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1.
Murat Durandurdu 《哲学杂志》2018,98(30):2723-2733
We report for the first time the atomic structure, electronic structure and mechanical properties of amorphous silicon hexaboride (a-SiB6) based on first-principles molecular dynamics simulation. The a-SiB6 model is generated from the melt and predominantly consists of pentagonal pyramid-like configurations and B12 icosahedral molecules, similar to what has been observed in most boron-rich materials. The mean coordination number of B and Si atoms are 5.47 and 4.55, respectively. The model shows a semiconducting behaviour with a theoretical bandgap energy of 0.3?eV. The conduction tail states are found to be highly localised and hence the n-type doping is suggested to be more difficult than the p-type doping for a-SiB6. The bulk modulus and Vickers hardness of a-SiB6 are estimated to be about 118 and 13–17?GPa, respectively.  相似文献   

2.
《Physics letters. A》2020,384(3):126075
Using first-principles calculations, we propose a novel metallic silicon hexaboride, Cmca-B6Si. This structure is dynamically and mechanically stable at ambient pressure. Furthermore, the formation energy of Cmca-B6Si is lower than those of synthesized Pnnm-SiB6 and SiB4, indicating that Cmca-B6Si is thermodynamically stable. The calculations of thermodynamic stability and structural inheritance indicate that Cmca-B6Si may be obtained by removing Li from the synthesized LiB6Si, that is, by imitating the preparation of Si24 and T-Graphene C4. The structure of Cmca-B6Si is a covalent three dimensional framework linked by B12 icosahedrons and Si atoms. The analysis of band structure and density of states reveals that Cmca-B6Si is metallic because of the partially occupied 2p and 3p electrons of B and Si atoms, respectively. The electron orbitals reveal that Cmca-B6Si has a novel two-dimensional metallicity. This study gives a fundamental investigation on the structural, phonon, and electronic properties of Cmca-B6Si.  相似文献   

3.
Polyamorphic transformations of silicon have been investigated by constant-pressure first-principles molecular-dynamics simulations. By pressurizing a normal amorphous Si with tetrahedral coordination, a new high density amorphous (HDA) form that has a strong resemblance to HDA water is obtained. We find that the HDA form can be also obtained through vitrification of liquid Si under pressure. Both HDA and liquid Si contain deformed tetrahedral configurations with interstitial atoms. These findings indicate that HDA Si is directly connected with liquid Si, which is of particular importance in understanding phase relations of polyamorphs of Si.  相似文献   

4.
The transformation of low‐density amorphous (LDA) ice produced from high‐density amorphous (HDA) ice was studied up to 400 MPa as a function of temperature by in situ Raman spectroscopy and optical microscopy. Changes in these amorphous states of H2O were directly tracked without using emulsions to just above the crystallization temperature Tx. The spectra show significant changes occurring above ∼125 K. The results are compared with data reported for the relaxation behavior of HDA, to form what we call relaxed HDA, or rHDA. We find a close connection with expanded HDA (eHDA), which is reported to exist as another metastable form in this P–T region. The observation of this temperature‐induced LDA transition under pressure complements the previously observed pressure‐induced reversible transition between LDA and HDA at 120–140 K. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

5.
The A15Nb3Si with a composition closed to stoichiometric compound has been synthesized under high pressure from a starting material of Nb77Si23 amorphous alloys. High pressure annealing was carried out in Bridgman anvils apparatus. The amorphous alloy would decompose into A15Nb3Si, bcc Nb solid solution and hexagonal phase when it was annealed under a pressure lower or a temperature higher than that for forming single phase A15Nb3Si. The yielded A15Nb3Si exhibited a superconducting transition temperatureT c of 19.1 K, and has been indexed unambiguously with a lattice parameter ofa=0.5093 nm. Moreover, a nonlinear relationship betweenT c anda has been constructed from our experimental data, and aT c of 27 K for stoichiometric A15Nb3Si can be expected.  相似文献   

