Si Coordination Number Research Articles

Pressure impact on the structure, elasticity, and electron density distribution of CaSi2O5

Ab initio molecular dynamics simulations were used to reveal the mechanism of the fivefold to sixfold transition in Si coordination numbers of CaSi${}_{2}$O${}_{5}$. The longest first-neighbor Si-O distance drops from 2.8 to 1.8 \AA{} upon the triclinic to monoclinic transition. We find significant bulk modulus softening during the structure crossover, which is due to appearance of intermediate Si-O connections in the triclinic phase under slightly nonhydrostatic stress. Nonetheless, no soft phonon modes were found in either structure, indicating that both structures are dynamically stable. Across the transition, ${c}_{33}$ doubles and ${c}_{35}$ increases sixfold in magnitude due to the formation of new Si-O bonds. Chemical bonding analysis reveals distinctions in the electron localization function and bond ellipticity between the regular (1.8 \AA{}) and the dangling Si-O bonds (2.8 \AA{}), both of which suggest an impending disassociation of the dangling Si-O bond.

Argon solubility in SiO2 melt under high pressures: A new experimental result using laser-heated diamond anvil cell

We have measured the solubility of argon in SiO2 melt under high pressure by melting a SiO2 crystal at the argon pressure as high as 19GPa using laser-heated diamond anvil cell (LHDAC). SiO2 was successfully melted at the pressure of about 12GPa and recovered with a completely glassy phase. Above 15GPa, the sample contained a large amount of crystalline SiO2, although it was completely melted and spherical due to the surface tension. This may arise from the change in the coordination number of Si in SiO2 melt at around 12–15GPa, which is close to the triple point of coesite, stishovite, and liquid SiO2. Argon content in the recovered SiO2 was measured by energy-dispersive spectroscopy and mass spectrometry. It increased at pressures up to approximately 5–7GPa and exhibited a constant value up to at least 12GPa; this implies that, at these conditions, the noble gas behaves as an incompatible element. The result is completely different from those of previous LHDAC studies, which claimed that the increased solubility then dramatically drops above the pressure of about 5GPa. Here, we report the new details of experimentally determined pressure dependence of noble gas solubility by LHDAC, and discuss the experimental difficulties and the structure change of silicate melt under high pressures.

Atomic structure and electronic properties of the SixSb100−xphase-change memory material

The electronic and structural properties of Si${}_{x}$Sb${}_{100\ensuremath{-}x}$ ($x$\ensuremath{\sim}16) materials are investigated using first-principles molecular dynamics simulations. Crystalline-liquid-amorphous phase transitions are examined and remarkable changes in the local structure around the Si atoms are found. The average Si coordination number 6 (3 long $+$ 3 short Si-Sb bonds) of the crystalline phase changes to 4 (3 long Si-Sb $+$ 1 short Si-Si bonds) by preserving three Si-Sb bonds in both the liquid and the amorphous phases. In the amorphous phase \ensuremath{\sim}90% of the Si atoms are fourfold coordinated compared to 40% in the liquid. The electronic density of states is metal-like in both the crystalline and the liquid phases, but it exhibits a pseudogap at the Fermi level in the amorphous phase, reflecting the strong abundance of fourfold coordinated Si in the amorphous phase.

Atomistic modeling of multiple amorphous-amorphous transitions in SiO2and GeO2glasses at megabar pressures

We present molecular dynamics simulations of SiO${}_{2}$ and GeO${}_{2}$ glasses up to megabar pressures, 120 and 160 GPa, respectively, and show direct parallels between polyamorphism of glasses and polymorphism of their crystalline counterparts under compression. The glasses undergo a set of several smooth transformations in much the same manner as the corresponding crystals at nearly the same pressures, where coordination numbers of Si and Ge atoms considerably exceed 6, reaching 6.4 and 7.6, respectively, at maximum simulation pressures. The transformations in glasses, unlike those in crystals, occur with rather small hysteresis. High coordination states are not retained in a metastable form at room pressure.

Structural properties and phase transitions in a silica clathrate

Melanophlogite, a low-pressure silica polymorph, has been extensively studied at different temperatures and pressures by molecular dynamics simulations. While the high-temperature form is confirmed as cubic, the low-temperature phase is found to be slightly distorted, in agreement with experiments. With increasing pressure, the crystalline character is gradually lost. At 8 GPa, the radial distribution function is consistent with an amorphous state. Like pristine glass, the topology changes, plastic behavior, and permanent densification appear above ∼12 GPa, triggered by Si coordination number changes. We predict that a partial crystalline and amorphous sample can be obtained by recovering the sample from a pressure of ∼12-16 GPa.

