Local motifs in GeS2–Ga2S3 glasses Pethes, I.; Nazabal, V.; Chahal, R. ...
Journal of alloys and compounds,
07/2016, Letnik:
673
Journal Article
Recenzirano
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The structure of (GeS2)0.75(Ga2S3)0.25 and (GeS2)0.83(Ga2S3)0.17 glasses was investigated by Raman scattering, high energy X-ray diffraction and extended X-ray absorption fine structure (EXAFS) ...measurements at the Ga and Ge K-edges. The reverse Monte Carlo simulation technique (RMC) was used to obtain structural models compatible with diffraction and EXAFS datasets. It was found that the coordination number of Ga is close to four. While Ge atoms have only S neighbors, Ga binds to S as well as to Ga atoms showing a violation of chemical ordering in GeS2–Ga2S3 glasses. Analysis of the corner- and edge-sharing between GeS4/2 units revealed that about 30% of germanium atoms participate in the edge-shared tetrahedra.
•Structural models of GeS2–Ga2S3 glasses consistent with XRD + EXAFS data are created.•Chemical order is respected but Ga–Ga bonds are present caused by S-deficiency.•The coordination number of Ga is 3.7 ± 0.3.•The frequency and geometry of corner/edge-sharing GeS4/2 units were determined.
Chemical short range order and topology of GexGaxTe100-2x glasses was investigated by neutron- and x-ray diffraction as well as Ge and Ga K-edge extended x-ray absorption fine structure (EXAFS) ...measurements. Large scale structural models were obtained by fitting experimental datasets simultaneously with the reverse Monte Carlo simulation technique. Models, relying only on experimental data and basic physical information without constraining the average coordination numbers, give 3.9–4.1 for the number of the atoms in the first coordination sphere of Ge atoms, while the average number of first neighbors of Ga atoms scatters around 3.8. The average coordination number of Te atoms is significantly higher than 2 for x = 12.5 and 14.3. It is found that the vast majority of MTe4 (M = Ge or Ga) tetrahedra have at least one corner sharing MTe4 neighbor.
•Models of GexGaxTe100-2x glasses consistent with multiple datasets are constructed.•The compositions investigated are characterized by strong chemical ordering.•Ge and Ga atoms have 4 Te nearest neighbors.•Average coordination number of Te atoms increases with Ge/Ga content.•Majority of the Ge(Ga)Te4 tetrahedra are connected by their corners or edges.
Short range order and topology of GexS100-x glasses over a broad composition range (20 ≤ x ≤ 42 in at%) was investigated by neutron diffraction, X-ray diffraction, and Ge K-edge extended X-ray ...absorption fine structure (EXAFS) measurements. The experimental data sets were fitted simultaneously in the framework of the reverse Monte Carlo simulation method. It was found that both constituents (Ge and S) satisfy the Mott-rule in all investigated glasses: Ge and S atoms have 4 and 2 neighbours, respectively. The structure of these glasses can be described with the chemically ordered network model: Ge-S bonds are preferred; S-S bonds are present only in S-rich glasses. Dedicated simulations showed that Ge-Ge bonds are necessary in Ge-rich glasses. Connections between Ge atoms (such as edge-sharing GeS4/2 tetrahedra) in stoichiometric and S-rich glasses were analysed. The frequency of primitive rings was also calculated.
•Ge atoms have 4, S atoms 2 nearest neighbor in all compositions investigated.•Chemically ordered network model is consistent with the experimental data.•Ge-Ge bonds are present in Ge-rich Ge42S58 glass.•Alternating 4 and 6 membered Ge-S rings are preferred in S-rich glasses.•membered rings are the most frequent in stoichiometric Ge33S67 glass.
We report on a detailed experimental and simulation study of the short- and medium-range order of a potential phase change material – glassy KSb5S8. On the experimental side, diffraction techniques ...and EXAFS have been employed to record accurate structural data. Structural models have been generated by fitting multiple datasets simultaneously with the reverse Monte Carlo simulation technique. In addition, density functional theory was employed to study the structure and vibrational modes of selected clusters, representative of the glass structure. Unconstrained RMC simulation runs revealed that the average Sb-S coordination number is 3.18 ± 0.2, thus Sb is mostly threefold coordinated in the glassy state. The fraction of edge and corner sharing SbSn polyhedra and distribution of bridging S atoms (Qn distribution) have also been obtained. Distribution of bridging S atoms around Sb is similar in the crystalline and glassy states. DFT calculations assisted in the identification of a Raman mode at ∼468 cm−1, assigned to hypervalent bonding (quasi-tetrahedral units) in the glass structure.
Different experimental techniques (X-ray diffraction, neutron diffraction with isotopic substitution, extended X-ray absorption spectroscopy) and theoretical methods (reverse Monte-Carlo simulation, ...molecular dynamics modelling, Voronoi analysis) were applied to elucidate the atomic structure of Ni–Zr and Cu–Zr alloys in glassy and crystalline states and to explain differences in the glass-forming abilities of the Ni64Zr36 and Cu65Zr35 compositions. Both glasses show similar strong topological ordering, but it is established that the degree of chemical ordering is much more pronounced in Ni64Zr36 glass than in Cu65Zr35 glass. The short-range atomic order and topology in the glassy and crystalline structures are remarkably different, and these differences are presumed to hinder crystal nucleation and growth, hence promoting glass formation upon fast cooling of the Ni64Zr36 and Cu65Zr35 liquid alloys. The larger differences observed for the Cu65Zr35 alloy in glassy and crystalline states are suggested to play a decisive role in increasing its bulk-glass-forming ability.