6.
Eu3+-doped Na2Ti6O13 (Na2Ti6O13:Eu) nanorods with diameters of 30 nm and lengths 400 nm were synthesized by hydrothermal and heat treatment methods. Raman spectra at ambient conditions indicated a pure monoclinic phase (space group C2/m) of the nanorods. The relations between structural and optical properties of Na2Ti6O13:Eu nanorods under high pressures were obtained by photoluminescence and Raman spectra. Two structural transition points at 1.39 and 15.48 GPa were observed when the samples were pressurized. The first transition point was attributed to the crystalline structural distortion. The later transition point was the result of pressure-induced amorphization, and the high-density amorphous (HDA) phase formed after 15.48 GPa was structurally related to the monoclinic baddeleyite structured TiO2 (P21/c). However, the site symmetry of the local environment around the Eu3+ ions in Na2Ti6O13 increased with the rising pressure. These above results indicate the occurrence of short-range order for the local asymmetry around the Eu3+ ions and long-range disorder for the crystalline structure of Na2Ti6O13:Eu nanorods by applying pressure. After releasing the pressure from 22.74 GPa, the HDA phase is transformed to low-density amorphous form, which is attributed to be structurally related to the α-PbO2-type TiO2.  相似文献   

7.
ZrO2–mullite nanocomposites were fabricated by in-situ-controlled crystallization of Si–Al–Zr–O amorphous bulk at 800–1250°C. The structural evolution of the Si–Al–Zr–O amorphous, annealed at different temperatures, was studied by X-ray diffraction, infrared, Laser Raman spectroscopy, field emission scanning electron microscopy, and high-resolution transmission electron microscopy. The materials consisted of an amorphous phase up to 920°C at which phase separation of Si-rich and Al, Zr-rich clusters occurred. The crystalline phases of t-ZrO2 and mullite were observed at 950°C and 1000°C, respectively. Mullite with a tetragonal structure, formed by the reaction between Al–Si spinel and amorphous silica at low temperature, changed into an orthorhombic structure with the increase of temperature. It was the phase segregation that improved crystallization of the Si–Al–Zr–O amorphous bulk. The anisotropic growth of mullite was observed and the phase transformation from t-ZrO2 to m-ZrO2 occurred when the temperature was higher than 1100°C.  相似文献   

8.
非晶合金Fe78Si9B13在脉冲电流作用下的单相晶化   总被引:3,自引:0,他引:3       下载免费PDF全文
对非晶合金Fe78Si9B13进行了超短脉冲电流处理,实现了晶化时α-Fe(Si)单相结构析出.可以认为,脉冲电流作用时,电子运动与非晶中空位型结构缺陷间的周期性排斥效应促进了类金属原子从非晶结构单元中析出,使Fe(Si)原子局部富集,导致基体金属相在较低温度下优先成核.而在空位的定向迁移的同时,将伴随B原子的扩散,则B原子局域富集,Fe-B化合物的形核析出就要受到这两个因素的抑制 关键词:  相似文献   

9.
An ultrasonic investigation of pressure-induced direct and reverse transformations lda-hda between the amorphous phases of low-and high-density ice is performed and the change in the shear modulus is determined. It is found that elastic softening of the amorphous network precedes both transformations, and the behavior of lda phase is similar to that of hexagonal 1h ice before amorphization. At the same time a number of features of the lda-hda transformation are due to the topological disordering of the amorphous phases, and the kinetics of the transformation is different from the standard behavior at a first-order phase transition and also from the compression kinetics of “ classic” glasses of the type a-SiO2 and a-GeO2. It is shown that the difference in the behavior of the lda and hda amorphous phases under pressure can be attributed to characteristic features of their spectra of low-energy two-level states. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 9, 653–658 (10 May 1999)  相似文献   

10.
Murat Durandurdu 《哲学杂志》2016,96(18):1950-1964
The pressure-induced phase transformation in hexagonal boron nitrite and amorphous boron nitrite is studied using ab initio molecular dynamics simulations. The hexagonal-to-wurtzite phase transformation is successfully reproduced in the simulation with a transformation mechanism similar to one suggested in experiment. Amorphous boron nitrite, on the other hand, gradually transforms to a high-density amorphous phase with the application of pressure. This phase transformation is irreversible because a densified amorphous state having both sp3 and sp2 bonds is recovered upon pressure release. The high-density amorphous state mainly consists of sp3 bonds and its local structure is quite similar to recently proposed intermediate boron nitrite phases, in particular tetragonal structure (P42/mnm), rather than the known the wurtzite or cubic boron nitrite due to the existence of four membered rings and edge sharing connectivity. On the basis of this finding we propose that amorphous boron nitrite might be best candidate as a starting structure to synthesize the intermediate phase(s) at high pressure and temperature (probably below 800 °C) conditions.  相似文献   

