Edge-sharing Research Articles

Structural and vibrational characteristics of Ge-S-Br glass system: ab initio calculation and Raman study

To improve the understanding of the structure network of chalcogenide glass, we report an investigation of the structure and vibrational property of the Ge-S-Br system chalcogenide glass. The structural origin was interpreted by the result of ab initio calculation performed on the basic units such as ethane-like(ETH) structure(Ge2S6 and Ge2Br6),edge-sharing(ES) structure Ge2S2SnBr4-n (n=0,1,2,3,4) and isolated GeSnBr4-n tetrahedron (n=0,1,2,3,4), which represent the local structure of Ge-S-Br glassy system. In addition, We have compared the experimental spectra of GeS2, GeS3Br, GeSBr2 glasses and liquid GeBr4 to the calculated main vibrational modes of basic units by ab initio calculation. The calculated vibrational modes of the GeSnBr4-n cluster are assigned by group theory. Moreover, the symmetrical stretching modes related to the Ge-S and Ge-Br bond are in accordance with the vibrational coupling theory. The variation of calculated main vibrational modes of GeSmBrn clusters reveals the structure formations of Ge-S-Br glassy system with different S/Br ratios, which can be in agreement with vibrational spectra of Ge-S-Br glass.

Effect of topologically close-packed clusters on the pair distribution function of amorphous Zr under different pressures

With the development of high-pressure technology, pressure as a thermodynamic parameter is an important variable to control and influence the glass transition of metal melt. Therefore, in order to explore the influence of pressure on the glass transition of zirconium (Zr) melt under rapid solidification, the molecular dynamics simulation method has been adopted to obtain Zr metallic glass (MGs) by rapid solidification at the cooling rate of 1.0 × 1011 K/s under five different pressures. The results indicate that the glass transition temperature (Tg) and the topologically close-packed (TCP) clusters have increased with the increasing pressure, resulting in the reduction of configurational entropy. The second peak of the pair distribution function (g(r)) gradually changes from the left lower than the right to the left higher than the right. A13[1/S444 10/S555 2/S666], Z12[12/S555], and Z14[12/S555 2/S666] clusters increase significantly with the increasing pressure, which is the reason for the second peak splitting of the g(r) curve. At 57.5 GPa (P5), Z12 and Z14 which account for 16.8% are concentrated in space and form large nanoclusters with a layered distribution. In the layered structure, Z12 and Z12 are connected by edge-sharing (ES), but Z12 and Z14 are connected by intercross-sharing (IS). It is confirmed that the second peak splitting of the g(r) curve is mainly caused by Z12, Z14, and both connecting. The distance from the peripheral atom to the central atom corresponds to R2/R1 = 1.56, R3/R1 = 2, and R4/R1 = 2.46 of the g(r) curve, and R1–4 represents different radial distances respectively. These results provide theoretical guidance for understanding the influence of pressure on the microstructure of MGs.

Raman scattering evidence on the correlation of middle range order and structural self-organization of As-S-Ge glasses in the intermediate phase region

The Raman scattering of bulk nonstoichiometric chalcogenide alloys along the pseudo-binary AsS3 – GeS4 tie-line, which completely lies in the intermediate phase (IP) region of As-S-Ge ternary system was investigated in order to reveal the structural transformations in charge of the unusual features of the middle range order, elastic and physical-chemical parameters of these glasses observed earlier. It is shown that very narrow compositional areas can form inside the IP with high level of structural self-organization, mainly due to sudden increase of the concentration of highly flexible species such as quasi-tetrahedral (QT) S=As(S1/2)3 s.u. that comprises 4-fold coordinated As atoms. In the (GeS4)x(AsS3)1-x ternary system such a compositional area appears around the Ge7.7As15.3S77 (x =0.33; < r > = 2.31) composition that is a strongly self-organized glass with an assessed concentration of QT S=As(S1/2)3 around 30 % of total atomic clusters being the building blocks of the structure. This composition consists of minimal number of atomic building species, apart from QT S=As(S1/2)3 s.u. and free sulfur only the AsS3 pyramids and edge-sharing (ES) GeS4 tetrahedra have been revealed. Actually this self-organized state seems to be compositionally “metastable” as the variation of Ge concentrations by approximately ±3.5at% transforms the most part of the QT S=As(S1/2)3 s.u. into either AsS3 pyramids or corner-sharing (CS) GeS4 tetrahedra, mixed with a number of others clusters of smaller concentration. A good correlation was found between the compositional dependence of Raman scattering normalized strengths and middle range ordering (MRO) parameters such as sizes of molecular domains (D) and correlated inter-domain distances (d), as well as the molar volume (Vm ) and longitudinal elastic modulus (CL). MRO parameters and molar volume exhibit global minima around composition (As15,3 S77 Ge7,7; x = 0.33; < r > = 2.31) corresponding to strong self - organized glass that assign it as glass with most “packed” MRO structure and meanwhile the elastic modulus reaches its maximum value.