Elasticity and pressure-induced structural changes in vitreous MgSiO 3-enstatite to lower mantle pressures

Knowledge of the pressure–density relations and elastic properties of silicate melts at high pressure and temperature conditions is fundamental for a better understanding of the dynamics and properties of the Earth's interior. Here we report the sound velocities and elastic properties of vitreous MgSiO 3-enstatite, a simple analog for melts in the Earth's mantle, determined up to 33.4 ± 1.2 GPa in a diamond anvil cell by Brillouin scattering spectroscopy. The adiabatic bulk and shear moduli of MgSiO 3 glass at ambient conditions are K S0 = 78.4(6) GPa and µ 0 = 41.1(3) GPa, corresponding to compressional and shear wave velocities of V P = 6.969(30) km/s and V S = 3.868(20) km/s. On compression both longitudinal V P and transverse V S modes exhibit anomalous change in slope at 8 GPa and between 15.0(1) and 17.9(2) GPa, that are associated with polyamorphic phase transitions in the glass. The acoustic data were used to determine the density and compressibility of MgSiO 3 glass at high pressure in both compression and decompression experiments. In contrast to velocities and elastic moduli, the density increases smoothly without singularities upon compression over the investigated pressure range. Densities of recovered glasses at room pressure are 1.5 to 2.4% higher than uncompressed glasses, indicating pressure-induced irreversible densification. The anomalous changes in velocities and compressibility with pressure are consistent with structural modifications that have been identified as the main densification mechanisms of the glass by in situ spectroscopic studies and molecular dynamic calculations. The results suggest that the residual densification of recovered glasses is associated with irreversible structural changes in the intermediate-range order (i.e., ring statistics) rather than with changes in the coordination number of Si upon pressurization. If the changes in compressibility of MgSiO 3 glasses identified in the present Brillouin study also occurs in the analog melts, they may have important implications for the present-day dynamics of the mantle and early evolution of the Earth during a magma ocean phase.

Simultaneous aluminum, silicon, and sodium coordination changes in 6 GPa sodium aluminosilicate glasses

We present the first direct observation of high-coordinated Si and Al occurring together in a series of high-pressure sodium aluminosilicate glasses, quenched from melts at 6 GPa. Using 29Si MAS NMR, we observe that a small amount of Al does not have a significant effect on the amount of VSi or VISi generated, but that larger Al concentrations lead to a gradual decrease in both these species. 27Al MAS NMR spectra show that samples with small amounts of Al have extremely high mean Al coordination values of up to 5.49, but that larger Al concentrations cause a gradual decrease in both VAl and VIAl. Although mean Al and Si coordination numbers both decrease with increasing Al contents, the weighted combined (Al+Si) coordination number increases. Silicon and Al resonances shift in frequency with increasing pressure or changing Al concentration, indicating additional structural changes, including compression of network bond angles. Increases in the 23Na isotropic chemical shifts indicate decreases in the mean Na-O bond lengths with increasing pressure, which are more dramatic at higher Al contents. Recovered glass densities are about 10 to 15% greater than those of similar ambient pressure samples. However, the density increases due to the combined coordination changes of Al and Si are estimated to total only about 1 to 2%, and are roughly constant with composition despite the large effects of Al content on the individual coordinations of the two cations. Thus, effects of other structural changes must be significant to the overall densification. Apparent equilibrium constants for reactions involving the generation of high-coordinated species show systematic behavior, which suggests an internal consistency to the observed Si and Al coordination number shifts.

Structure, thermodynamic, and transport properties of molten Mg2SiO4: Molecular dynamics simulations and model EOS

