517 research outputs found
Phonon Spectra in the Parent Superconducting Iron-tuned Telluride FeTe from Inelastic Neutron Scattering and Ab Initio Calculations
We report inelastic neutron scattering measurements of phonon spectra in the
parent superconductor iron-tuned chalcogenide FeTe, for two different x
contents (x 0.11), using neutron time-of-flight technique. Thermal
neutron spectroscopy allowed to collect the low-temperature Stokes spectra over
an extended Q-range, at 2, 40 and 120K - hence covering both the magnetic
monoclinic and the paramagnetic tetragonal phases. Whereas cold-neutrons
allowed to measure high-resolution anti-Stokes spectra at 140, 220 and 300K,
thus covering the tetragonal phase. Our results evidence a spin-phonon coupling
behaviour towards the observed noticeable temperature-dependent change of the
Stokes spectra across the transition temperatures. On the other hand, the
anti-Stokes spectra reveal a pronounced hardening of the low-energy, acoustic
region, of the phonon spectrum, upon heating, indicating a strong anharmonicity
and a subtle dependence of phonons on structural evolution within the
tetragonal phase. Experimental results are accompanied by ab initio
calculations of phonon spectra of the tetragonal stoichiometric phase for a
comparison with the high-resolution anti-Stokes spectra. Calculations included
different density functional methods. Spin polarization and van der Waals
interaction, were either considered or neglected, individually or
concomitantly, in order to study their respective effect on lattice dynamics
description. Our results suggest that including van der Waals interaction has
only a slight effect on phonon dynamics, however, phonon spectra are better
described when spin polarization is included, in a cooperative way with van der
Waals interactions
Physical properties of the thermoelectric cubic lanthanum chalcogenides La3-yX4 (X=S,Se,Te) from first-principles
We report ab-initio calculations of the stability, lattice dynamics,
electronic and thermoelectric properties of cubic La3-yX4 (X=S,Se,Te) materials
in view of analyzing their potential for thermoelectric applications. The
lanthanum motions are strongly coupled to the tellurium motions in the
telluride, whereas the motions of both types of atoms are decoupled in the
sulfides. Nevertheless, this has no impact on their thermal properties because
experimentally all compounds have low thermal conductivity. We believe that
this is due to Umklapp scattering of the acoustical modes, notably by the low
energy optical modes at about 7-8 meV found in all three chalcogenides, as in
cage compounds such as skutterudites or clathrates, even though there are no
cages in the cubic Th3P4 structure. We find that the energy bandgap increases
from the telluride to the sulfide in good agreement with the experiments.
However, due to their similar band structure, we find that all three compounds
have almost identical thermoelectric properties. Our results agree
qualitatively with the experiments, especially in the case of the telluride for
which a great amount of data exists. All our results indicate that the sulfides
have strong potential for thermoelectricity and could replace the tellurides if
the charge carrier concentration is optimized. Finally, we predict also a
larger maximum ZT for the p-type doped materials than for the n-type doped
ones, even though compounds with p-doping have still to be synthesized. Thus
our results indicate the possibility to make high temperature performing
thermo-generators based only on La3X4 compounds.Comment: 37 pages, 12 figure
Effect of doping on the thermoelectric properties of thallium tellurides using first principles calculations
We present a study of the electronic properties of Tl5Te3, BiTl9Te6 and
SbTl9Te6 compounds by means of density functional theory based calculations.
The optimized lattice constants of the compounds are in good agreement with the
experimental data. The band gap of BiTl9Te6 and SbTl9Te6 compounds are found to
be equal to 0.589 eV and 0.538 eV, respectively and are in agreement with the
available experimental data. To compare the thermoelectric properties of the
different compounds we calculate their thermopower using Mott's law and show,
as expected experimentally, that the substituted tellurides have much better
thermoelectric properties compared to the pure compound.Comment: PTM2010 Conferenc
Physical properties of Thallium-Tellurium based thermoelectric compounds using first-principles simulations
We present a study of the thermodynamic and physical properties of Tl5Te3,
BiTl9Te6 and SbTl9Te6 compounds by means of density functional theory based
calculations. The optimized lattice constants of the compounds are in good
agreement with the experimental data. The electronic density of states and band
structures are calculated to understand the bonding mechanism in the three
compounds. The indirect band gap of BiTl9Te6 and SbTl9Te6 compounds are found
to be equal to 0.256 eV and 0.374 eV, respectively. The spin-orbit coupling has
important effects on the electronic structure of the two semiconducting
compounds and should therefore be included for a good numerical description of
these materials. The elastic constants of the three compounds have been
calculated, and the bulk modulus, shear modulus, and young's modulus have been
determined. The change from ductile to brittle behavior after Sb or Bi alloying
is related to the change of the electronic properties. Finally, the Debye
temperature, longitudinal, transverse and average sound velocities have been
obtained
Lattice stability and formation energies of intrinsic defects in Mg2Si and Mg2Ge via first principles simulations
We report an ab initio study of the semiconducting Mg2X (with X = Si, Ge)
compounds and in particular we analyze the formation energy of the different
point defects with the aim to understand the intrinsic doping mechanisms. We
find that the formation energy of Mg2Ge is 50 % larger than the one of Mg2Si,
in agreement with the experimental tendency. From the study of the stability
and the electronic properties of the most stable defects taking into account
the growth conditions, we show that the main reason for the n-doping in these
materials comes from interstitial magnesium defects. Conversely, since other
defects acting like acceptors such as Mg vacancies or multivacancies are more
stable in Mg2Ge than in Mg2Si, this explains why Mg2Ge can be of n or p type,
contrary to Mg2Si. The finding that the most stable defects are different in
Mg2Si and Mg2Ge and depend on the growth conditions is important and must be
taken into account in the search of the optimal doping to improve the
thermoelectric properties of these materials.Comment: 25 pages, 6 Table
High-field irreversible moment reorientation in the antiferromagnet FeTe
Magnetization measurements have been performed on single-crystalline
FeTe in pulsed magnetic fields up to 53 T
and temperatures from 4.2 to 65 K. At K, a non-reversible reorientation
of the antiferromagnetic moments is observed at T as the pulsed
field is on the rise. No anomaly is observed at during the fall of the
field and, as long as the temperature is unchanged, during both rises and falls
of additional field pulses. The transition at is reactivated if the
sample is warmed up above the N\'{e}el temperature K and cooled
down again. The magnetic field-temperature phase diagram of FeTe in
is also investigated. We present the temperature
dependence of , as well as that of the antiferromagnetic-to-paramagnetic
borderline in temperatures above 40 K.Comment: 5 pages, 4 figure
Magnetic glassy phase in FeSeTe single crystals
The evolution of the magnetic order in FeSeTe crystals as a function of Se
content was investigated by means of ac/dc magnetometry and muon-spin
spectroscopy. Experimental results and self-consistent DFT calculations both
indicate that muons are implanted in vacant iron-excess sites, where they probe
a local field mainly of dipolar origin, resulting from an antiferromagnetic
(AFM) bicollinear arrangement of iron spins. This long-range AFM phase
disorders progressively with increasing Se content. At the same time all the
tested samples manifest a marked glassy character that vanishes for high Se
contents. The presence of local electronic/compositional inhomogeneities most
likely favours the growth of clusters whose magnetic moment "freezes" at low
temperature. This glassy magnetic phase justifies both the coherent muon
precession seen at short times in the asymmetry data, as well as the glassy
behaviour evidenced by both dc and ac magnetometry.Comment: Approved for publication in J. Phys.: Condens. Matte
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