Aging-Induced Structural Evolution of a GeSe2 Glass Network: The Role of Homopolar Bonds

The structural evolution of GeSe2 glass during aging is studied using Raman spectroscopy and density relaxation measurements. The Raman spectra indicate volume- and entropy-driven changes in the relative concentrations of the corner-sharing (CS) and edge-sharing (ES) GeSe4 tetrahedra and in the degree of chemical order of the tetrahedral network during aging at 65 °C below the nominal glass transition temperature. The attendant structural changes involve a progressive increase in the CS:ES ratio and in the chemical order that can be expressed in the form of a reaction Ge-Ge + Se-Se → 2 Ge-Se, which shifts to the right, with lowering of fictive temperature. The isothermal relaxation of both the structure and density during aging displays rather similar stretched exponential kinetics with a stretching exponent β ∼0.54 and an average relaxation time of ∼13.5 h. In situ high-temperature Raman spectroscopic measurements indicate that structural relaxation does not affect the anharmonicity of the vibrational potential wells in the energy landscape of GeSe2 glass.

Crystal growth and thermal annealing of AgGaGe5Se12 crystal

AgGaGe5Se12 is a new quaternary nonlinear optical crystal which is applied to laser frequency conversion, especially converting 1.064 µm light to mid-IR range. In this work, we have synthesized the AgGaGe5Se12 polycrystal by the two-zones temperature method. By the modified Bridgman method, we have grown high-quality AgGaGe5Se12 single crystal 30 mm in diameter and 50 mm in length. The structure refinement was carried out by the Rietveld method. Besides, the XRS results were analyzed by comparing with AgGaGeS4 and AgGaGe5S12. In addition, the non-polarized Raman spectra were recorded and the intense peaks were observed at 200 and 232 cm−1. The peaks were attributed to corner-sharing (CS) and edge-sharing (ES) clusters. At last, the crystal was characterized by a high transparency in the 0.6–15 µm spectral range and the absorptions at 3.4 and 7 µm, observed in the as-grown crystal, have been eliminated after annealing. Thus, AgGaGe5Se12 is appropriate for further optical experiments and more comprehensive applications.

Chemical and Structural Variety in Sodium Thioarsenate Glasses Studied by Neutron Diffraction and Supported by First-Principles Simulations.

Sodium-conducting sulfide glasses are promising materials for the next generation of solid-state batteries. Deep insight into the glass structure is required to ensure a functional design and tailoring of vitreous alloys for energy applications. Using pulsed neutron diffraction supported by first-principles molecular dynamics, we show a structural diversity of Na2S-As2S3 sodium thioarsenate glasses, consisting of long corner-sharing (CS) pyramidal chains CS-(AsSS2/2)k, small AspSq rings (p + q ≤ 11), mixed corner- and edge-sharing oligomers, edge-sharing (ES) dimers ES-As2S4, and isolated (ISO) pyramids ISO-AsS3, entirely or partially connected by sodium species. Polysulfide S-S bridges and structural units with homopolar As-As bonds complete the glass structure, which is basically different from structural motifs predicted by the equilibrium phase diagram. In contrast to superionic silver and sodium sulfide glasses, characterized by a significant population of isolated sulfur species Siso (0.20 < Siso/Stot < 0.28), that is, sulfur connected to only mobile cations M+ with a usual M/Siso stoichiometry of 2, poorly conducting Na2S-As2S3 alloys exhibit a modest Siso fraction of 6.2%.

Annealing effect on elastic and structural behavior of Tantalum monatomic metallic glass

