Comparison of the Site
Occupancies Determined by Combined
Rietveld Refinement and Density Functional Theory Calculations: Example
of the Ternary Mo–Ni–Re σ Phase
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Abstract
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