11.
Precision capacitance dilatometry provides a sensitive measure of the thermal strain developed in a sample undergoing a structural distortion with its varying temperature. The A15 structure compounds, V3Si and Nb3Sn, are well known to undergo distortion from their cubic structures at room temperature to tetragonal structures (c/a > 1 for V3Si and c/a < 1 for Nb3Sn) at low temperatures. In the past, highly anomalous thermal expansion behaviour recorded for these materials has been attributed to a strongly anharmonic lattice potential manifesting itself in unusually high, and strongly temperature-dependent, Grüneisen parameters. Further studies on polycrystalline material revealed this anomalous expansion to be highly anisotropic at temperatures for which, according to conventional diffraction data, the materials are cubic. This behaviour was linked to control of sample morphology by a residual stress field resulting from sample preparation.

More recent experiments, in which the transformation morphology has been controlled by the application of external stresses to single crystal V3Si and polycrystalline samples of Nb3Sn and Nb3(Sn1-x Sb x ), have confirmed the occurrence of significant anisotropy in the thermal strain in the cubic phase, well above the structural transformation.

We link this departure from cubic symmetry with the well-known soft-mode character of these materials and the associated “central peak” scattering which is also observed well above the transformation temperature. We are led to propose that the “central peak” is the precursor to a Bragg reflection for the transformation structure. This coincidence between “central peak” scattering and the reciprocal lattice for the transformed phase in Ti-Ni has been termed a “ghost lattice”.  相似文献   

12.
Structural transformations of the cumulene form of amorphous carbyne which are induced by heating at high pressure (7.7 GPa) are investigated. These can be described by the sequence amorphous phase — crystal — amorphous phase — disordered graphite. Raman scattering shows that predominately the chain structure of carbyne remains at the first three stages. It was found that the intermediate crystalline phase is an unknown modification of carbon whose structure is identified as cubic (a=3.145 Å). A mechanism of structural transformations in carbyne that involves the formation of new covalent bonds between chains is discussed. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 4, 237–242 (25 August 1997)  相似文献   

13.
The structural properties, electronic band structure and Bader charge of Sb2Te3 under hydrostatic pressure were simulated using density functional theory in order to study isostructural phase transitions (IPT) in Sb2Te3. The theoretical results showed that the axial ratio c /a did not exhibit any anomaly below 6 GPa. The variations of bond lengths were discontinuous at 2.5 GPa, which suggested considerable changes in interatomic interactions and provided sound support to the IPT. The effective charges of Sb and Te atoms showed significant discontinuous variations at 2.5 GPa, which revealed a strong redistribution of the electronic charge density and considerably changed interactions among bonding atoms. Thus, the IPT is originated from the considerable variation in the electronic charge density. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)  相似文献   

14.
An amorphous phase has been produced in a binary Nb-19 at .% Si alloy by rapidly quenching from the molten state. Primary decomposition of the amorphous phase by annealing resulted in the production of a small quantity of the A15 cubic Nb3Si phase. However, long annealing treatments led to the complete transformation to the equilibrium phases. Application of high pressure of 10 GPa during decomposition of the amorphous phase resulted firstly in the improved stability of the amorphous phase and secondly, in the production of a much larger quantity of the A15 cubic Nb3Si phase. The lattice parameter of this phase was calculated to be 0.515 nm, in conformity with the previous investigations.  相似文献   

15.
Structural and magnetic properties of silicon/aluminum-added and -free FeCoNi magnetic alloy nanofibers with nanogranular phases prepared by electrospinning and subsequent annealing of the PVP-blended ternary metal precursors in hydrogen atmosphere were investigated. The FeCoNi magnetic alloy nanofibers with evenly distributed nanocrystalline phases were formed, which are identified as γ-Fe1−x Ni x binary phase with face-centered cubic structure and α-CoFe phase with body-centered cubic structure. At elevated temperature, the α → γ structural martensitic transformation in the FeCoNi ternary alloys occurred due to the inhomogeneities in composition of the matrix during annealing of the alloy with metastable α-phase. In the Si/Al-added FeCoNi nanofibers, more than two phases with complicated-boundaries of the grains in and/or outside the nanofibers were formed as crystalline phases and amorphous phase. The amorphous phase consisted of Si and/or Al acted as an inhibitor diminishing α → γ transformation as well as an interparticle insulation layer. At low annealing temperature of 450 °C, the Si/Al-added nanofiber mainly consisted of metastable α-phase with a low-crystallinity surface structure and very small diameter of 13 nm was formed and showed an unexpectedly high coercivity, which attributed to the surface effects and/or high surface/volume ratio.  相似文献   