Molecular dynamics simulation with embedded atom method (EAM) interatomic potential has been devoted to investigating the elastic and structural behavior of Tantalum (Ta) monatomic metallic glass (MG). Ta-MG samples were heat treated for periods ranging from 0 to 10 ns at three aging temperatures (500 K, 900 K and 1100 K). Our results revealed an improvement in terms of elastic constants C11 and C44, and a reduction of C12 when annealing time and temperature increase. During annealing process, the Ta monatomic MG undergoes a hardening and its yield strength increases. This has been correlated to the increase of Young's modulus (E) and shear modulus (G) which suggests the improvement of the elastic behavior and the resistance to the shear bands (SBs), respectively. We found that during aging process, the isotropic property of Ta monatomic MG remains invariants. The self-diffusion coefficient (D) has been found to be more pronounced at 1100 K near the glass transition temperature, which leads to an atomic vibration by reducing the amount of free volume accumulated during the glass transition and inhibits the localization of certain defect such as shear bands (SBs). We have also found that the <0,0,12,0> full icosahedral short-range order (SRO) increases as annealing time increases. At medium-range order (MRO), the full icosahedral cluster <0,0,12,0> has been observed to be connected by intercross-sharing (IS), face-sharing (FS), Edge-sharing (ES) and vertex-sharing (VS). These enhancements in structural properties are the principal key to interpret the achieved hardening of Ta monatomic glass under annealing process.

Dimeric Molecular Structure of Molten Gallium Trichloride and a Hidden Evolution toward a Possible Liquid-Liquid Transition.

Group 13 trihalides MY3 (M = Al, Ga, and In; Y = Cl, Br, and I) mostly having a dimeric M2Y6 molecular structure in the solid state and a mixture of M2Y6 dimers and MY3 monomers in the vapor phase are potential candidates for entropy-driven liquid-liquid transition M2Y6 ⇄ 2MY3 at elevated temperatures. Using pulsed neutron diffraction and high-energy X-ray scattering supported by structural modeling, we show a dimer molecular structure of liquid GaCl3 above the melting point at 351 K and midway between the boiling point (474 K) and the critical temperature (694 K) with almost hidden characteristic evolution toward a possible liquid-liquid transition. In contrast to edge-sharing (ES) dimers in solid and vapor of D2h symmetry, the ES Ga2Cl6 molecules in the melt have a puckered structure of the central four-membered ring with shorter Cl-Cl (2.90-3.09 Å) and longer Ga-Ga (3.20-3.26 Å) second-neighbor correlations. The elongation of Ga-Ga intramolecular distances with increasing temperature simultaneously with diminished Cl-Cl nearest neighbor contacts destabilizes the ES dimers, indicating the first step toward dimer dissociation.

Structural behavior of Tantalum monatomic metallic glass

We simulated the Tantalum (Ta) system to form monatomic metallic glass (MG), using molecular dynamics (MD). The simulation is investigated within the framework of the embedded atom method (EAM) potential. Owing to studying the local atomic structure of Ta monoatomic MG, several structural analysis techniques are adopted to characterize the polyhedral network in terms of short-range order (SRO) and medium-range order (MRO). The simulation results confirm that the splitting in the second peak of the radial distribution function (RDF) proving the formation of amorphous phase. We found that the distorted Voronoi icosahedra <0,1,10,2> is the most dominant polyhedral cluster in the formed Ta monatomic MG. From another side, in terms of MRO, the icosahedral clusters are found to be connected via intercross-sharing (IS), face-sharing (FS), edge-sharing (ES) and vertex-sharing (VS). We have also found that the five-fold symmetry governs the formation of the amorphous state and may behaves as a principal indication of the formation of the glassy state during the cooling process. The ratio Ri/R1 of RDF obtained showed the presence of a hidden crystalline order in Ta monatomic MG, which can be interpreted by a combination between spherical- periodic order (SPO) and local translational symmetry (LTS). Furthermore, the atomic-level analysis of the structure revealed that the glass has a fractal dimension (Df = 2.59) between the center atom and the first coordination shell, while beyond the first coordination shell Df = 3.01, which suggests the development of fractal dimension at short distance.

Identification and tracking of different types of crystalline nucleiduring isothermal crystallization of amorphous Ag