Molecular dynamics simulations have been used to study the structure, equation of state (EOS), self-diffusion, and shear viscosity of molten Mg 2 SiO 4 for pressures and temperatures in the range 2.5–110 GPa and 2100–5060 K, respectively. The transferable pair-potential parameters of Matsui (1998) for the system Na 2 O-CaO-MgO-Al 2 O-SiO 2 have been used accounting for Coulomb, Born, and van der Waals forces. Simulations have been carried out in the microcanonical (NEV) ensemble at 63 state points along 12 isochores spanning the density range 2754–4500 kg/m 3 . Thermodynamic properties including the isochoric heat capacity, isobaric expansivity, isothermal compressibility, thermal pressure, and the Gruneisen parameter (γ) are computed directly from MD results. A density crossover between molten Mg 2 SiO 4 and forsterite crystals occurs at ~15 GPa at 2100 K. We find the Gruneisen parameter to be a function of temperature ( T ), increasing with increasing T at low density (ρ 3 ) but decreasing as T rises at high density (ρ > 3400 kg/m 3 ); hence, the integrated form of the Mie-Gruneisen EOS is only approximately valid for liquid Mg 2 SiO 4 since γ varies by ~20% over the T range along an isochore. Radial distribution functions for all atoms around all other atoms were used to generate coordination statistics as a function of pressure ( P ) and T . Oxygen about Si coordination increases from fourfold coordination at low pressure to sixfold at higher pressure; the abundance of distorted trigonal bipyramidal fivefold polyhedra, Si(V) maximizes at 30 GPa at 3500 K. Interestingly, O about O increases to a maximum of 13 at low P before decreasing with increasing pressure to ~10. The mean coordination number (CN) of Si around oxygen increases from 1.2 to 1.5 consistent with an increasing abundance of Si 2 O 7 dimers as pressure increases. Self-diffusion of Mg, Si, and O was calculated at each state point giving activation energies of 67, 79, and 76 kJ/mol and activation volumes of 1.42, 1.10, and 1.32 cm 3 /mol, respectively. Shear viscosity of the liquid calculated at 12 state points using the Green-Kubo formulation provides an excellent Arrhenian fit. Viscosity varies by a factor of ~20 (1.5 × 10 −3 Pa s to 0.03 Pa s) from 1 to 100 GPa. The validity of the Stokes-Einstein and Eyring expressions for atom mobility and shear viscosity is examined in detail. Characteristic lengths for atom mobility are consistent with ionic radii to within a factor of ~1.5–2 for all atoms. An equation of state and thermodynamic model for Mg 2 SiO 4 liquid is developed consistent with the fundamental measure functional theory of Rosenfeld and Tarazona (1998). Our model reproduces the E-P-V-T relations and the derived thermodynamic properties obtained from the MD simulations to within the reported uncertainty.

Molecular dynamic studies on MgO–Al2O3–SiO2glass-ceramics

Three sets of original dynamics model parameters for MgO–Al2O3–SiO2(MAS) system were reported for the first time in this paper; moreover, a new parameter optimization standard was put forward to study three different molecular dynamic models of MAS glass-ceramics. The limitations of the conventional parameter optimization methods were also studied. The results indicate: (i) Born-Mayer-Huggins (BMH) model can be only used to simulate amorphous MAS systems. Furthermore, both static optimization and a dynamics test are necessary; (ii) for structure optimization or macroproperties calculation, high accuracy has been achieved relative to the experimental results by using the core-shell (CS) model; (iii) partialQ model computes at a high speed, about twelve times that of the CS model; (iv) for a bulk system, the partialQ model can be first used to obtain an initial structure rapidly, followed by the CS model for high accuracy calculation. In this way, both accuracy and efficiency are achieved. When the model was used to simulate the cordierite crystal and the amorphous in the cordierite glass-ceramic, the results were consistent with the experiments and the structure data from the ab initio calculation. Simulations on amorphous structures in the cordierite glass-ceramic with various compositions displayed that the bond length or coordination numbers (CN) of Si–O and Al–O remained the same with increasing content of MgO, suggesting no change in the tetrahedral configuration of short-range structure. Although the bond length of Mg–O stays almost the same with the increasing content of MgO, the coordination number increases to a certain extent, and the content of O-bridge in SiO2glass drops from 100%–60% in pyrope glass.

Effect of hydrogen on ground state properties of silicon clusters(SinHm;n = 11–15,m = 0–4): a density functional based tight binding study

The effect of hydrogen addition to silicon clustersSin (n = 11–15) is examined in terms of the change in their ground state geometry, bonding characteristics(bond lengths, bond angles and Si coordination numbers), structural stability, vibrationalspectra and electronic structure. We found that addition of hydrogen caused a drasticchange in the structural geometry of silicon clusters. Upon hydrogenation, some clustersbecome highly symmetric, while some others are completely distorted. Hydrogenationcaused a structural transition in silicon clusters; from spherical to elongated inSi11 and Si12, from one side capped elongated to both side capped elongated inSi13 and Si14 and from both side capped elongated to one side capped elongated inSi15.Hydrogen preferred to bond with either under-coordinated or over-coordinated Si atoms. Each Hatom is bonded to only one Si atom and localized outside the silicon cluster skeleton. Hydrogenationcaused a significant change in the vibrational spectra and electronic structures of bareSin silicon clusters. There exists a band gap near the Fermi level in EDOS in all the bare andhydrogenated silicon clusters. The band gap of a silicon cluster depends on its size andamount of hydrogenation.

