共查询到20条相似文献,搜索用时 687 毫秒
1.
A. Bouhemadou R. Khenata M. Chegaar 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,56(3):209-215
Using first-principles density functional calculations, the effect of high
pressures, up to 20 GPa, on the structural and elastic properties of
Zr2AlX and Ti2AlX, with X = C and N, were studied by means of the
pseudo-potential plane-waves method. Calculations were performed within the
local density approximation to the exchange-correlation approximation
energy. The lattice constants and the internal parameters are in agreement
with the available results. The elastic constants and their pressure
dependence are calculated using the static finite strain technique. We
derived the bulk and shear moduli, Young's moduli and Poisson's ratio for
ideal polycrystalline Zr2AlX and Ti2AlX aggregates. We estimated
the Debye temperature of Zr2AlX and Ti2AlX from the average sound
velocity. This is the first quantitative theoretical prediction of the
elastic properties of Zr2AlC, Zr2AlN and Ti2AlN compounds,
and it still awaits experimental confirmation. 相似文献
2.
V. Srivastava M. Rajagopalan S. P. Sanyal 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,61(2):131-139
A first-principles tight-binding linear muffin tin orbital (TB-LMTO) method
within the local-density approximation is used to calculate the total
energy, lattice parameter, bulk modulus, magnetic moment, density of states
and energy band structures of half-metallic CrO2 at ambient as well as
at high pressure. The magnetic and structural stabilities are determined
from the total energy calculations. From the present study we predict a
magnetic transition from ferromagnetic (FM) state to a non-magnetic (NM)
state at 65 GPa, which is of second order in nature. We also observe from
our calculations that CrO2 is more stable in tetragonal phase
(rutile-type) at ambient conditions and undergoes a transition to an
orthorhombic structure (CaCl2-type) at 9.6 GPa, which is in good
agreement with the experimental results. We predict a second structural
phase transition from CaCl2- to fluorite-type structure at 89.6 GPa
with a volume collapse of 7.3%, which is yet to be confirmed
experimentally. Interestingly, CrO2 shows half metallicity under
ambient conditions. After the first structural phase transition from
tetragonal to orthorhombic, half metallicity has been retained in CrO2 and it vanishes at a pressure of 41.6 GPa. Ferromagnetism is quenched at a
pressure of 65 GPa. 相似文献
3.
B. Pedrini S. Wessel J. L. Gavilano H. R. Ott S. M. Kazakov J. Karpinski 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,55(3):219-228
We report results of susceptibility χ and 7Li NMR measurements on LiVSi2O6. The temperature dependence of the magnetic susceptibility χ(T) exhibits a broad maximum, typical for low-dimensional magnetic
systems. Quantitatively it is in agreement with the expectation for an S=1 spin chain, represented by the structural arrangement
of V ions.
The NMR results indicate antiferromagnetic ordering below TN=24 K. The intra- and interchain coupling J and Jp for LiVSi2O6, and also for its sister compounds LiVGe2O6, NaVSi2O6 and NaVGe2O6, are obtained via a modified random phase approximation which takes into account results of quantum Monte Carlo calculations.
While Jp is almost constant across the series, J varies by a factor of 5, decreasing with increasing lattice constant along the chain
direction. The comparison between experimental and theoretical susceptibility data suggests the presence of an easy-axis magnetic
anisotropy, which explains the formation of an energy gap in the magnetic excitation spectrum below TN, indicated by the variation of the NMR spin-lattice relaxation rate at T≪TN. 相似文献
4.
We have studied structural, thermodynamic, elastic, and electronic
properties of cubic IrO2 polymorph via ab initio calculations within the
LDA and GGA approximations. Basic physical properties, such as lattice
constant, bulk modulus, second-order elastic constants (Cij), and the
electronic band structures are calculated, and compared with available
experimental values. We have, also, predicted the Young's modulus, Poison's
ratio (ν), Anisotropy factor (A), sound velocities, and Debye
temperature. 相似文献
5.
B. Xu X. Li J. Sun L. Yi 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,66(4):483-487
Using first-principles calculations based on density-functional theory in
its local-density approximation, we investigated the Electronic structure,
ferroelectricity and optical properties of CaBi2Ta2O9 (CBT)
for the first time. It is found that CBT compound has an indirect band gap
of 3.114 eV and the O 2s and 2p states are strongly hybridized with the 6s
states of Bi which belong to the (Bi2O2)2+ planes. The quite
strong Ta–O and Bi–O hybridization is the primary source for
ferroelectricity. Our results imply that the interaction between Bi and O is
highly covalent. The anisotropy occurs mainly above 4 eV in the optical
properties. The different optical properties have been discussed. 相似文献
6.