The isothermal crystallization of amorphous Ag is investigated by a molecular dynamics (MD) simulation, and the heredity and evolution of different types of crystalline clusters aretracked and analyzed by a reverse tracking method of atom trajectories with the help of cluster-type index method (CTIM) based on Honeycutt-Anderson (H-A) bond-type index. According to the difference in the type of crystalline cluster and the linkage mode, i.e., vertex-sharing (VS), edge-sharing (ES), face-sharing (FS) and intercross-sharing (IS), a cluster analysis method which can efficiently characterize fcc single-crystal, fcc poly-crystal and fcc hydrid-crystal, is proposed. That is, the IS-linkage of fcc basic clusters, i.e., a fcc medium range order, is defined as a fcc single-crystal cluster. The extended cluster of fccbasic clusterslinked by ISand FS modes is named fcc poly-crystal clusters. In the case of IS-linkages, if the majority of core atoms arefcc atoms, the extended cluster composed of fcc, hcp and bcc basic clusters will be regarded as a fcc hydrid-crystal cluster. Moreover, a structural analysis method of critical nuclei distinguishing embryosis also developed in terms of the hereditary characteristics of various crystalline clusters. In this scheme, the extended cluster which has only transient heredity and no continuous heredity is defined as an embryo, while it will be named nuclei if part of core atoms in extended clusters can keep cluster type of atoms unchanged and be continuously passed down in the early stage of crystallization. Thus, corresponding to the onset time/temperature of continuous heredity, the critical nuclei of fcc singe-crystals, fcc poly-crystals and fcc hybrid-crystals can be identified and characterized. It is found that the nuclei of fcc crystalsemerge after the steep drop of total energy of system and before the abrupt increase of sizesof tracked clusters. And regardless of critical sizes or geometric configurations, an evident difference exists among fcc singe-crystal, hybrid-crystal clusters and fcc poly-crystal clusters, of which the fcc single-crystal nucleus is the smallest (~1.6 nm ×1.0 nm × 1.1 nm), followed by poly-crystal nucleus (~1.7 nm × 1.0 nm × 1.6 nm) and hydrid-crystal nucleus (~2.3 nm × 2.0 nm × 2.4 nm) in sequence. There are a few hcp and bcc atoms at surfaces, i.e. shells, of single-crystal and poly-crystal nucleus, but neither hcp nor bcc atom can be detected at the shell of fcc hydrid-crystal nucleus. And theconfiguration of fcc single-crystal, poly-crystal and hydrid-crystal critical nuclei are all non-spherical.

Short and medium-range orders in Co3Al metallic glass

This brief overview presents classical molecular dynamics simulations that we employed to examine the glass formation and cluster evolution during the rapid solidification of Co3Al metallic alloy. The structural characteristics at short-range order (SRO) and medium-range order (MRO) in Co3Al metallic glass are investigated by means of several structural analysis methods, radical distribution functions (RDF), structure factor, coordination number (CN) and Voronoi tessellation analysis (VTA). The well-known characteristics of glassy metallic alloys have been observed in Co3Al indicating the glass formation in this system. The Voronoi tessellation analysis showed that, although the decrease of temperature enlarges the population of <0,3,6,4> which is identified as the basic local structural motif in Co3Al glass state. The partial RDF of the icosahedral-like clusters is then further analyzed, providing insight into the packing of the MRO. Based on the perspective of different connection schemes of atomic packing motifs including vertex-sharing (VS), edge-sharing (ES), face-sharing (FS) and intercross-sharing (IS), the splitting of the second peak in the (RDF) of metallic glasses is further characterized, suggesting that the first and second sub-peaks result from the formation of <0,0,12,0> and <0,1,10,2>respectively.

Observation of photobleaching in Ge-deficient Ge16.8Se83.2 chalcogenide thin film with prolonged irradiation

We presented the unusual result of photobleaching (PB) in Ge-deficient Ge16.8Se83.2 thin films with continuous irradiation with 560 nm laser for 12000 s, which is contradicted with the previous reports that the PB only occurs in GexSe100-x films with x > 30. Observation of the dynamics variations of the photo-induced effects indicated that, photodarkening (PD) appears almost instantaneously upon light irradiation, saturates faster in a shorter time scale, and then photobleaching (PB) becomes dominant. Moreover, both PD and PB process accelerates with increasing irradiation power density. Raman spectra provided the evidence on the change of the photostructure of the samples, e.g. the structural transformation from Ge(Se1/2)4 edge-sharing (ES) to corner-sharing (CS) tetrahedral and homopolar Ge-Ge and Se-Se bonds to heteropolar Ge-Se bonds.

Atomic packing and medium-range order in Ni3Al metallic glass

In this report, classic molecular dynamics simulations based on the embedded atom method (EAM) are carried out to study the atomic arrangement in Ni3Al metallic glass. The short-range order (SRO) and medium-range order (MRO) in the rapidly quenched Ni3Al alloy are characterized by the both structural analysis methods; Radial distribution function (RDF) and Voronoi tessellation. From RDF, we found a splitting in the second peak that reflects the atomic packing beyond the nearest neighbors and it is a characteristic of the MRO. Furthermore, the transition from the supercooled state to the glassy state of Ni3Al alloy leads to an increased population of icosahedral-like clusters and their spatial connectivity via vertex-sharing (VS), edge-sharing (ES), face-sharing (FS) and intercross-sharing (IS). Interestingly, by comparing the position ratios of subpeaks in RDF with those of the shared cluster distance to the first peak in RDF, we suggest that the first subpeak originates from the face-sharing clusters while the second one results from the vertex-sharing clusters. However, the edge-sharing is hidden between the two subpeaks due to its smaller fraction in the system.

