2,522 research outputs found
Structural Phase Stability in Fluorinated Calcium Hydride
In order to improve the hydrogen storage properties of calcium hydride
(CaH2), we have tuned its thermodynamical properties through fluorination.
Using ab-initio total energy calculations based on density functional theory,
the structural stability, electronic structure and chemical bonding of CaH2-xFx
systems are investigated. The phase transition of fluorinated systems from
orthorhombic to cubic structure has been observed at 18% fluorine doped CaH2.
The phase stability analysis shows that CaH2-xFx systems are highly stable and
the stability is directly correlating with their ionicity. Density of states
(DOS) plot reveals that CaH2-xFx systems are insulators. Partial DOS and charge
density analyses conclude that these systems are governed by ionic bonding. Our
results show that H closer to F can be removed more easily than that far away
from F and this is due to disproportionation induced in the bonding interaction
by fluorination
Chemical Bonding Analysis on Amphoteric Hydrogen - Alkaline Earth Ammine Borohydrides
Usually the ions in solid are in the positive oxidation states or in the
negative oxidation state depending upon the chemical environment. It is highly
unusual for an ion having both positive as well as negative oxidation state in
a particular compound. Structural analysis suggest that the alkaline earth
ammine borohydrides (AABH) with the chemical formula M (BH4)2(NH3)2 (M = Mg,
Ca, or Sr) where hydrogen is present in +1 and -1 oxidation states. In order to
understand the oxidation states of hydrogen and also the character of chemical
bond present in AABH we have made charge density, electron localization
function, Born effective charge, Bader effective charge, and density of states
analyses using result from the density functional calculations. Our detailed
analyses show that hydrogen is in amphoteric behavior with hydrogen closer to
boron is in negative oxidation state and that closer to nitrogen is in the
positive oxidation state. Due to the presence of finite covalent bonding
between the consitutents in AABH the oxidation state of hydrogen is
non-interger value. The confirmation of the presence of amphtoric behavior of
hydrogen in AABH has implication in hydrogen storage applications
Magnetoelectric Properties of Pb Free Bi2FeTiO6: A Theoretical Investigation
The structural, electronic, magnetic and ferroelectric properties of Pb free
double perovskite multiferroic Bi2FeTiO6 are investigated using density
functional theory within the general gradient approximation (GGA) method. Our
structural optimization using total energy calculations for different potential
structures show a minimum energy for a non-centrosymmetric rhombohedral
structure with R3 space group. Bi2FeTiO6 is found to be an antiferromagnetic
insulator with C-type magnetic ordering with bandgap value of 0.3 eV. The
calculated magnetic moment of 3.52 \mu_B at Fe site shows the high spin
arrangement of 3d electrons which is also confirmed by our orbital projected
density of states analysis. We have analyzed the characteristics of bonding
present between the constituents of Bi2FeTiO6 with the help of calculated
partial density of states and Born effective charges. The ground state of the
nearest centrosymmetric structure is found to be a G-type antiferromagnet with
half metallicity showing that by the application of external electric field we
can not only get a polarized state but also change the magnetic ordering and
electronic structure in the present compound indicating strong magnetoelectric
coupling. The cation sites the coexistence of Bi 6s lone pair (bring
disproportionate charge distribution) and Ti4+ d0 ions which brings covalency
produces off-center displacement and favors a non-centrosymmetric ground state
and thus ferroelectricity. Our Berry phase calculation gives a polarization of
48 \muCcm-2 for Bi2FeTiO6.Comment: 4 pages, 5 picture
Search for Thermoelectrics with High Figure of Merit in half-Heusler compounds with multinary substitution
In order to improve the thermoelectric performance of TiCoSb we have
substituted 50% of Ti equally with Zr and Hf at Ti site and Sb with Sn and Se
equally at Sb site. The electronic structure of Ti0.5Zr0.25Hf0.25CoSn0.5Se0.5
is investigated using the full potential linearized augmented plane wave method
and the thermoelectric transport properties are calculated on the basis of
semi-classical Boltzmann transport theory. Our band structure calculations show
that Ti0.5Zr0.25Hf0.25CoSn0.5Se0.5 has semiconducting behavior with indirect
band gap value of 0.98 eV which follow the empirical rule of 18
valence-electron content to bring semiconductivity in half Heusler compounds,
indicating that one can have semiconducting behavior in multinary phase of half
Heusler compounds if they full fill the 18 VEC rule and this open-up the
possibility of designing thermoelectrics with high figure of merit in half
Heusler compounds. We show that at high temperature of around 700K
Ti0.5Zr0.25Hf0.25CoSn0.5Se0.5 has high thermoelectric figure of merit of ZT =
1.05 which is higher than that of TiCoSb (~ 0.95) suggesting that by going from
ternary to multinary phase system one can enhance the thermoelectric figure of
merit at higher temperatures
Theoretical Investigation on the Effect of multinary Isoelectronic Substitution on TiCoSb based half-Heusler alloys
To understand the effect of isoelectronic substitution on thermoelectric
properties of TiCoSb based half - Heusler (HH) alloys, we have systematically
studied the transport properties with substitution of Zr at Ti and Bi at Sb
sites. The electronic structure of TixZr1-xCoSbxBi1-x (x = 0.25, 0.5, 0.75) and
parent TiCoSb are investigated using the full potential linearized augmented
plane wave method and the thermoelectric transport properties are calculated on
the basis of semiclassical Boltzmann transport theory. The band analysis of the
calculated band structures reveal that TixZr1-xCoSbxBi1-x has semiconducting
behavior with indirect band gap at x = 0.25, 0.5 concentration and direct band
gap behavior at x = 0.75 concentration. The TixZr1-xCoSbxBi1-x (x = 0.25, 0.5,
0.75) compounds show smaller band gap values as compared to the pure TiCoSb.