Amorphous GeCu2Te3 was investigated by X-ray diffraction and extended X-ray absorption fine structure measurements at the Ge, Cu and Te K-edges. Structural models were obtained by fitting the four ...experimental datasets simultaneously by reverse Monte Carlo simulation. It was found that Ge–Ge and Cu–Cu bonding are both significant. The average coordination numbers of Cu and Te, as well as Ge, are close to four. The high average coordination number of the network contributes to the enhanced thermal stability of amorphous GeCu2Te3.
The atomic structure of Cu
35Zr
65, Cu
50Zr
50, and Cu
65Zr
35 (at.%) metallic glasses was investigated by means of high-energy X-ray diffraction, neutron diffraction and extended X-ray absorption ...fine structure spectroscopy. Three-dimensional structure models were developed by applying the reverse Monte-Carlo method and analyzed in terms of the local atomic arrangements. The geometric short-range order of Cu–Zr glasses is characterized by a variety of polyhedra. The atomic sites of the nearest neighbourhood around a Cu or Zr atom are statistically occupied by Cu or Zr atoms. The composition dependence of the short- and the medium-range order in the Cu–Zr glasses points to a statistical replacement of Cu and Zr atoms in the whole composition range. No indications were observed for the existence of a dominant structural arrangement in the Cu–Zr glasses.
The structure of Ge22Ga3Sb10S65 and Ge15Ga10Sb10S65 glasses was investigated by neutron diffraction (ND), X-ray diffraction (XRD), and extended X-ray absorption fine structure (EXAFS) measurements at ...the Ge, Ga and Sb K-edges. Experimental data sets were fitted simultaneously in the framework of the reverse Monte Carlo (RMC) simulation technique. Short range order parameters were determined from the obtained large-scale configurations. It was found that the coordination numbers of Ge, Sb and S are around the values predicted by the Mott-rule (4, 3 and 2, respectively). The Ga atoms have on average 4 nearest neighbors. The structure of these stoichiometric glasses can be described by the chemically ordered network model: Ge-S, Ga-S and Sb-S bonds are the most important. Long Sb-S distances (0.3–0.4 Å higher than the usual covalent bond lengths) are observed, suggesting that Sb atoms can be found in various local environments.
•Structure of Ge22Ga3Sb10S65 and Ge15Ga10Sb10S65 glasses was investigated.•Structural models consistent with diffraction (ND, XRD) and EXAFS data was created.•The coordination numbers of Ge, Ga, Sb and S atoms are 4, 4, 3 and 2, respectively.•Structure of these glasses can be described by the chemically ordered network model.
Short range order of glassy Ge20Ga10Se70 and Ge20Ga5Se75 was investigated by neutron diffraction and extended X-ray absorption fine structure spectroscopy (EXAFS) at Ge, Ga and Se K-edges. For each ...composition large scale structural models were obtained by fitting simultaneously the four experimental datasets in the framework of the reverse Monte Carlo simulation technique. It was found that both Ge and Ga are predominantly fourfold coordinated. The quality of the fits was strongly improved by introducing Ge–Ga bonding. Models giving the best agreement with experimental data show that Ga has a complex effect on the Ge–Se host matrix: i) it enters the covalent network by forming Ga–Ge bonds ii) by decreasing the number of Se atoms around Ge, it contributes to the formation of Se–Se bonds, which may explain the higher solubility of lanthanide ions iii) the average coordination number of Se increases due to the Ga–Se ‘extra’ bonds. The higher average coordination of the network may be responsible for the increase of Tg upon adding Ga to Ge–Se glasses.
•The coordination number of Se is significantly higher than 2.•Ga–Ge bonds can be found even in Se-rich compositions.•The formation of Se–Se bonds increases the solubility of rare earth ions.•The coordination number of Ga is 4.
Chemical order in Ge-Ga-Sb-Se glasses Pethes, I.; Chahal, R.; Nazabal, V. ...
Journal of non-crystalline solids,
03/2018, Letnik:
484
Journal Article
Recenzirano
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The short range order in Ge30Ga5Sb10Se55 and Ge21Ga5Sb10Se64 glasses was investigated by X-ray (XRD) and neutron diffraction (ND) as well as extended X-ray absorption fine structure (EXAFS) ...measurements at the Ge, Ga, Sb and Se K-edges. Large scale structural models were obtained by fitting simultaneously the experimental data sets by reverse Monte Carlo (RMC) simulation technique. It was found that Ge, Sb and Se atoms follow the Mott-rule and have 4, 3 and 2 nearest neighbors, respectively. The average coordination number of the Ga atoms was around 4. The structure of these glasses can be described by the chemically ordered network model: the Ge–Se, Ga–Se and Sb–Se bonds are the most prominent while Ge–Ge and Ge–Sb bonds are formed only in Se-poor compositions. Models generated by RMC contained some long distances (0.3–0.4 Å higher than the usual covalent bond lengths) between Ge–Se and/or Ge–Ge pairs. Dedicated simulation runs confirm the existence of these bonds.
•Structural models of GeGaSbSe glasses consistent with multiple datasets are created.•These glasses can be described by the chemically ordered network model.•Ge, Sb, Se and Ga atoms have 4, 3, 2 and 4 nearest neighbors, respectively.•Ge-Ge (or Ge–Ga) bonds can be found in both Se deficient glasses.•In Ge30Ga5Sb10Se55 glass Ge–Sb bonds also exist.
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