16.
Many works have been devoted to describing mechanisms of pressure-induced polyamorphism. This phenomenon is apparent in the phase transition between low- and high-density amorphous states (LDA and HDA) upon the application of pressure, resulting in substantial changes in the structure and physical properties of the amorphous state. The HDA–LDA transition in Si nanocrystals is observed when recording Raman spectra in situ during decompression at 6.68 GPa.  相似文献   

17.
G. Will  H. Berndt 《高压研究》2013,33(3-4):215-225
Abstract

We have studied at room temperature the kinetics of the phase transformation in RbJ from the 6-coordinated NaCl to 8-coordinated CsCl type structure by energy dispersive diffraction technique with synchrotron radiation. The time resolution was between 0.3 and 1.0 sec. The measurements were analyzed by fitting the Avrami equation to the measured data. The final transformation pressure is ptr = 3.74 ± 0.05 kbar obtained by extrapolating k, the transformation constant in the Avrami equation, to k=0.The growth constant n is (n) = 1 indicating growth of the new high pressure phase on the crystal faces. The transformation can be interpreted as a dilatational transformation, according to Buerger.  相似文献   

18.
Abstract

The phase transition of orthorhombic sulphur α-S8 to a high pressure amorphous sulphur allotrope (a-S) has been investigated by Raman spectroscopy. The conversion is found to be induced by the absorption of laser light and can be discussed in terms of ring opening followed by cis-trans conversion of the dihedral angle of S8 molecules. Laser energy and transition pressure are correlated due to the pressure tuned red shift of the absorption edge of α-S8. The amorphous (a-S) phase is observed up to 15 GPa at laser intensities below 30μW/μm2 at 514.5 and 488.0 nm. Above this threshold power a-S transforms into a second photo-induced phase (p-S), whose discrete Raman spectrum implies an ordered molecular and crystalline structure. By further increasing pressure crystalline S6 can be created which is found to be the dominant molecular species at pressures above 10 GPa and low temperatures. A phase diagram in the range T < 300 K and p < 15 GPa is also presented.  相似文献   

19.
ABSTRACT

The structural properties of pyrochlore Eu2Zr2O7 under high pressure have been studied by using Raman spectroscopy and in situ angle-dispersive X-ray diffraction (ADXRD). The results of Raman spectra indicate that Eu2Zr2O7 undergoes a reversible structural change around 21.2?GPa. The results of Rietveld refinements from in situ ADXRD data indicate that the ordered pyrochlore structure (Fd-3m) transforms to the defect-cotunnite structure (Pnma) at 26.5?GPa. The phase transition is irreversible and the transformation process is mainly induced by the accumulations of anti-site defects of the cation sublattice and Frenkel defects on the anion sublattice. Besides, the <Zr–O> bonds should play a more important role than the <Eu–O> bonds in the process of the phase transformation.  相似文献   

20.
Abstract

The crystal structure of the TiH0·74 alloy was studied by the energy dispersive X-ray diffraction technique in the pressure range to 30·5 GPa at temperatures to 630 K. A phase transformation to the (η + ω) two-phase state was found to occur above 7 GPa at room temperature, then (η+ω)-TiH0·74 remained stable up to P=30·5 GPa. Another phase transformation resulting in a single-phase state, ζ-TiH0·74, was found to occur upon heating (η+ω)-TiH0·74 above T ? 560 K. Both high-pressure phases, η and ζ, were indexed on the basis of the tetragonal sublattices of the Ti atoms with nearly the same specific volumes. It is assumed from the relation of the specific volumes that the hydrogen atoms occupy the tetrahedral interstices in the ζ-phase and the octahedral interstices in the η-phase.  相似文献   

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