334 research outputs found
Exploring the possibility of enhancing the figure-of-merit ( 2) of NaCoO: A combined experimental and theoretical study
Search of new thermoelectric (TE) materials with high
\textit{figure-of-merit} (ZT) is always inspired the researcher in TE field.
Here, we present a combined experimental and theoretical study of TE properties
of NaCoO compound in high-temperature region. The experimental
Seebeck coefficient (S) is found to vary from 64 to 118 V/K in the
temperature range K. The positive values of S are indicating the
dominating p-type behaviour of the compound. The observed value of thermal
conductivity () is 2.2 W/m-K at 300 K. In the temperature region
K, the value of increases up to 2.6 W/m-K and then
decreases slowly till 620 K with the corresponding value of 2.4 W/m-K.
We have also carried out the theoretical calculations and the best matching
between experimental and calculated values of transport properties are observed
in spin-polarized calculation within DFT+\textit{U} by chosen \textit{U} = 4
eV. The maximum calculated value of ZT is found to be 0.67 at 1200 K for
p-type conduction. Our computational study suggests that the possibility of
n-type behaviour of the compound which can lead to a large value of ZT at
higher temperature region. Electron doping of 5.110
cm is expected to give rise the high ZT value of 2.7 at 1200 K.
Using these temperature-dependent ZT values, we have calculated the maximum
possible values of efficiency () of thermoelectric generator (TEG) made
by p and n-type NaCoO. The present study suggests that one can
get the efficiency of a TE cell as high as 11 when the cold and hot
end temperature are fixed at 300 K and 1200 K, respectively. Such high values
of ZT and efficiency suggest that NaCoO can be used as a
potential candidate for high-temperature TE applications
Influence of Ni doping on the electronic structure of Ni_2MnGa
The modifications in the electronic structure of Ni_{2+x}Mn_{1-x}Ga by Ni
doping have been studied using full potential linearized augmented plane wave
method and ultra-violet photoemission spectroscopy. Ni 3d related electron
states appear due to formation of Ni clusters. We show the possibility of
changing the minority-spin DOS with Ni doping, while the majority-spin DOS
remains almost unchanged. The total magnetic moment decreases with excess Ni.
The total energy calculations corroborate the experimentally reported changes
in the Curie temperature and the martensitic transition temperature with x.Comment: 4 pages, 4 figures, accepted in Phys. Rev.
Evolution of complex magnetic phases and metal-insulator transition through Nb substitution in LaSrCoNbO
We report the evolution of structural, magnetic, transport, and electronic
properties of bulk polycrystalline LaSrCoNbO
( 0.025--0.25) samples. The Rietveld refinement of the x-ray diffraction
patterns with Rc space group reveals that the lattice parameters and
rhombohedral distortion monotonously increase with the Nb(4)
substitution (). The magnetic susceptibility exhibits a decrease in the
magnetic ordering temperature and net magnetization with , which manifests
that the Nb substitution dilutes the ferromagnetic (FM) double exchange
interaction and enhances the antiferromagnetic (AFM) super-exchange
interaction. Interestingly, for the 0.1 samples the FM order is completely
suppressed and the emergence of a glassy state is clearly evident. Moreover,
the decrease in the coercivity (H) and remanence (M) with
in the magnetic isotherms measured at 5~K further confirms the dominance of AFM
interactions and reduction of FM volume fraction for the 0.1 samples. More
interestingly, we observe resistivity minima for the 0.025 and 0.05
samples, which are analyzed using the quantum corrections in the conductivity,
and found that the weak localization effect dominates over the renormalized
electron-electron interactions in the 3D limit. Further, a semiconducting
resistivity behavior is obtained for 0.05, which follows the Arrhenius law
at high temperatures (160--320~K), and the 3D-variable range hopping
prevails in the low-temperature region (160~K). The core-level photoemission
spectra confirm the valence state of constituent elements and the absence of
Co is discernible.Comment: submitte
Correlation between the exchange bias effect and antisite disorders in SrLaCoNbO
We unravel the effect of La substitution and hence antisite disorders on the
exchange bias (EB) mechanism in SrLaCoNbO ( 0, 0.2) double
perovskite samples using the detailed analysis of the field cooled
magnetization isotherms (M--H) and training effect. The field dependence of the
freezing temperature deviates from both Gabay-Toulouse (GT) and de
Almeida-Thouless (AT) lines and the analysis suggests that the 0 sample
follows a different universality class with moderate anisotropy in the frozen
spins. Interestingly, we find that the EB effect is significantly suppressed in
the 0.2 sample due to increase [decrease] in the size of ferromagnetic
(FM) [cluster glass (CG)] domain, which reduces the effective disordered
interface responsible for the EB. The changes in fraction of FM, AFM, and CG
like interactions with the La substitution and applied magnetic field are found
to be crucial in governing the EB effect in these samples. Further, the
training effect measurements show the unequal shift in the left and right
branches of the M--H loops and their different evolution with the field cycles
(). The analysis reveals that the rotatable spins relax approximately one
order of magnitude faster than the frozen spins at the disordered interface. We
find a possible correlation between the observed EB effect and the antisite
disorders in these samples.Comment: submitte
Investigation of lattice dynamics, magnetism and electronic transport in -NaVO
We investigate the electronic and magnetic properties as well as lattice
dynamics and spin-phonon coupling of -NaVO using
temperature-dependent Raman scattering, dc-magnetization and dc-resistivity,
x-ray photoemission, and absorption spectroscopy. The Rietveld refinement of
XRD pattern with space group C2/m confirms the monoclinic structure. The
analysis of temperature-dependent Raman spectra in a temperature range of
13--673~K reveals an anharmonic dependence of the phonon frequency and full
width at half maximum, which is accredited to the symmetric phonon decay.
However, below about 40 K, the hardening of the phonon frequency beyond
anharmonicity is attributed to the spin-phonon coupling. Interestingly, the
estimated effective magnetic moment 0.63~ from the
magnetization data manifests a mixed-valence state of V ions in 4+ (181\%)
and 5+ (821\%). A similar ratio of V to V is also observed
in the x-ray photoemission and x-ray absorption near-edge spectra and that is
found to be consistent with the sample stoichiometry. In addition, the V
ions are distributed between different vanadium (V1 and V3) sites. The analysis
of extended x-ray absorption fine structure at different V-sites gives the
corresponding V--O bond lengths, which are utilized in the assignment of Raman
modes. Moreover, the temperature-dependent resistivity resembles a
semiconducting behavior where the charge carrier transport is facilitated by
the band conduction at higher temperatures and via hopping 260~K.Comment: to be published in PR
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