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1.
An alternative model has been proposed for the phase transition from cubic argentite ß-Ag2S to monoclinic acanthite α-Ag2S in silver sulfide as a disorder–order transition. It has been shown that, as the temperature decreases below the transition temperature Ttrans, S atoms equiprobably occupying the sites of the body centered cubic (bcc) nonmetal sublattice of argentite are concentrated at four sites of the monoclinic nonmetal sublattice, whereas the other sites remain vacant. A disorder-order transition channel including three superstructure vectors of k9 and k4 stars has been determined. The distribution function of sulfur atoms in monoclinic acanthite α-Ag2S has been calculated. It has been shown that displacements of sulfur atoms distort the bcc nonmetal sublattice of argentite, forming a monoclinic lattice, where silver atoms are spaced by quite large distances and occupy their crystallographic positions with a probability of 1. The region of allowed values of the long-range order parameters η9 and η4 for the model monoclinic ordered phase α-Ag2S has been determined. 相似文献
2.
This study is dedicated to the growth of bcc Mn by molecular beam epitaxy, in order to look at the magnetic properties of
bcc Mn near room temperature. For this purpose, Mn is deposited on bcc M xV 1-x(001) alloy lattices (M = Fe or Nb) for which the lattice spacing is tunable by varying the concentration x. We first show that the parameter of the M xV 1-x alloy's buffer layers can be adjusted from 2.95 ? to 3.3 ? depending on x and M. Three different structures in Mn films grown on these buffer layers are observed depending on the in-plane spacing
of the initial M xV 1-x lattice. Thick Mn films are always found to grow epitaxially in the Mnstructure. For moderate thicknesses larger than 4 atomic planes, Mn grows in an unidentified structure. Finally, up to four
deposited atomic planes, Mn is found to grow in a tetragonal structure close to a bcc one on Fe(001), Fe xV 1-x(001) and Nb xV 1-x(001) for . This tetragonal structure is shown to be a distorsion of a Mn bcc structure with . Except for ultra-thin Mn films deposited on Fe(001), no macroscopic magnetization is detected in our strained bcc Mn samples.
These results are compared to theoretical predictions.
Received 21 June 1999 相似文献
3.
利用扩展x射线吸收精细结构和x射线衍射研究了机械合金化制备的体心立方(bcc)的亚稳态Fe 80Cu 20合金固溶体的结构随退火温度的变化特点.结果表明,在300—873 K温度范围内,随着退火温度的升高,bcc结构物相的晶格常数近于线性降低,这主要是由于Cu原子从bcc结构Fe 80Cu 20合金固溶体中逐渐偏析出来,生成面心立方(fcc)结构的Cu物相所致.经603K退火后,Cu原子的平均键长R Cu—Cu增加了0.003 nm左右,大约有50%的Cu原子从bcc结构的Fe 80Cu 20合金固溶体中偏析出来.在773 K退火后,bcc结构Fe 80Cu 20合金固溶体近于完全相分离,生成了bcc结构的α-Fe与fcc结构的Cu物相.
关键词:
扩展x射线吸收精细结构
x射线衍射
80Cu 20合金')" href="#">Fe 80Cu 20合金
机械合金化 相似文献
4.
The transition phase of GaAs from the zincblende (ZB)
structure to the rocksalt (RS) structure is investigated by ab initio plane-wave
pseudopotential density functional theory method, and the thermodynamic
properties of the ZB and RS structures are obtained through the
quasi-harmonic Debye model. It is found that the transition from the ZB
structure to the RS structure occurs at the pressure of about 16.3\,GPa,
this fact is well consistent with the experimental data and other theoretical results. The
dependences of the relative volume V/V 0 on the pressure P, the Debye
temperature \Th and specific heat C V on the pressure P, as well as
the specific heat C V on the temperature T are also obtained
successfully. 相似文献
5.
The quadratic, cubic and semi-diagonal quartic force field of ethyl cyanide has been calculated at the B3LYP level of theory employing a basis set of triple-ζ quality. A semi-experimental equilibrium structure has been derived from experimental ground state rotational constants and rovibrational interaction parameters calculated from the ab initio force field. This structure is in excellent agreement with the ab initio structure calculated at the CCSD(T) level of theory using a basis set of quadruple-ζ quality and a core correlation correction. The empirical structures are also determined and their accuracy is discussed. The potential barrier V3 hindering internal rotation of the methyl group has been calculated from 23 rotational transitions of CH 3CH 2C 15N which were found split into doublets, giving V3 = 3074(27) cal mol −1. 相似文献
6.