A high energy X-ray diffraction study of sol–gel derived (Ta2O5) x (SiO2)1−x glasses (x = 0.05, 0.11 and 0.25)—elucidating the role of tantalum in silica

High energy X-ray diffraction (HEXRD) has been used to study the atomic structure of (Ta2O5) x (SiO2)1−x (x = 0.05, 0.11 and 0.25) xerogels; the direct interpretation of the resultant data has been augmented using reverse Monte Carlo (RMC) modelling. For the first time in this type of material, two Ta–O correlations have been identified (at ∼1.8 Å and 2.0 Å). The RMC modelling approach explicitly used MAS-NMR data to define its constraints; when combined with HEXRD data, it helps to confirm the more directly determined total coordination of five for the two Ta–O correlation distances and suggests Ta···Si and Ta···Ta coordination distances of ∼3.3 Å and ∼3.8 Å, respectively. The O···O and Si···Si distances and coordination numbers associated with the host silica network suggest that the Ta (V) is acting as a network modifier. The way in which the Ta–O correlations are affected by composition and calcination temperature suggest some phase separation in the (Ta2O5)0.25(SiO2)0.75 sample. However, in general, the results indicate good mixing of the component oxides.

Classification of metal-oxide bonded interactions based on local potential- and kinetic-energy densities

A classification of the hydrogen fluoride H-F-bonded interactions comprising a large number of molecules has been proposed by Espinosa et al. [J. Chem. Phys. 117, 5529 (2002)] based on the ratio /Vr(c)/ / Gr(c) where /Vr(c)/ is the magnitude of the local potential-energy density and Gr(c) is the local kinetic-energy density, each evaluated at a bond critical point r(c). A calculation of the ratio for the M-O bonded interactions comprising a relatively large number of oxide molecules and earth materials, together with the constraints imposed by the values of inverted Delta2rho r(c) and the local electronic energy density, Hr(c) = Gr(c) + Vr(c), in the H-F study, yielded practically the same classification for the oxides. This is true despite the different trends that hold between the bond critical point and local energy density properties with the bond lengths displayed by the H-F and M-O bonded interactions. On the basis of the ratio, Li-O, Na-O, and Mg-O bonded interactions classify as closed-shell ionic bonds, Be-O, Al-O, Si-O, B-O, and P-O interactions classify as bonds of intermediate character with the covalent character increasing from Be-O to P-O. N-O interactions classify as shared covalent bonds. C-O and S-O bonded interactions classify as both intermediate and covalent bonded interactions. The C-O double- and triple-bonded interactions classify as intermediate-bonded interactions, each with a substantial component of covalent character and the C-O single-bonded interaction classifies as a covalent bond whereas their local electronic energy density values indicate that they are each covalent bonded interactions. The ratios for the Be-O, Al-O, and Si-O bonded interactions indicate that they have a substantial component of ionic character despite their classification as bonds of intermediate character. The trend between the ratio and the character of the bonded interactions is consistent with trends expected from electronegativity considerations. The ratio increases as the net charges and the coordination numbers for the atoms for several Ni-sulfides decrease. On the contrary, the ratio for the Si-O bonded interactions for the orthosilicate, forsterite, Mg2SiO4, and the high-pressure silica polymorph, stishovite, decreases as the observed net atomic charges and the coordination numbers of Si and O increase in value. The ratio for the Ni-Ni bonded interactions for the Ni-sulfides and bulk Ni metal indicate that the interactions are intermediate in character with a substantial component of ionic character.

Spontaneous disordering of nm-grain-sized polycrystals and clusters of silica stishovite

We investigate spontaneous structural disordering of nm-grain-sized polycrystals and clusters of stishovite at ambient conditions using molecular dynamics, and compare to disordering in stishovite nanocrystals and single-crystal α-quartz at high pressures. For nanocrystals and clusters at ambient conditions and sub-ns time scales, the extent and rate of disordering increase with the surface-area-to-volume ratio, and the structure becomes completely amorphous if the size is less than about 2 nm. Within the stability field of stishovite, disordering at the grain boundaries is less pronounced. Low-pressure disordering of stishovite structure involves Si coordination number reduction from six to five and finally to four, a process opposite to the pressure-induced amorphization of quartz.

