glass system. These bands were identified on the basis of the reported data in [30][31][32] and represented in Fig. 1(b). The observed band centered at around 4.24 μm and 6.14 μm can be assigned to the stretching mode of Se-H and Se-O bonds, respectively [30,32] while two vibrational bands of the hydroxyl groups Se-OH are situated at 2.9 μm and 3.45 μm [31,32]. ...
... These bands were identified on the basis of the reported data in [30][31][32] and represented in Fig. 1(b). The observed band centered at around 4.24 μm and 6.14 μm can be assigned to the stretching mode of Se-H and Se-O bonds, respectively [30,32] while two vibrational bands of the hydroxyl groups Se-OH are situated at 2.9 μm and 3.45 μm [31,32]. The broad band positioned around 9.29 μm could be due to a combined frequency of hetero-polar bonds like Se − Sb and Se − Te bonds [33]. ...
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... medium range order (MRO). The first sharp diffraction peak (maximum) (FSDP) in the distribution of Xray diffraction intensity or neutrons, the structural factor determined from them and the repeated maxima in the correlation function [15][16][17][18][19][20], as well as the appearance of the "boson" peak in Raman scattering spectra [21][22][23][24] is explained by the existence of the MRO region in the CVS materials. According to literature data [25], in semiconductors with covalent bonds between constituent atoms, the dominant factor determining the local structural order is the so-called "8-N" rule. ...
... The third weak peaks are at 4.75 Å (As 40 Se 60 ), 4.3 Å (As 40 Se 30 S 30 ) and 5.6 Å (As 40 Se 30 Te 30 ). The obtained values of the bond length for the As 40 Se 60 composition are in good agreement with the results of [5,16,17]. According to the results of [35,36], the second peaks in the g Se-Se (r) and g As-As (r) graphs could give a forecast about the existence of medium- In Table 2 are summarized the short-range order parameters, i.e. "the partial coordination numbers" N ij , i.e. the average number of j atoms around i atom, average number of neighbors, total coordination number, average coordination number. ...
... In the RMC procedure, starting from the initial atomic configuration, the initial atomic configurations are iteratively adjusted for increasingly improved convergence with the S(Q) provided by the experiment. For the starting configuration, we have used the initial atomic configuration constructed according to the protocol already established in our earlier work on similar glassy systems [4,51]. During the RMC runs, cut-off constraints were used, constraining the minimum inter-atomic distances between two atoms. ...
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a b s t r a c t New quaternary chalcogenide Ge x Sb 40Àx S 50 Te 10 (x = 10, 20 and 27 at.%) and Ge x Sb 40Àx S 55 Te 5 (x = 20 and 27 at.%) glasses have been synthesized and the compositions have been characterized applying prompt gamma-ray activation analyses, neutron diffraction, and material density measurements. Using the experimental data, the basic physical parameters, such as average atomic volume, packing density, com-pactness, average coordination number, number of constrains, average heat of atomization and cohesive energy, of the synthesized glasses are evaluated and the results are discussed in a function of glass composition. Ó 2009 Elsevier B.V. All rights reserved.
Article
The use of hard X-rays (60–300 keV) for diffraction studies of disordered materials has several advantages: higher resolution in direct space, smaller correction terms, removal of truncation effects, the possibility for operating in extreme environments and for direct comparison between X-ray and neutron data. A feasibility study of amorphous silica has been performed at 95 keV, using a wiggler synchrotron beam-line at HASYLAB and a cylindrical sample, 3 mm in diameter. The range of Q between 0.8 and 32 Å−1 was covered. A thorough discussion of the experimental challenges is given. The resulting systematic error intrinsic to the scattering process (not including errors in the form-factors) is found to be of the order of 0.2%. The data have been analyzed in terms of a model of the short-range order. The OSiO bond angle distribution is found to be nearly Gaussian, centered around 109.3(3)° with a rms value of 4.2(3)°. For the SiOSi bond angle, several types of distribution V(α) = V1(α) sin(α) were investigated. Best fits were obtained for rather broad distributions with V having its maximum at 147° and V1 at 180°.
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