101 research outputs found
A model for the formation energies of alanates and boranates
We develop a simple model for the formation energies (FEs) of alkali and
lkaline earth alanates and boranates, based upon ionic bonding between metal
cations and (AlH4)- or (BH4)- anions. The FEs agree well with values obtained
from first principles calculations and with experimental FEs. The model shows
that details of the crystal structure are relatively unimportant. The small
size of the (BH4)- anion causes a strong bonding in the crystal, which makes
boranates more stable than alanates. Smaller alkali or alkaline earth cations
do not give an increased FE. They involve a larger ionization potential that
compensates for the increased crystal bonding.Comment: 3 pages, 2 figure
Pressure-induced structural transitions in MgH
The stability of MgH has been studied up to 20~GPa using
density-functional total-energy calculations. At ambient pressure
-MgH takes a TiO-rutile-type structure. -MgH is
predicted to transform into -MgH at 0.39~GPa. The calculated
structural data for - and -MgH are in very good agreement
with experimental values. At equilibrium the energy difference between these
modifications is very small, and as a result both phases coexist in a certain
volume and pressure field. Above 3.84~GPa -MgH transforms into
-MgH; consistent with experimental findings. Two further
transformations have been identified at still higher pressure: i) - to
-MgH at 6.73 GPa and (ii) - to -MgH at
10.26~GPa.Comment: 4 pages, 4 figure
Two-Dimensional Sigma-Hole Systems in Boron Layers: A First-Principles Study on Mg_{1-x}Na_xB_2 and Mg_{1-x}Al_xB_2
We study two-dimensional sigma-hole systems in boron layers by calculating
the electronic structures of Mg_{1-x}Na_xB_2 and Mg_{1-x}Al_xB_2. In
Mg_{1-x}Na_xB_2, it is found that the concentration of sigma holes is
approximately described by (0.8 + 0.8 x) * 10^{22} cm^{-3} and the largest
attainable concentration is about 1.6 * 10^{22} cm^{-3} in NaB_2. In
Mg_{1-x}Al_xB_2, on the other hand, it is found that the concentration of sigma
holes is approximately described by (0.8 - 1.4 x) * 10^{22} cm^{-3} and sigma
holes are disappeared at x of about 0.6. These relations can be used for
experimental studies on the sigma-hole systems in these materials.Comment: 5 pages, 5 figure
Theoretical Study of Electronic Structure and Superconductivity in Nb_(1-x)B_2 Alloys
Using the Korringa-Kohn-Rostoker coherent-potential approximation in the
atomic-sphere approximation (KKR-ASA CPA) we have studied the changes in the
electronic structure and the superconducting transition temperature T_{c} in
Nb_{1-x}B_{2} alloys as a function of x. We find that the variation in the
electronic structure of Nb_{1-x}B_{2} alloys as a function of x is consistent
with the rigid-band model. However, the variation of T_{c}, obtained using the
Allen-Dynes equation within the Gaspari-Gyorffy formalism to estimate the
electron-phonon matrix elements, does not follow the expected trend. We
associate this disagreement to the use of a constant \omega_{rms} in the
Allen-Dynes equation over the whole range of vacancy concentration, thereby
indicating the importance of lattice dynamical effects in these systems.Comment: 10 pages, 4 figure
Bonding mechanism and magnetic ordering in Laves phase λ1−MgCo2 intermetallic compound from theoretical and experimental studies
publishedVersio
Electronic structure, phase stability and chemical bonding in ThAl and ThAlH
We present the results of theoretical investigation on the electronic
structure, bonding nature and ground state properties of ThAl and
ThAlH using generalized-gradient-corrected first-principles
full-potential density-functional calculations. ThAlH has been reported
to violate the "2 \AA rule" of H-H separation in hydrides. From our total
energy as well as force-minimization calculations, we found a shortest H-H
separation of 1.95 {\AA} in accordance with recent high resolution powder
neutron diffraction experiments. When the ThAl matrix is hydrogenated, the
volume expansion is highly anisotropic, which is quite opposite to other
hydrides having the same crystal structure. The bonding nature of these
materials are analyzed from the density of states, crystal-orbital Hamiltonian
population and valence-charge-density analyses. Our calculation predicts
different nature of bonding for the H atoms along and . The strongest
bonding in ThAlH is between Th and H along which form dumb-bell
shaped H-Th-H subunits. Due to this strong covalent interaction there is very
small amount of electrons present between H atoms along which makes
repulsive interaction between the H atoms smaller and this is the precise
reason why the 2 {\AA} rule is violated. The large difference in the
interatomic distances between the interstitial region where one can accommodate
H in the and planes along with the strong covalent interaction
between Th and H are the main reasons for highly anisotropic volume expansion
on hydrogenation of ThAl.Comment: 14 pages, 9 figure
Detailed electronic structure studies on superconducting MgB and related compounds
In order to understand the unexpected superconducting behavior of MgB
compound we have made electronic structure calculations for MgB and closely
related systems. Our calculated Debye temperature from the elastic properties
indicate that the average phonon frequency is very large in MgB compared
with other superconducting intermetallics and the exceptionally high in
this material can be explained through BCS mechanism only if phonon softening
occurs or the phonon modes are highly anisotropic. We identified a
doubly-degenerate quasi-two dimensional key-energy band in the vicinity of
along -A direction of BZ which play an important role in
deciding the superconducting behavior of this material. Based on this result,
we have searched for similar kinds of electronic feature in a series of
isoelectronic compounds such as BeB, CaB, SrB, LiBC and
MgBC and found that MgBC is one potential material from the
superconductivity point of view. There are contradictory experimental results
regarding the anisotropy in the elastic properties of MgB ranging from
isotropic, moderately anisotropic to highly anisotropic. In order to settle
this issue we have calculated the single crystal elastic constants for MgB
by the accurate full-potential method and derived the directional dependent
linear compressibility, Young's modulus, shear modulus and relevant elastic
properties. We have observed large anisotropy in the elastic properties. Our
calculated polarized optical dielectric tensor shows highly anisotropic
behavior even though it possesses isotropic transport property. MgB
possesses a mixed bonding character and this has been verified from density of
states, charge density and crystal orbital Hamiltonian population analyses
Antiferromagnetic vs ferromagnetic interactions and spin-glass-like behavior in ruthenates
We have made a series of gradient-corrected relativistic full-potential
density-functional calculation for Ca-substituted and hole-doped SrRuO in
para, ferro, and -, -, and -type antiferromagnetic states. Magnetic
phase-diagram data for SrCaRuO at 0 K are presented. Neutron
diffraction measurement combined with total energy calculations show that
spin-glass behavior with short-range antiferromagnetic interactions rules in
CaRuO. The substitution of Sr by Ca in SrRuO decreases the
ferromagnetic interaction and enhances the -type antiferromagnetic
interaction; the -AF state is found to stabilize around = 0.75
consistent with experimental observations. Inclusion of spin-orbit coupling is
found to be important in order to arrive at the correct magnetic ground state
in ruthenates.Comment: 5 pages, 3 figures. Solid state communications (in press
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