6 research outputs found
Comparison of the Site Occupancies Determined by Combined Rietveld Refinement and Density Functional Theory Calculations: Example of the Ternary Mo–Ni–Re σ Phase
The site occupancies of the Mo–Ni–Re σ
phase
have been studied as a function of the composition in the ternary
homogeneity domain by both experimental measurements and calculations.
Because of the possible simultaneous occupancy of three elements on
the five sites of the crystal structure, the experimental determination
of the site occupancies was achieved by using combined Rietveld refinement
of X-ray and neutron diffraction data, whereas calculation of the
site occupancies was carried out by using the density functional theory
results of every ordered (i.e., 3<sup>5</sup> = 243) configuration
appearing in the ternary system. A comparison of the experimental
and calculation results showed good agreement, which suggests that
the topologically close-packed phases, such as the σ phase,
could be described by the Bragg–Williams approximation (i.e.,
ignoring the short-range-order contributions). On the other hand,
the atomic distribution on different crystallographic sites of the
Mo–Ni–Re σ phase was found to be governed by the
atomic sizes. Ni, having the smallest atomic size, showed a preference
for low-coordination-number (CN) sites, whereas Mo, being the largest
in atomic size, preferred occupying high-CN sites. However, the preference
of Re, having intermediate atomic size, varied depending on the composition,
and a clear reversal in the preference of Re as a function of the
composition was evidenced in both the calculated and experimental
site-occupancy results
χ and σ Phases in Binary Rhenium–Transition Metal Systems: a Systematic First-Principles Investigation
The
Frank–Kasper phases, known as topologically close-packed (tcp)
phases, are interesting examples of intermetallic compounds able to
accommodate large homogeneity ranges by atom mixing on different sites.
Among them, the χ and σ phases present two competing complex
crystallographic structures, the stability of which is driven by both
geometric and electronic factors. Rhenium (Re) is the element forming
the largest number of binary χ and σ phases. Its central
position among the transition metals in the periodic table plays an
important role in the element ordering in tcp phases. Indeed, it has
been shown that Re shows an opposite site preference depending on
which elements it is alloyed with. In the present work, χ- and
σ-phase stability in binary Re–<i>X</i> systems
is systematically studied by a first-principles investigation. The
heats of formation of the complete set of ordered configurations (16
for χ and 32 for σ) have been calculated in 16 well-chosen
systems to identify stability criteria. They include not only the
systems in which χ-Re–<i>X</i> (<i>X</i> = Ti, Mn, Zr, Nb, Mo, Hf, Ta, W) or σ-Re–<i>X</i> (<i>X</i> = V, Cr, Mn, Fe, Nb, Mo, Ta, W) exist but also
the systems in which both phases are not stable, including systems
in which <i>X</i> is a 3<i>d</i> element from
Ti to Ni, a 4<i>d</i> element from Zr to Ru, and a 5<i>d</i> element from Hf to Os. Careful analysis is done of the
energetic tendencies as a function of recomposition, size effect,
and electron concentration. Moreover, the site preference and other
crystallographic properties are discussed. Conclusions are drawn concerning
the relative stability of the two phases in comparison with the available
experimental knowledge on the systems
χ and σ Phases in Binary Rhenium–Transition Metal Systems: a Systematic First-Principles Investigation
The
Frank–Kasper phases, known as topologically close-packed (tcp)
phases, are interesting examples of intermetallic compounds able to
accommodate large homogeneity ranges by atom mixing on different sites.