The structural, mechanical, electronic and thermoelectric properties of the low temperature orthorhombic perovskite phase of CH3NH3PbI3 have been investigated using density functional theory (DFT). Elastic parameters bulk modulus B, Young’s modulus E, shear modulus G, Poisson’s ratio ν and anisotropy value A have been calculated by the Voigt–Reuss–Hill averaging scheme. Phonon dispersions of the structure were investigated using a finite displacement method. The relaxed system is dynamically stable, and the equilibrium elastic constants satisfy all the mechanical stability criteria for orthorhombic crystals, showing stability against the influence of external forces. The lattice thermal conductivity was calculated within the single-mode relaxation-time approximation of the Boltzmann equation from first-principles anharmonic lattice dynamics calculations. Our results show that lattice thermal conductivity is anisotropic, and the corresponding lattice thermal conductivity at 150 K was found to be 0.189, 0.138, and 0.530 Wm?1K?1 in the a, b, and c directions. Electronic structure calculations demonstrate that this compound has a DFT direct band gap at the gamma point of about 1.57 eV. The electronic transport properties have been calculated by solving the semiclassical Boltzmann transport equation on top of DFT calculations, within the constant relaxation time approximation. The Seebeck coefficient S is almost constant from 50 to 150 K. At temperatures 100 and 150 K, the maximal figure of merit is found to be 0.06 and 0.122 in the direction of the c-axis, respectively. 相似文献
7.
G. Rakotovelo P. S. Moussounda M. F. Haroun P. Légaré A. Rakotomahevitra J. C. Parlebas 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,57(3):291-297
Progress of scanning tunneling microscopy (STM) allowed to handle various molecules adsorbed on a given surface. New concepts
emerged with molecules on surfaces considered as nano machines by themselves. In this context, a thorough knowledge of surfaces
and adsorbed molecules at an atomic scale is thus particularly invaluable. In this work, within the framework of density functional
theory (DFT), we present an electronic and structural ab initio study of a BaTiO3 (001) surface (perovskite structure) in its paraelectric phase. As far as we know the atomic and molecular adsorption of
oxygen at surface is then analyzed for the first time in the literature. Relaxation is taken into account for several layers.
Its analysis for a depth of at least four layers enables us to conclude that a reasonable approximation for a BaTiO3 (001) surface is provided with a slab made up of nine plans. The relative stability of two possible terminations is considered.
By using a kinetic energy cut off of 400 eV, we found that a surface with BaO termination is more stable than with TiO2 termination. Consequently, a surface with BaO termination was chosen to adsorb either O atom or O2 molecule and the corresponding calculations were performed with a coverage 1 on a (1×1) cell. A series of cases with O2 molecule adsorbed in various geometrical configurations are also analyzed. For O2, the most favorable adsorption is obtained when the molecule is placed horizontally, with its axis, directed along the Ba-Ba
axis and with its centre of gravity located above a Ba atom. The corresponding value of the adsorption energy is -9.70 eV
per molecule (-4.85 eV per O atom). The molecule is then rather extended since the O–O distance measures 1.829 ?. By comparison,
the adsorption energy of an O atom directly located above a Ba atom is only -3.50 eV. Therefore we are allowed to conclude
that the O–O interaction stabilizes atomic adsorption. Also the local densities of states (LDOS) corresponding to various
situations are discussed in the present paper. Up to now, we are not aware of experimental data to be compared to our calculated
results. 相似文献
8.
Z. Lu Z. Yang 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,63(4):455-460
First-principles electronic structure calculations of noble metals (NM=Pd, Pt)/Ce0.75Zr0.25O2 systems are presented. It is found that: the NM adatoms do not prefer to stay at the atop or the bridge sites of the cations
(Ce and Zr), but prefer to be adsorbed at or around the anion sites. The most preferable adsorption sites for both the Pd
and Pt adatoms are the O-bridge sites neighboring the Zr dopant. The Pt adatom show much stronger interaction with the Ce0.75Zr0.25O2(111) surface than does the Pd adatom. The interactions of the NM/Ce0.75Zr0.25O2(111) interfaces are stronger than those of the corresponding NM/ceria(111) interfaces. There are some metal induced gap states
(MIGS) appeared in the gaps of the NM/Ce0.75Zr0.25O2(111) interfaces, which are important to catalytic properties of the NM/Ce0.75Zr0.25O2(111) catalysts. 相似文献
9.
