Computational Study of Y NMR Shielding in Intermetallic Yttrium Compounds

Density functional theory (DFT) calculations of the magnetic shielding for solid state nuclear magnetic resonance (NMR) provide an important contribution for the understanding of experimentally observed signals. In this work, we present calculations of the Y NMR shielding in intermetallic compounds (YMg, YT, YTX, YT2X, YT2X2, Y2TB6, and Y2TSi3 where T represents various transition metals and X refers to group IV elements C, Si, Ge, Sn, Pb). The total shielding σ of this selection varies by about 2500 ppm and correlates very well with the experimentally observed shifts except for YMg and YZn. These two simple compounds have a spike in the DOS at EF and a corresponding huge spin susceptibility which leads to the disagreement. It could be a problem of DFT (neglect of spin fluctuations), but we would interpret the discrepancies as caused by disorder which could be present in the experimental samples because disorder removes the spike in the DOS. The diamagnetic contribution σo (chemical shift) is by no means constant as often assumed when interpreting experimental metallic shifts and varies up to 1500 ppm, but still the dominating term is the spin contact term σc. Although all compounds are metals, only half of them have a paramagnetic (negative) σc due to the reoccupation of the valence Y-5s electrons, while for others the large induced Y-4d magnetic moment induces a diamagnetic core polarization. In most of our cases, the spin dipolar contribution σsd is fairly small with |σsd| less than 100 ppm and often even much smaller except in a few very asymmetric compounds like YCo2Si2 and YRu2Si2 (σsd ≈ 320 ppm).