DFT-GIPAW Al-27 NMR Simulations for Intermetallics: Accuracy Issues and Magnetic Screening Mechanisms

摘要

This study reports first-principles simulations of Al-27 solid-state nuclear magnetic resonance (ssNMR) shifts (delta(exp)(iso)) 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 Al-27 delta(exp)(iso) 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 Al-27 nuclide has proved to be more critical than the frozen-core approximation, especially in the case of wealdy 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 Al-27 delta(exp)(iso) 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.