Molecular dynamics study of atomic-level structure in monatomic metallic glass

Molecular dynamics simulations have been performed to study the vitrification process and the local atomic structures in Ni monatomic metallic glasses (MG). The interatomic interactions are described by embedded atom method (EAM) potentials. Short-range order (SRO) and medium range order (MRO) in nickel monatomic MG is studied using several structural techniques such as the Radial distribution function (RDF); the Common neighbor analysis method (CNA) and the Voronoi tessellation. As a result, we found that the atomic packing in metallic glasses can be described globally as a superposition of the spherical-periodic order (SPO) and the local translational symmetry (LTS). From CNA method, we found that the majority of icosahedral clusters in Ni monatomic MG are connected together by vertex-sharing (VS), edge-sharing (ES), face-sharing (FS) and intercross-sharing (IS) rather than isolated clusters. These typical cluster connections constitute to the partial RDF of icosahedral atoms except the edge sharing which is hidden between the second and the third subpeaks of RDF. Furthermore, the visualization technologies are applied to follow the formation of clusters during rapid quenching. It is found that the clusters ⟨0,1,10,2⟩ and ⟨0,2,8,4⟩ are more dominant than the full icosahedral polyhedron ⟨0,0,12,0⟩ in our monatomic system. Furthermore, the splitting of the second peak in RDF curve in Ni glass is not only caused by the icosahedral clusters, but also by the Voronoi polyhedrons ⟨0,1,10,2⟩ and ⟨0,2,8,4⟩.

Midgap states, Raman scattering, glass homogeneity, percolative rigidity and stress transitions in chalcogenides

AbstractRaman scattering from binary GexSe100 − x glasses excited using 1064 nm radiation display vibrational modes whose linewidths significantly exceed those observed using 647 nm radiation. In these glasses, 1064 nm radiation excites midgap states while 647 nm radiation excites conduction band tail states. Presence of midgap states in glasses, ascribed to coordination defects, is responsible for vibrational mode broadening that smears glass compositional variation of mode frequency using 1064 nm radiation but not with the 647 nm radiation. In the latter, mode‐frequency variation of Corner‐ (CS) and Edge‐sharing (ES) tetrahedral units, in specially prepared homogeneous glasses, display thresholds near the rigidity (x = 19.5%) and stress (x = 26.0%) transitions, opening an intermediate phase (IP) that correlates well with the reversibility window (blue region) observed in calorimetric measurements (figure on the right). magnified imageCompositional trends in the non‐reversing enthalpy of relaxation at Tg, ΔHnr(x), in binary GexSe100 − x glasses examined in 1997, 2009, and 2011. The reversibility window (blue region) progressively sharpens to acquire a square‐well like behaviour as melts/glasses homogenize. Figure taken from Ref. 1.

Stiffness transitions inSixSe1−xglasses from Raman scattering and temperature-modulated differential scanning calorimetry

Temperature-modulated differential scanning calorimetry (MDSC) measurements on ${\mathrm{Si}}_{x}{\mathrm{Se}}_{1\ensuremath{-}x}$ glasses show glass transitions to be thermally reversing in character in the composition window $0.20&lt;x&lt;0.27.$ Raman scattering shows a trimodal distribution of vibrational modes, identified with corner-sharing (CS), edge-sharing (ES), and Se-chain modes (CM). In the floppy region, $0&lt;x&lt;0.19,$ these modes are symmetric and are characteristic of a random network. In the transition region, $0.20&lt;x&lt;0.27,$ the ES and CM split into doublets suggesting appearance of extended range structural correlations. In the percolatively rigid region, $x&gt;0.27,$ the CS mode in addition to the ES and CM also splits into a doublet indicating growth of substantial medium range structure. The large compositional width $(0.20&lt;x&lt;0.27)$ associated with Raman elastic thresholds coincides with the thermally reversing window from MDSC.

Crystalline fragments in glasses.

The nature of tetrahedral molecular fragments is investigated in ${\mathrm{SiSe}}_{2}$ glasses using the molecular-dynamics method. The glass consists of both edge-sharing (ES) and corner-sharing tetrahedra. The ES tetrahedra are the building blocks of chain-like-molecular fragments. The two-edge-sharing tetrahedra are the nucleus, and corner-sharing configurations provide connecting hinges between fragments. Statistics of rings and fragments reveals that threefold and eightfold rings are most abundant, chainlike fragments that are typically 10--15 \AA{} long occur mostly in eightfold rings, and the longest fragments occur in elevenfold rings.