An equation of state for silicate melts. I. Formulation of a general model

A simple, empirical, volume-explicit equation of state (EOS) is developed for use in describing the volumetric properties of materials that exhibit large thermal expansivities and strongly non-linear, pressure-dependent compressibilities. Existing and commonly used EOS expressions, like the Universal and Birch-Murnaghan equations, develop singularities at critical values of temperature and pressure that prevent the calculation of derived thermodynamic properties for this class of substances. The proposed EOS reduces to a simple polynomial form at low-pressure; this form is utilized in the literature to describe the properties of silicate melts at 10⁵ Pa. At high-pressures, the proposed EOS can be parameterized to yield a finite, non-zero volume limit, unlike other formulations. Thermodynamic properties can be calculated using the proposed EOS without the need for iterative solutions and all derivative and integral representations of the equation are analytic. A detailed example is developed demonstrating how the proposed EOS can be utilized in modeling the Gibbs free energy of amorphous silica at elevated temperature and pressure. The thermodynamic model described in the example accounts for <i>vibrational</i> contributions to the energy of the system as well as <i>configurational</i> effects arising from increases in the coordination number of Si with temperature and pressure. Vibrational contributions are modeled at elevated pressure by the proposed EOS. Configurational contributions are described by a simple associated solution model for the entropy of mixing. Parameters are calibrated from molecular dynamical simulations of the configurational and volumetric properties of amorphous silica. The calibration is used to calculate the phase diagram of silica to 3000 K and 15 GPa, which compares quite favorably with experimental data on melting.

Molecular dynamics simulation of disordered zircon

The melting of zircon and the amorphous state produced by quenching from the melt were simulated by molecular dynamics using a new partial charge model combined with the Ziegler-Biersack-Littmark potential. The model has been established for the description of the crystalline and aperiodic structures of zircon in order to be used for the simulation of displacement cascades. It provides an excellent fit to the structure, and accounts with convenient precision the mechanical and thermodynamic properties of zircon. The calculated melting temperature is about 2100 K. The activation energy for self-diffusion of ions in the liquid state was determined to be 190-200 kJ/mole. Melt quenching was employed to produce two different disordered states with distinct densities and structures. In the high density disordered state, the zircon structure is intact but the bond angle distributions are broader, 4% of the Si units are polymerized, and the volume swelling is about 8%. In the low density amorphous state, the Zr and Si coordination numbers are lower, and the Zr-O and Si-O bond lengths are shorter than corresponding values for the crystal. In addition, a highly polymerized Si network, with average connectivity of two, is observed in the low density amorphous state. These featuresmore » have all been experimentally observed in natural metamict zircon. The present findings, when considered in light of experimental radiation effects studies, suggest that the swelling in zircon arises initially from disorder in the zircon crystal, and at high doses the disordered crystal is unable to accommodate the volume expansion and transforms to the amorphous state.« less

NMR evidence for formation of octahedral and tetrahedral Al and repolymerization of the Si network during dissolution of aluminosilicate glass and crystal

Five sodium aluminosilicate glasses in the series Na2O: x Al2O3:(3– x ) SiO2 and Na2O:Al2O3: y SiO2 were prepared and subjected to leaching at pH 2 under ambient conditions for up to 1000 h. Solid-state nuclear magnetic resonance (NMR) spectroscopy revealed the identity of aluminate and silicate environments in the leached surface layers of these glasses as well as a sample of albite crystal subjected to the same aqueous leaching. While 29Si and 27Al magic-angle spinning (MAS) NMR experiments report on the bulk structures of the samples, cross-polarization from hydrogen atoms that are present only in the surface layers provides structural information from the regions of the sample transformed during treatment. Aluminum in octahedral coordination by O atoms (AlVI) is confirmed for the first time on the near-surface region of an albite crystal under the treatment conditions of this study. A quantification of the change in the amount of six-coordinate aluminum as a function of bulk Al/Si ratio is made possible by comparing the relative amounts of 1H → 27Al CPMAS signals from AlIV and AlVI obtained from different samples under reproducible experimental conditions. The relative contribution of AlVI to total Al in the hydrated layers increases with the Al/Si ratio of the glasses studied. The leached albite crystal sample has an anomalously high concentration of AlVI given its Al/Si ratio. This anomaly is probably related to the relatively low thickness of the leached layer developed on this phase: little hydrogen penetrates the crystal surface and almost all of the Al in the thin leached layer is octahedrally coordinated, similar to Al in solution. These data suggest that hydrolysis of bridging O atoms around Al atoms in the glass or crystal hydrated layer is accompanied by a change in the coordination number of the Al atom. Aging of surfaces documents no formation of AlVI during storage after leaching. The MAS data, coupled with 27Al → 29Si CPMAS experiments, describe the bulk network structure and provide further insight into the surface structures, including documentation of repolymerization of the silicon network in the surface layer of a nepheline glass via formation of condensed Q4 units. Further triple-resonance experiments correlate 1H, 29Si, and 27Al environments in the glasses, and indicate that the repolymerized structures in nepheline glass are not phase-separated from aluminum-containing network structures. These data for acid dissolution under ambient conditions yield the first picture of the complicated series of reactions relating connectivity and coordination number of Al and Si at the altered surfaces of geologically interesting aluminosilicates.

