Structural and electronic transformations of GeSe2 glass under high pressures studied by X-ray absorption spectroscopy.

Pressure-induced transformations in an archetypal chalcogenide glass (GeSe2) have been investigated up to 157 GPa by X-ray absorption spectroscopy (XAS) and molecular dynamics (MD) simulations. Ge and Se K-edge XAS data allowed simultaneous tracking of the correlated local structural and electronic changes at both Ge and Se sites. Thanks to the simultaneous analysis of extended X-ray absorption fine structure (EXAFS) signals of both edges, reliable quantitative information about the evolution of the first neighbor Ge-Se distribution could be obtained. It also allowed to account for contributions of the Ge-Ge and Se-Se bond distributions (chemical disorder). The low-density to high-density amorphous-amorphous transformation was found to occur within 10 to 30 GPa pressure range, but the conversion from tetrahedral to octahedral coordination of the Ge sites is completed above [Formula: see text] 80 GPa. No convincing evidence of another high-density amorphous state with coordination number larger than six was found within the investigated pressure range. The number of short Ge-Ge and Se-Se "wrong" bonds was found to increase upon pressurization. Experimental XAS results are confirmed by MD simulations, indicating the increase of chemical disorder under high pressure.

Possible boron-rich amorphous silicon borides from ab initio simulations.

CONTEXT: By means of ab initio molecular dynamics simulations, possible boron-rich amorphous silicon borides (BnSi1-n, 0.5 ≤ n ≤ 0.95) are generated and their microstructure, electrical properties and mechanical characters are scrutinized in details. As expected, the mean coordination number of each species increases progressively and more closed packed structures form with increasing B concentration. In all amorphous models, pentagonal pyramid-like configurations are observed and some of which lead to the development of B12 and B11Si icosahedrons. It should be noted that the B11Si icosahedron does not form in any crystalline silicon borides. Due to the affinity of B atoms to form cage-like clusters, phase separations (Si:B) are perceived in the most models. All simulated amorphous configurations are a semiconducting material on the basis of GGA+U calculations. The bulk modulus of the computer-generated amorphous compounds is in the range of 90 GPa to 182 GPa. As predictable, the Vickers hardness increases with increasing B content and reaches values of 25-33 GPa at 95% B concentration. Due to their electrical and mechanical properties, these materials might offer some practical applications in semiconductor technologies. METHOD: The density functional theory (DFT) based ab initio molecular dynamics (AIMD) simulations were used to generate B-rich amorphous configurations.

Theoretical investigation of substituent effects on the relative stabilities and electronic structure of [BnXn]2- clusters.

In this study, we provide a theoretical evaluation of relative stabilities and electronic structure for [BnXn]2- clusters (n = 10, 12, 13, 14, 15, 16). Structural and electronic characteristics of [BnXn]2- clusters are examined by comparison with the [B12X12]2- counterparts with a focus on the substituent effects (X = H, F, Cl, Br, CN, BO, OH, NH2) on the electronic structure, electron detachment energies, formation enthalpies, and charge distributions. For the electronic structure and electron detachment energies, substituent effects on boron clusters are shown to follow a very similar trend to the mesomeric and inductive effects (± M and ± I) of π-conjugated systems, and the most stable derivatives in terms of HOMO/LUMO and electron detachment energies are calculated for CN and BO substituents due to strong -M effects. In the case of formation enthalpies for larger boron clusters (n ≥ 13), the icosahedral barrier is shown to increase with the halogen and CN substitution, whereas it is possible to reduce the icosahedral barrier for the cases of X = OH and NH2. It is shown that this reduction results from destabilizing the [B12X12]2- cluster with electronic (+ M) and symmetry effects induced by OH and NH2 ligands.

Nanosize icosahedral quasicrystal in Mg90Ca10 glass: an ab initio molecular dynamics study.

Rapid solidification of Mg(90)Ca(10) from its liquid state is studied by means of an ab initio molecular dynamics technique, and its local structure is investigated by various analyzing methods. The liquid and amorphous states are found to have slightly different short range order even though the perfect and defective icosahedral bonding environments are major bonding elements of both liquid and amorphous states. Perfect icosahedrons with a small frequency exist in the liquid state, more develop during the cooling process and they become the leading building units in the glass state, indicating an icosahedral short range order in Mg(90)Ca(10) glass. Also the linked icosahedrons lead to an icosahedral medium range order. Furthermore, an ordered arrangement of some icosahedrons in the hexagonal symmetry is observed in the glass model, representing a nanoscale icosahedral quasicrystalline phase in Mg(90)Ca(10) glass.

New transformation mechanism for a zinc-blende to rocksalt phase transformation in MgS.

The stability of the zinc-blende structured MgS is studied using a constant pressure ab initio molecular dynamics technique. A phase transition into a rocksalt structure is observed through the simulation. The zinc-blende to rocksalt phase transformation proceeds via two rhombohedral intermediate phases within R3m (No:160) and [Formula: see text] (No:166) symmetries and does not involve any bond breaking. This mechanism is different from the previously observed mechanism in molecular dynamics simulations.

The structural phase transition of ZnSe under hydrostatic and nonhydrostatic compressions: an ab initio molecular dynamics study.

Ab initio constant pressure molecular dynamics simulations within a generalized gradient approximation (GGA) are carried out to study the structural phase transformation of ZnSe under hydrostatic and nonhydrostatic conditions. ZnSe undergoes a first-order phase transition from the zinc-blende structure to a rocksalt structure having practically identical transformation mechanisms under hydrostatic and nonhydrostatic compressions. This phase transformation is also analyzed using the enthalpy calculations. Our transition parameters and bulk properties are comparable with experimental and theoretical data. Furthermore, the influence of pressure on the electronic structure of ZnSe is investigated. It is found that the band gap energy increases nonlinearly under both hydrostatic and nonhydrostatic conditions and the effect of stress deviations on the band gap energy is small. The computed pressure coefficients and deformation potential of the band gap are in good agreement with experiments.

Transformation pathways of silica under high pressure.

Network-forming oxides with rigid polyhedral building blocks often possess significant capacity for densification under pressure owing to their open structures. The high-pressure behaviour of these oxides is key to the mechanical properties of engineering materials and geological processes in the Earth's interior. Concurrent molecular-dynamics simulations and first-principles calculations reveal that this densification follows a ubiquitous two-stage mechanism. First, a compact high-symmetry anion sublattice forms, as controlled by strong repulsion between the large oxygen anions, and second, cations redistribute onto the newly created interstices. The same mechanism is observed for two different polymorphs of silica, and in the particular case of cristobalite, is corroborated by the experimental finding of a previously unidentified metastable phase. Our simulations not only clarify the nature of this phase, but also identify its occurrence as key evidence in support of this densification mechanism.