284 research outputs found
Theory of the ferromagnetism in TiCrN solid solutions
First-principles calculations are used to investigate the magnetic properties
of TiCrN solid solutions. We show that the magnetic interactions
between Cr spins that favour antiferromagnetism in CrN is changed upon alloying
with TiN leading to the appearance of ferromagnetism in the system at
approximately in agreement with experimental reports. Furthermore
we suggest that this effect originates in an electron density redistribution
from Ti to Cr that decreases the polarization of Cr d-states with t
symmetry while it increases the polarization of Cr d-states with e
symmetry, both changes working in favour of ferromagnetism.Comment: 21 pages, 7 figure
Effect of magnetic disorder and strong electron correlations on the thermodynamics of CrN
We use first-principles calculations to study the effect of magnetic disorder
and electron correlations on the structural and thermodynamic properties of
CrN. We illustrate the usability of a special quasirandom structure supercell
treatment of the magnetic disorder by comparing with coherent potential
approximation calculations and with a complementary magnetic sampling method.
The need of a treatment of electron correlations effects beyond the local
density approximation is proven by a comparison of LDA+U calculations of
structural and electronic properties with experimental results. When magnetic
disorder and strong electron correlations are taken into account
simultaneously, pressure and temperature induced structural and magnetic
transitions in CrN can be understood.Comment: 23 pages, 7 figure
Configurational order-disorder induced metal-nonmetal transition in BC studied with first-principles superatom-special quasirandom structure method
Due to a large discrepancy between theory and experiment, the electronic
character of crystalline boron carbide BC has been a controversial
topic in the field of icosahedral boron-rich solids. We demonstrate that this
discrepancy is removed when configurational disorder is accurately considered
in the theoretical calculations. We find that while ordered ground state
BC is metallic, configurationally disordered BC,
modeled with a superatom-special quasirandom structure method, goes through a
metal to non-metal transition as the degree of disorder is increased with
increasing temperature. Specifically, one of the chain-end carbon atoms in the
CBC chains substitutes a neighboring equatorial boron atom in a B
icosahedron bonded to it, giving rise to a BC(BBC) unit. The
atomic configuration of the substitutionally disordered BC thus
tends to be dominated by a mixture between B(CBC) and
BC(BBC). Due to splitting of valence states in
BC(BBC), the electron deficiency in B(CBC) is gradually
compensated
Exchange Interactions in Paramagnetic Amorphous and Disordered Crystalline CrN-based Systems
We present a first principles supercell methodology for the calculation of
exchange interactions of magnetic materials with arbitrary degrees of
structural and chemical disorder in their high temperature paramagnetic state.
It is based on a projection of the total magnetic energy of the system onto
local pair clusters, allowing the interactions to vary independently as a
response to their local environments. We demonstrate our method by deriving the
distance dependent exchange interactions in vibrating crystalline CrN, a
TiCrN solid solution as well as in amorphous CrN. Our method
reveals strong local environment effects in all three systems. In the amorphous
case we use the full set of exchange interactions in a search for the
non-collinear magnetic ground state.Comment: 5 pages, 3 figure
Phase composition and transformations in magnetron-sputtered (Al,V)2O3 coatings
Coatings of (Al1-xVx)2O3, with x ranging from 0 to 1, were deposited by
pulsed DC reactive sputter deposition on Si(100) at a temperature of 550
{\deg}C. XRD showed three different crystal structures depending on V-metal
fraction in the coating: {\alpha}-V2O3 rhombohedral structure for 100 at.% V, a
defect spinel structure for the intermediate region, 63 - 42 at.% V. At lower
V-content, 18 and 7 at.%, a gamma-alumina-like solid solution was observed,
shifted to larger d-spacing compared to pure {\gamma}-Al2O3. The microstructure
changes from large columnar faceted grains for {\alpha}-V2O3 to smaller
equiaxed grains when lowering the vanadium content toward pure {\gamma}-Al2O3.
Annealing in air resulted in formation of V2O5 crystals on the surface of the
coating after annealing to 500 {\deg}C for 42 at.% V and 700 {\deg}C for 18
at.% V metal fraction respectively. The highest thermal stability was shown for
pure {\gamma}-Al2O3-coating, which transformed to {\alpha}-Al2O3 after
annealing to 1100{\deg} C. Highest hardness was observed for the Al-rich
oxides, ~24 GPa. The latter decreased with increasing V-content, larger than 7
at.% V metal fraction. The measured hardness after annealing in air decreased
in conjunction with the onset of further oxidation of the coatings
A unified cluster expansion method applied to the configurational thermodynamics of cubic TiAlN
We study the thermodynamics of cubic Ti1-xAlxN using a unified cluster
expansion approach for the alloy problem. The purely configurational part of
the alloy Hamiltonian is expanded in terms of concentration and volume
dependent effective cluster interactions. By separate expansions of the
chemical fixed-lattice, and local lattice relaxation terms of the ordering
energies, we demonstrate how the screened generalized perturbation method can
be fruitfully combined with a concentration dependent Connolly-Williams cluster
expansion method. Utilising the obtained Hamiltonian in Monte Carlo simulations
we access the free energy of Ti1-xAlxN alloys and construct the isostructural
phase diagram. The results show surprising similarities with the previously
obtained mean-field results: The metastable c-TiAlN is subject to coherent
spinodal decomposition over a larger part of the concentration range, e.g. from
x >= 0.33 at 2000 K.Comment: 21 pages, 7 figure
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