Neutron diffraction study of liquid Si XTe 100− X ( [formula omitted]) alloys

The structures of liquid Si X Te 100− X alloys with 0<X ≦ 20 have been investigated by neutron diffraction measurements. The structure factor, S( q), of liquid Si X Te 100− X alloys with X=10, 15 and 20 has a prepeak at around q=1.0 Å −1 at lower temperatures, which suggests the existence of medium-range order in these liquids. The prepeak of S( q) for liquid Si 10Te 90 and Si 15Te 85 alloys disappears at higher temperatures, while that for liquid Si 20Te 80 remains. The coordination numbers of Si and Te are about 4 and 2, respectively. It seems that the tetrahedral configuration of SiTe 4 in the Si 2Te 3 crystal may partly remain in the liquid state.

Silicon clustering in Si–GaAs δ-doped layers and superlattices

To probe the development of Si–GaAs superlattices, we performed Si K-edge x-ray absorption measurements on periodic Si δ-doped structures in GaAs, for equivalent Si thicknesses of 0.02, 0.2, 0.5, 1, 2, and 4 monolayers (ML). We detected the presence of both Si–Si and Si–As (or Si–Ga) bonds and followed the variation of the coordination numbers as a function of the equivalent thickness of the Si layers. The Si–Si coordination number was found to gradually increase with increasing thickness, but Si–Si bonds were always detected, even at equivalent Si coverages of 0.02 ML. This indicates that the preferred growth conditions for Si–GaAs superlattices lead to widespread Si clustering and self-compensation, and suggests that lateral growth of such clusters to achieve coalescence may be the main mechanism for Si quantum well development.

Evolution of the intermixing process in Ge/Si(111) self-assembled islands

In this contribution we present a study of the Ge–Si intermixing process that arises during the growth of Ge/Si(111) self-assembled islands. The samples, grown by means of a molecular beam epitaxy (MBE)-like technique, have been characterized by in-situ scanning tunneling microscopy (STM) and atomic force microscopy (AFM) complemented by ex-situ X-ray absorption fine structure (XAFS). We have observed by STM a change in the island morphological evolution, from truncated tetrahedra to atoll-like islands, that can be related to the misfit the reduction effect induced by the intermixing process. We have evaluated the intermixing by measuring the average coordination numbers of Si and Ge around a Ge atom by XAFS. We show that the Si content in the nominally pure Ge wetting layer reaches 50% while in the three-dimensional (3-D) islands it is about 25%, and that the intermixing increases with increasing deposition temperature in the 450–530 °C range.

EXAFS and Mössbauer studies on local atomic structure in an amorphous Fe–B–Si alloy

Local atomic structures of an amorphous Fe 79B 16Si 5 alloy and the structural changes by thermal annealing (structural relaxation) have been studied by extended X-ray absorption fine structure (EXAFS) and Mössbauer measurements. The structural analyses indicate that the interatomic distance of Fe–Si is larger than that of Fe–B and is close to the Fe–Fe distance. Such geometry becomes more obvious after structural relaxation. Discussions based on the structural model composed of capped trigonal prisms suggest that Si atoms do not locate at the central site of trigonal prisms, but substitutionally occupy the sites of Fe atoms at the vertices. During structural relaxation, the distance of Fe–B decreases and that of Fe–Fe is slightly shortened, while the Fe–Si distance remains unchanged. The coordination number of B atoms around Fe decreases but that of Fe atoms increases. The coordination number of Si around Fe is almost constant. The increase in coordination number of Fe around Fe during the structural relaxation is also confirmed by the Mössbauer effect study.