The transition metal pair potential (TMPP) is used to study band structure energy of Rh and Ir. Both metals are found to be
most stable in fcc structure down to atomic volume 0.5 V
0. The pressure at 0.5 V
0 is found to be 5.235 Mbar and 9.216 Mbar in Rh and Ir, respectively. The TMPP is also used to study other properties of these
metals like cohesive energy, phonon frequencies at observed volume. The bulk moduli and elastic constants of these metals
at observed volume are calculated by including the volume contribution. 相似文献
7.
The transition phase of PtN from zincblende (ZB) structure to rocksalt (RS) structure is investigated by ab initio plane-wave pseudopotential density functional theory method, and the thermodynamic properties of the ZB and RS structures under high pressure and temperature are obtained through the quasi-harmonic Debye model. The transition phase from the ZB structure to the RS structure occurs at the pressure of 18.2 GPa, which agrees well with other calculated values. Moreover, the dependences of the relative volume V/ V0 on the pressure P, the Debye temperature Θ and heat capacity CV on the pressure P, together with the heat capacity CV on the temperature T are also successfully obtained. 相似文献
8.
The phase transition of gallium phosphide (GaP) from zinc-blende (ZB) to a rocksalt (RS) structure is investigated by the plane-wave pseudopotential density functional theory (DFT). Lattice constant a0, elastic constants cij, bulk modulus B0 and the pressure derivative of bulk modulus B0' are calculated. The results are in good agreement with numerous experimental and theoretical data. From the usual condition of equal enthalpies, the phase transition from the ZB to the RS structure occurs at 21.9 GPa, which is close to the experimental value of 22.0 GPa. The elastic properties of GaP with the ZB structure in a pressure range from 0 GPa to 21.9 GPa and those of the RS structure in a pressure range of pressures from 21.9 GPa to 40 GPa are obtained. According to the quasi-harmonic Debye model, in which the phononic effects are considered, the normalized volume V/ V0, the Debye temperature θ, the heat capacity Cv and the thermal expansion coefficient α are also discussed in a pressure range from 0 GPa to 40 GPa and a temperature range from 0 K to 1500 K. 相似文献
9.
The monoclinic (space group C2/ m) superstructure of V 14O 6, which is formed in the atom-vacancy ordering of the tetragonal solid solution of oxygen in vanadium, is studied by the methods
of x-ray diffraction and symmetry analysis. It has been found that the channel of the order-disorder phase transition attributed
to the formation of the monoclinic suboxide V 14O 6 includes six superstructure vectors belonging to three non-Lifshitz stars { k
1−1}, { k
1−2}, and { k
1–3} of one type { k
1}. The distribution function of the O atoms in the V 14O 6 monoclinic superstructure has been calculated. It has been shown that the displacements of V atoms distort the body-centered
tetragonal metal sublattice, thus preparing the formation of the fcc sublattice and the transition from the suboxide V 14O 6 to the cubic vanadium monoxide with the B1 structure. 相似文献
10.
The transition phase of GaN from zincblende (ZB) structure to rocksalt structure (RS) is investigated by ab initio plane-wave pseudopotential density functional theory method, and the thermodynamic properties of the ZB and RS structures are obtained through the quasi-harmonic Debye model. We find that the transition phase from the ZB structure to the RS structure occurs at the pressure of 42.2 GPa, which is in good agreement with other calculated values. Moreover, the dependences of the relative volume V/ V0 on the pressure P, the Debye temperature Θ and heat capacity CV on the pressure P, as well as the heat capacity CV on the temperature T are also successfully obtained. 相似文献
11.
The phase transition of ZnS from the zincblende (ZB) structure to the rocksalt (RS) structure is investigated by the ab initio plane-wave pseudopotential density functional theory method. It is found that the pressures for transition from the ZB structure to the RS structure are 17.5 GPa from total energy-volume data and 15.4 GPa from equal enthalpies, consistent with the experimental data. From the high pressure elastic constants obtained, we find that the ZB structure ZnS is unstable when the applied pressure is larger than 17 GPa. Moreover, the dependence of the normalized primitive cell volume V/ V0 on pressure P can also be successfully obtained. 相似文献
12.
对Co 100-xMn x合金在GaAs(001)表面的分子束外延生长、晶体结构和磁学性质进行了研究.结果表明,当0 100-xMnx合金薄膜是体材料中不存在的体心立方(bcc)结构,并且具有较强的铁磁性,当44100-xMnx合金薄膜最初为bcc结构,随着厚度的增加,逐渐从bcc向面心立方(fcc)结构转化,最后成为完全的fcc结构,薄膜具有较
关键词: 相似文献
13.
Abstract We report here a calculation of the band structure and superconductivity of Arsenic in the simple cubic phase under pressure. The effect of pressure on the band structure is obtained using Andersen's linear muffin-tin orbital method under atomic sphere approximation. McMillan's formula is used to calculate the superconducting transition temperature (T c). The theoretically calculated valve of T c in sc phase at 26.6 GPa is 3.62 K. Further increase in pressure decreases the T c values. 相似文献
14.
