Computational Study of Al and Sc NMR Shielding in Metallic ScTT'Al Heusler Phases

The results of first-principles calculations of Al and Sc NMR shielding for a set of ScTT'Al Heusler phase compounds are reported and compared to recently published experimental NMR shifts. The orbital component of the shielding (chemical shift) is computed using density functional perturbation formalism. The spin part (Knight shift) is evaluated using a direct self-consistent approach, where a finite external magnetic field acting on the electron spin is added to the density functional theory (DFT) Hamiltonian. Both approaches are implemented in the full potential linearized augmented plane wave method. We show that, for Al, the calculated and measured shifts match quite well except for the ScNiAuAl case, which is probably due to an experimental problem as the reported lattice parameter is also unrealistic. For Sc nuclei, we find three such unmatched cases between theory and experiment and we discuss possible sources of this discrepancy. For selected compounds, the effects of composition and lattice size on the computed shielding are determined. In addition, we performed a detailed analysis of the paramagnetic and diamagnetic components of the Knight shift. Most notably, the Knight shift is found to be fairly unimportant for Sc shieldings as the large valence and core contributions cancel each other and the large variations in the shifts originate solely from orbital contributions, a quantity usually assumed to be constant in metallic systems. Finally, we compare our results with recently published data for Al shielding in ScT2Al obtained using a plane-wave pseudopotential based method.