S. Ramasubramanian M. Rajagoplan R. Thangavel J. Kumar 《The European Physical Journal B - Condensed Matter and Complex Systems》2009,69(2):265-268
Elastic and Thermodynamical properties of Ti1-xZrxC have been investigated using LAPW + lo within the density-functional theory with the generalized gradient approximation.
We have studied the stability of the alloy Ti1-xZrxC as a function of Zr composition in rocksalt (B1) structure by calculating the elastic constants C11, C12 and C44 using the tetragonal and trigonal distortions. Mechanical properties such as Poisson ratio, bulk, shear and Young’s modulii
of Ti1-xZrxC are calculated. The Debye temperature and hardness are also computed for the first time to our knowledge for Ti1-xZrxC in various compositions. 相似文献
10.
Q. Sun Q. Wang X.G. Gong V. Kumar Y. Kawazoe 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2002,18(1):77-81
We report results of the atomic and electronic structures of Al7C cluster using ab initio molecular dynamics with ultrasoft pseudopotentials and generalized gradient approximation. The lowest energy structure is
found to be the one in which carbon atom occupies an interstitial position in Al7 cluster. The electronic structure shows that the recent observation [Chem. Phys. Lett. 316, 31 (2000)] of magic behavior of Al7C- cluster is due to a large highest occupied and lowest unoccupied molecular orbital (HOMO-LUMO) gap which makes Al7C- chemically inert. These results have further led us to the finding of a new neutral magic cluster Al7N which has the same number of valence electrons as in Al7C- and a large HOMO-LUMO gap of 1.99 eV. Further, calculations have been carried out on (Al7N)2 to study interaction between magic clusters.
Received 28 July 2001 相似文献
11.
B. H. Yang P. C. Stancil 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2008,47(3):351-358
Quantum close-coupling and coupled-states approximation
scattering calculations for rotational energy transfer
of rotationally excited NH3 and ND3 due to collisions with He
are presented. Calculations were performed for collision
energies between 10-5 and 10 000 cm-1
using the NH3-He potential of Hodges and Wheatley [J. Chem. Phys.
114, 8836 (2001)]. State-to-state and total quenching cross sections from
some selected initial states are presented.
Rate coefficients for ortho-NH3 for the quenching 10+→00+
transition were obtained on potentials scaled to reproduce measurements
of second virial coefficients with the results showing strong sensitivity
to the potential, especially at low temperatures. Significant isotope
effects are found in quenching cross sections in the cold to ultracold
regime particularly in the region dominated by quasi-bound resonances,
~0.1 to 10 cm-1. As ammonia has been translationally cooled
via Stark deceleration methods, it is a good candidate for experimental
study of such effects at cold temperatures.
Comparison of rate coefficients with available theoretical results
are also presented. 相似文献
12.
H. He W. Y. Zhang 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,61(4):457-463
The spin-states of cobalt based perovskite compounds depend sensitively
on the valence state and local crystal environment of Co ions and the
rich physical properties arise from strong coupling among charge, spin,
and orbital degrees of freedom. While extensive studies have been carried
out in the past, most of them concentrated on the isotropic compound
LaCoO3. In this paper, using the unrestricted Hartree-Fock approximation
and the real-space recursion method, we have investigated the competition
of various magnetically ordered spin-states of anisotropic double-layered
perovskite Sr2Y0.5Ca0.5Co2O7. The energy
comparison among these states shows that the nearest-neighbor
high-spin-intermediate-spin ferromagnetically ordered state is the relevant
magnetic ground state of the compound. The magnetic structure and sizes of
magnetic moments are consistent with the recent experimental observation. 相似文献
13.
K. Araki T. Goto Y. Nemoto T. Yanagisawa B. Lüthi 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,61(3):257-259
The temperature dependence of the elastic constants in
Tb3Ga5O12 was measured and analysed with a simple crystal
field model. The magneto-elastic coupling constants have been deduced from
this experiment. The coupling constant gΓ5, related to the
c44 mode, is anomalously large. These coupling constants are important
for the interpretation of the phonon Hall effect. 相似文献
14.
H. M. Huang K. L. Yao 《The European Physical Journal B - Condensed Matter and Complex Systems》2011,83(3):319-323
The electronic structures and magnetic properties of Si3CaC4 in zinc-blende phase has been studied by employing the first-principles method based on density functional theory (DFT).