Among them, the χ and σ phases present two competing complex
crystallographic structures, the stability of which is driven by both
geometric and electronic factors. Rhenium (Re) is the element forming
the largest number of binary χ and σ phases. Its central
position among the transition metals in the periodic table plays an
important role in the element ordering in tcp phases. Indeed, it has
been shown that Re shows an opposite site preference depending on
which elements it is alloyed with. In the present work, χ- and
σ-phase stability in binary Re–<i>X</i> systems
is systematically studied by a first-principles investigation. The
heats of formation of the complete set of ordered configurations (16
for χ and 32 for σ) have been calculated in 16 well-chosen
systems to identify stability criteria. They include not only the
systems in which χ-Re–<i>X</i> (<i>X</i> = Ti, Mn, Zr, Nb, Mo, Hf, Ta, W) or σ-Re–<i>X</i> (<i>X</i> = V, Cr, Mn, Fe, Nb, Mo, Ta, W) exist but also
the systems in which both phases are not stable, including systems
in which <i>X</i> is a 3<i>d</i> element from
Ti to Ni, a 4<i>d</i> element from Zr to Ru, and a 5<i>d</i> element from Hf to Os. Careful analysis is done of the
energetic tendencies as a function of recomposition, size effect,
and electron concentration. Moreover, the site preference and other
crystallographic properties are discussed. Conclusions are drawn concerning
the relative stability of the two phases in comparison with the available
experimental knowledge on the systems
Comparison of the Site Occupancies Determined by Combined Rietveld Refinement and Density Functional Theory Calculations: Example of the Ternary Mo–Ni–Re σ Phase
The site occupancies of the Mo–Ni–Re σ
phase
have been studied as a function of the composition in the ternary
homogeneity domain by both experimental measurements and calculations.
Because of the possible simultaneous occupancy of three elements on
the five sites of the crystal structure, the experimental determination
of the site occupancies was achieved by using combined Rietveld refinement
of X-ray and neutron diffraction data, whereas calculation of the
site occupancies was carried out by using the density functional theory
results of every ordered (i.e., 3<sup>5</sup> = 243) configuration
appearing in the ternary system. A comparison of the experimental
and calculation results showed good agreement, which suggests that
the topologically close-packed phases, such as the σ phase,
could be described by the Bragg–Williams approximation (i.e.,
ignoring the short-range-order contributions). On the other hand,
the atomic distribution on different crystallographic sites of the
Mo–Ni–Re σ phase was found to be governed by the
atomic sizes. Ni, having the smallest atomic size, showed a preference
for low-coordination-number (CN) sites, whereas Mo, being the largest
in atomic size, preferred occupying high-CN sites. However, the preference
of Re, having intermediate atomic size, varied depending on the composition,
and a clear reversal in the preference of Re as a function of the
composition was evidenced in both the calculated and experimental
site-occupancy results
Systematic First-Principles Study of Binary Metal Hydrides
First-principles
calculations were systematically performed for
31 binary metal–hydrogen (<i>M</i>–H) systems
on a set of 30 potential crystal structures selected on the basis
of experimental data and possible interstitial sites. For each <i>M</i>–H system, the calculated enthalpies of formation
were represented as functions of H composition. The zero-point energy
correction was considered for the most stable hydrides via additional
harmonic phonon calculations. The sequence of stable hydrides (ground-state)
given by the convex hull was found in satisfactory agreement with
the experimental data. In addition, new high pressure dihydrides and
trihydrides were predicted, providing orientations for new materials
synthesis. The overall results provide a global overview of hydride
relative stabilities and relevant input data for thermodynamic modeling
methods
Thermodynamic Modeling of the Ni–H System
A new
thermodynamic assessment of the Ni–H system has been
carried out, providing a complete description valid up to 6 ×
10<sup>9</sup> Pa of this system at the center of hydrogen storage
issues. The study includes the hydride formation reaction and the
presence of a miscibility gap in the fcc interstitial solid solution
of hydrogen in nickel. In addition to a complete literature review,
first-principles calculations were carried out and combined with other
modeling approaches. The cluster expansion method allowed us to describe
the energetic interactions between atoms leading to the miscibility
gap in the fcc solid solution. Besides, the compressibility at high
pressure was characterized for each phase including the condensed
phases. For this purpose, a specific high-pressure model was assessed
with the contribution of quasi-harmonic phonon calculations. The obtained
consistent model allows us to characterize entirely the thermochemical
behavior of the Ni–H system