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. ■ INTRODUCTION Density functional theory (DFT) calculations of nuclear magnetic resonance (NMR) shielding in solids present in principle a relatively straightforward way to aid the interpretation of measured spectra. In the case of insulating solids, where only the response of orbital motion contributes to the screening of the external magnetic field, the approach is well-established and has been used for years. The situation is much more complicated for metallic systems, where the interaction of the external field with the spin of the electrons leads to repopulation of electronic states around the Fermi level, resulting in a spin current that contributes directly to the screening. The resulting spin density modifies the existing potential and polarizes the lower (core) eigenstates, constituting a source of technical difficulties that may lead to potential issues. For these reasons, there are only very few reported studies of NMR calculations in metals. However, these studies also show that carefully approaching such problems can result in reliable computed NMR shieldings. In order to expand the application base of our recently published computational scheme for calculating NMR shielding in metals, we report on the isotropic shielding at Sc and Al sites in Heusler phases with general composition ScTT′Al, where T and T′ are transition metals. We compare our results to recently published experimental data. The availability of experimental NMR data presents a valuable opportunity for studying the relation between NMR parameters and the atomic and electronic structures as well as the specific material composition, since this is in general not very well understood in solids. The NMR shielding at the Al site has been recently computed by another group using a similar approach but with a different DFT code. Unfortunately, these authors did not present their results for Sc shieldings; however, a comparison of Al shieldings from these two approaches is still valuable for the validation of computational methodologies. The Heusler phases XTT′Y are cubic compounds spanning a wide range of chemical compositions. They exhibit several physical properties such as magnetic phases, giant magnetoresistance, enhanced thermoelectricity, and superconductivity that may be considered as technologically relevant. In a recent study, the ternary compounds ScT2Al, where T = Ni, Pd, Cu, Ag, Au, and quaternary compounds ScTT′Al, with TT′ = NiCu, NiAg, PdCu, NiPd, NiAu, PdAu, and AgAu, have been synthesized and their NMR chemical shieldings were reported. Powder X-ray diffraction measurements have shown that the ternary ScT2Al Heusler alloys crystallize in the cubic MnCu2Altype structure (Fm3̅m), where the Al and Sc atoms are arranged in an NaCl fashion and the T atoms are placed inside the voids of the Sc−Al sublattice such that each Sc/Al atom is surrounded by eight transition metal atoms arranged in the Received: April 13, 2017 Revised: May 19, 2017 Published: May 19, 2017 Article