The calculations predict stable ferromagnetic ground state in Si3CaC4, resulting from calcium substitution for silicon. The calculated total magnetic moment is 2.00 μ
B per supercell, which mainly arises from the Ca and neighboring C atoms. Band structures and density of states studies show
half-metallic (HM) ferromagnetic property for Si3CaC4. The ferromagnetic coupling is generally observed between the Ca and C atoms. The ferromagnetism of Si3CaC4 can be explained by the hole-mediated double exchange mechanism. The sensitivity of half-metallicity of Si3CaC4 as a function of lattice constant is also discussed, and the half-metallicity can be kept in a wider lattice constant range. 相似文献
15.
Nishant N. Patel K. B. Joshi 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,59(1):19-23
Using empirical pseudopotential method Γ-L crossover is found for
the Ga0.74Al0.26Sb. The conduction band minimum is observed to
switch at the (0.87, 0, 0) point for Ga0.51Al0.49Sb which shifts to
the X point for Ga0.21Al0.79Sb and remains at X leading finally to
indirect band gap in AlSb. Band structure calculations for a large number of
alloys are performed and bowing parameters bX and bL are proposed
for the EX and EL respectively. Our findings may serve as
directive to select the materials in a range of composition to examine the
bowing parameters and thereby effective mass experimentally for the
GaxAl1-xSb alloys. 相似文献
16.
Y. Duan G. Tang L. Qin L. Shi 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,66(2):211-215
Elasticity and polarization of GaxAl1-xN alloys subjected
to uniaxial and homogeneous biaxial compression are calculated using
first-principles methods. The uniaxial compression along the c-axis
reduces Young’s modulus along the c-axis, and enhances bulk
modulus and total polarization, whereas the biaxial compression in
the plane perpendicular to the c-axis enhances bulk and Young’s
moduli. It is also found that when the in-plane biaxial compression
is applied by constraining the a-axis lattice constant to that of
AlN, the bulk and Young’s moduli dramatically increase with
increasing Ga concentration, and the total polarization could be
suppressed, even annihilated, and finally enhanced by controlling Ga
concentration. 相似文献
17.
Yu. M. Basalaev P. V. Demushin E. V. Nikolaeva A. S. Poplavnoi A. V. Silinin 《Russian Physics Journal》2012,54(10):1145-1151
It is demonstrated that the MgSiN2 crystal structure can be reduced to a superposition of the Bravais approximate highly symmetric sublattices. The combined
Brillouin zones are constructed for the crystal lattice and sublattices. Results of calculations of the electronic MgSiN2 structure in the context of the local approximation of density functional theory are presented. The influence of the existing
hidden symmetry of the crystal on its band spectrum is described. 相似文献
18.
B.-T. Liou 《Applied Physics A: Materials Science & Processing》2007,86(4):539-543
Numerical calculations based on first-principles are applied to study the electronic and structural properties of zincblende
AlxGa1-xN. The results show that the lattice constant has a very small deviation from the linear Vegard’s law. The direct and indirect
bowing parameters of 0.295±0.034 eV and -0.125±0.060 eV are obtained, respectively, and there is a direct-indirect crossover
near x=0.692. Besides, the bulk moduli and their pressure derivatives are monotonically increased with an increase of the
aluminum composition x. The deviation parameter of the bulk modulus of -5.32±1.60 GPa is obtained.
PACS 71.15.-m; 71.15.Nc; 71.55.Eq; 71.20.Nr; 42.70.Qs 相似文献
19.
B. H. Yanga P. C. Stancil 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2008,49(3):317-324
Quantum close-coupling and coupled-state approximation scattering calculations for rotational energy transfer of rotationally
excited CH4 due to collisions with He are presented for collision energies between 10−7 and 3000 cm−1 using the MP4 potential of Calderoni et al. [J. Chem. Phys. 121, 8261 (2004)]. State-to-state cross sections and rate coefficients
from selected initial rotational states of CH4 in symmetries A, E, and F are studied from the ultra-cold to the thermal regime. Comparison of the cross sections with available theoretical results
and experimental data show good agreement. Applications to astrophysics and cold laboratory environments are briefly addressed. 相似文献
20.
T. Rander M. Lundwall A. Lindblad G. Öhrwall M. Tchaplyguine S. Svensson O. Björneholm 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2007,42(2):253-257
Resonant Auger spectra of O2 clusters excited at the O1s edge are reported. After excitation to the repulsive 1s-13σ* state, the resulting resonant Auger spectrum displays features that remain constant in kinetic energy as the photon energy
is detuned. The shift between known atomic fragment features and these features is consistent with that observed for atoms
and clusters in singly charged states in direct photoemission. These findings are strong evidence for the existence of molecular
ultrafast dissociation processes within the clusters or on their surface. 相似文献