701 research outputs found
Interplay of force constants in the lattice dynamics of disordered alloys : An ab-initio study
A reliable prediction of interatomic force constants in disordered alloys is
an outstanding problem. This is due to the need for a proper treatment of
multisite (atleast pair) correlation within a random environment. The situation
becomes even more challenging for systems with large difference in atomic size
and mass. We propose a systematic density functional theory (DFT) based study
to predict the ab-initio force constants in random alloys. The method is based
on a marriage between special quasirandom structures (SQS) and the augmented
space recursion (ASR) to calculate phonon spectra, density of states (DOS) etc.
bcc TaW and fcc NiPt alloys are considered as the two distinct test cases.
Ta-Ta (W-W) bond distance in the alloy is predicted to be smaller (larger) than
those in pure Ta (W), which, in turn, yields stiffer (softer) force constants
for Ta (W). Pt-Pt force constants in the alloy, however, are predicted to be
softer compared to Ni-Ni, due to a large bond distance of the former. Our
calculated force constants, phonon spectra and DOS are compared with
experiments and other theoretical results, wherever available. Correct trend of
present results for the two alloys pave a path for further future studies in
more complex alloy systems
Thermal conductivity and diffusion-mediated localization in Fe_{1-x}Cr_{x} Alloys
We apply a new Kubo-Greenwood type formula combined with a generalized
Feynman diagram- matic technique to report a first principles calculation of
the thermal transport properties of disordered Fe_{1-x}Cr_{x} alloys. The
diagrammatic approach simplifies the inclusion of disorder-induced scattering
effects on the two particle correlation functions and hence renormalizes the
heat current operator to calculate configuration averaged lattice thermal
conductivity and diffusivity. The thermal conductivity K(T) in the present case
shows an approximate quadratic T-dependence in the low temperature regime (T <
20 K), which subsequently rises smoothly to a T-independent saturated value at
high T . A numerical estimate of mobility edge from the thermal diffusivity
data yields the fraction of localized states. It is concluded that the complex
disorder scattering processes, in force-constant dominated disorder alloys such
as Fe-Cr, tend to localize the vibrational modes quite significantly.Comment: 5 pages, 5 figure
Anomalous random correlations of force constants on the lattice dynamical properties of disordered Au-Fe alloys
Au-Fe alloys are of immense interest due to their biocompatibility, anomalous
hall conductivity, and applications in various medical treatment. However,
irrespective of the method of preparation, they often exhibit a high-level of
disorder, with properties sensitive to the thermal or magnetic annealing
temperatures. We calculate lattice dynamical properties of AuFe
alloys using density functional theory methods, where, being a multisite
property, reliable interatomic force constant (IFC) calculations in disordered
alloys remain a challenge. We follow a two fold approach: (1) an accurate IFC
calculation in an environment with nominally zero chemical pair correlations to
mimic the homogeneously disordered alloy; and (2) a configurational averaging
for the desired phonon properties (e.g., dispersion, density of states, and
entropy). We find an anomalous change in the IFC's and phonon dispersion (split
bands) near =0.19, which is attributed to the local stiffening of the Au-Au
bonds when Au is in the vicinity of Fe. Other results based on mechanical and
thermo-physical properties reflect a similar anomaly: Phonon entropy, e.g.,
becomes negative below =0.19, suggesting a tendency for chemical unmixing,
reflecting the onset of miscibility gap in the phase diagram. Our results match
fairly well with reported data, wherever available
Revealing the nature of antiferro-quadrupolar ordering in Cerium Hexaboride: CeB
Cerium-hexaboride (CeB) f-electron compound displays a rich array of
low-temperature magnetic phenomena, including `magnetically hidden' order,
identified as multipolar in origin via advanced x-ray scattering. From
first-principles electronic-structure results, we find that the
\emph{antiferro-quadrupolar} (AFQ) ordering in CeB arises from
crystal-field splitting and yields band structure in agreement with
experiments. With interactions of -electrons between Ce and B being
small, the electronic state of CeB is suitably described as
Ce(4)(e)(B). The AFQ state of orbital spins is
caused by an exchange interaction induced through spin-orbit interaction, which
also splits J=5/2 state into ground state and excited
state. Within the smallest antiferromagnetic (111) configuration, an
orbital-ordered AFQ state appears during charge self-consistency, and supports
the appearance of `hidden' order. Hydrostatic pressure (either applied or
chemically induced) stabilizes the AFM (AFQ) states over a ferromagnetic one,
as observed at low temperatures.Comment: 6 pages, 4 figure
- …