The d electrons of Ti/Zr and Co dominate the electronic transport properties of
TixZr1-xCoSbxBi1-x system. All these systems follow the empirical rule of 18
valence-electron content to bring semiconductivity in HH alloys. The
isoelectronic substitution in TiCoSb can tune the band structure by shifting
the Fermi level. This provides us lot of possibilities to get the desired band
gap values for designing thermoelectrics with high efficiency. In this study we
have showed that the isoelectronic substitution at both Ti and Sb site of
TiCoSb has very small effect for increasing the ZT values and one should go for
isoelectronic substitution at any one sites of TiCoSb HH alloys alone to
improve ZT
Effect of multinary substitution on electronic and transport properties of TiCoSb based half-Heusler alloys
The electronic structures of TixZrx/2CoPbxTex, TixZrx/2Hfx/2CoPbxTex (x =
0.5), and the parent compound TiCoSb were investigated using the full potential
linearized augmented plane wave method. The thermoelectric transport properties
of these alloys are calculated on the basis of semi-classical Boltzmann
transport theory. From the band structure calculations we show that the
substitution of Zr,Hf in the Ti site and Pb and Te in the Sb site lower the
band gap value and also change the indirect band (IB) gap of TiCoSb to the
direct band (DB) gap. The calculated band gap of TiCoSb, TixZrx/2CoPbxTex, and
TixZrx/2Hfx/2CoPbxTex are 1.04 eV (IB), 0.92 eV (DB), and 0.93 eV (DB),
respectively. All these alloys follow the empirical rule of 18 valence-electron
content which is essential for bringing semiconductivity in half Heusler
alloys. It is shown that the substitution of Hf at the Ti site improve the ZT
value (~1.05) at room temperature, whereas there is no significant difference
in ZT is found at higher temperature. Based on the calculated thermoelectric
transport properties, we conclude that the appropriate concentration of Hf
substitution can further improve the thermoelectric performance of
TixZrx/2Hfx/2CoPbxTex
Phosphorene-AsP Heterostructure as a Potential Excitonic Solar Cell Material - A First Principles Study
Solar energy conversion to produce electricity using photovoltaics is an
emerging area in alternative energy research. Herein, we report on the basis of
density functional calculations, phosphorene/AsP heterostructure could be a
promising material for excitonic solar cells (XSCs). Our HSE06 functional
calculations show that the band gap of both phosphorene and AsP fall exactly
into the optimum value range according to XSCs requirement. The calculated
effective mass of electrons and holes show anisotropic in nature with effective
masses along -X direction is lower than the -Y direction
and hence the charge transport will be faster along -X direction. The
wide energy range of light absorption confirms the potential use of these
materials for solar cell applications. Interestingly, phosphorene and AsP
monolayer forms a type-II band alignment which will enhance the separation of
photogenerated charge carriers and hence the recombination rate will be lower
which can further improve its photo-conversion efficiency if one use it in
XSCs
Ti4+ Substituted Magnesium Hydride as Promising Material for Hydrogen Storage and Photovoltaic Applications
In order to overcome the disadvantages of MgH2 towards its applications in
on-board hydrogen storage, first principle calculations have been performed for
Ti (2+, 3+, and 4+) substituted MgH2. Our calculated enthalpy of formation and
H site energy implies that Ti substitution in Mg site reduces the stability of
MgH2 which improve the hydrogen storage properties and Ti prefers to be in +4
oxidation state in MgH2. The bonding analyses through partial density of
states, electron localization function and Bader charge of these systems
confirm the existence of iono-covalent bonding. Electronic structure obtained
from hybrid functional calculations show that intermediate bands (IB) are
formed in Ti4+ substituted MgH2 which could improve the solar cell efficiencies
due to multiple photon absorption from valence band to conduction band via IBs
and converts low energy photons in the solar spectrum also into electricity.
Further, our calculated carrier effective masses and optical absorption spectra
show that Ti4+ substituted MgH2 is suitable for higher efficiency photovoltaic
applications. Our results suggest that Ti4+ substituted MgH2 can be considered
as a promising material for hydrogen storage as well as photovoltaic
applications
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