An irreversible pressure induced semiconductor-to-metal transition in bulk Ge 20Te 80 glass is observed at about 5 GPa pressure. The high pressure phase has a face centered cubic structure with a lattice constant 6.42 A° as deduced by X-ray diffraction studies on the pressure quenched samples. The temperature and pressure dependence of the electrical resistivity confirms the observed transition to be a semiconductor-to-metal transition. The temperature dependence of thermo electric power is also reported. 相似文献
15.
The question raised recently about whether the high-pressure phase transitions of Mg follow a hexagonal close-packed (hcp) → body centered cubic (bcc) or hcp → double hexagonal close-packed (dhcp) → bcc sequence at room temperature is examined by the use of first principles density functional methods. Enthalpy calculations show that the bcc structure replaces the hcp structure to become the most stable structure near 48 GPa, whereas the dhcp structure is never the most stable structure in the pressure range of interest. The characterized phase-transition mechanisms indicate that the hcp → dhcp transition is also associated with a higher enthalpy barrier. At room temperature, the structural sequence hcp → bcc is therefore more energetically favorable for Mg. The same conclusion is also reached from the simulations of the phase transitions using metadynamics methods. At room temperature, the metadynamics simulations predict the onset of a hcp → bcc transition at 40 GPa and the transition becomes more prominent upon further compression. At high temperatures, the metadynamics simulations reveal a structural fluctuation among the hcp, dhcp, and bcc structures at 15 GPa. With increasing pressure, the structural evolution at high temperatures becomes more unambiguous and eventually settles to a bcc structure once sufficient pressure is applied. 相似文献
16.
We have calculated the quantum quadrupolar interaction due to charge density fluctuations of localized 4 f-electrons in Ce by taking into account the angular dependence, the degeneracy of the localized 4 f
-orbitals and the spin-orbit coupling. The calculated crystal field of 4
f electronic states is in good agreement with neutron diffraction measurements. We show that orientational ordering of quantum
quadrupoles drives a phase transition at K which we assign with the transformation. In the phase the centers of mass of the Ce atoms still form a face centered cubic lattice. The theory accounts for the first order
character of the transition and for the cubic lattice contraction which accompanies the transition. The transition temperature
increases linearly with pressure. Our approach does not involve Kondo spin fluctuations as the significant process for the
phase transition.
Received 19 October 1998 相似文献
17.
Abstract Here we report a theoretical calculation of the band structure and superconductivity of Se in the bcc phase. The energy band structure and the effect of pressure on the band structure is obtained by means of the Linear Muffin-Tin Orbital method within the atomic sphere approximation. The superconducting transition temperature (Tc) is calculated using McMillan's formula and we predict the value of Tc at 115.3 Gpa as 2.3 K. Further increase in presssure decreases the Tc values. The normal state electrical resistivity at 115.3 Gpa is 1.43 fl cm, with further increase in pressure the resistivity decreases, which is a typical behaviour of number of elemental metals under pressure. 相似文献
18.
Single crystalline C 60 nanotubes having face‐centered‐cubic structure with diameters in the nanometer range were synthesized by a solution method. In situ Raman and photoluminescence spectroscopy under high pressure were employed to study the structural stabilities and transitions of the pristine C 60 nanotubes. A phase transition, probably because of the orientational ordering of C 60 molecules, from face‐centered‐cubic structure to simple cubic structure occurred at the pressure between 1.46 and 2.26 GPa. At above 20.41 GPa, the Raman spectrum became very diffuse and lost its fine structure in all wavenumber regions, and only two broad and asymmetry peaks initially centered at 1469 and 1570 cm –1 were observed, indicating an occurrence of amorphization. This amorphous phase remained to be reversible until 31.1 GPa, and it became irreversible to the ambient pressure after the pressure cycle of 34.3 GPa was applied. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
19.
Thermodynamic properties of the half-filled-band Hubbard model are calculated in the strong-coupling regime for a simple cubic structure, using the Bogoliubov variational principle. When the Coulomb-interaction V
o exceeds the bandwidth J, we find a phase transition from a condensed phase of itinerant electrons into a state of localized ones at a transition temperature which exhibits the asymptotic behaviour T
0J
2/ V
0 at large V
0. 相似文献
20.
Thermodynamic properties of the half-filled-band Hubbard model are calculated in the strong-coupling regime for a simple cubic structure, using the Bogoliubov variational principle. When the Coulomb-interaction V
o exceeds the bandwidth J, we find a phase transition from a condensed phase of itinerant electrons into a state of localized ones at a transition temperature which exhibits the asymptotic behaviour T
0J
2/ V
0 at large V
0. 相似文献
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