This
study reports first-principles simulations of 27Al solid-state
nuclear magnetic resonance (ssNMR) shifts (δiso
exp) carried out
for a set of seven Al-containing intermetallic compounds. With the
aim of assessing accuracy issues in such calculations using the gauge-including
projector augmented waves method, the referred set was sought to cover
the wide range of experimental 27Al δiso
exp shifts reported
in the literature for this type of compound in a representative way
(from about −200 to 1600 ppm). From a technical/computational
perspective, the findings allowed the detection of critical sources
of inaccuracy and they were addressed to different approximations
inherent in the computational methods. Among those, it is possible
to highlight the formulation of the exchange-correlation energy functional
in density functional theory, which at least for the 27Al nuclide has proved to be more critical than the frozen-core approximation,
especially in the case of weakly magnetic aluminides. From a practical
point of view, particularly regarding a possible use of such simulations
for a more efficient interpretation of experimental ssNMR spectra,
key physical insights into the magnetic screening mechanisms underlying
the 27Al δiso
exp shifts are reported, in which theory supports
that in addition to the contributions of closed orbitals, those coming
from unpaired electronic spins arise from two main competing hyperfine
interactions: the first one with local magnetic moments at specific
sites, resulting in a shielding effect and even negative shifts, and
the second mechanism in which large Knight shifts to downfield gradually
dominate in those compounds having a rather metallic character.