Orientation-dependent NMR study of the giant-unit-cell intermetallicsβ−Al3Mg2, Bergman-phaseMg32(Al,Zn)49, andξ′−Al74Pd22Mn4

Abstract

We present a $^{27}\mathrm{Al}$ NMR study of three giant-unit-cell complex metallic compounds, $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Al}}_{3}{\mathrm{Mg}}_{2}$, the ``Bergman-phase'' ${\mathrm{Mg}}_{32}{(\mathrm{Al},\mathrm{Zn})}_{49}$, and ${\ensuremath{\xi}}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{Al}\text{\ensuremath{-}}\mathrm{Pd}\text{\ensuremath{-}}\mathrm{Mn}$, which contain some hundreds up to more than a thousand atoms in the unit cell. The NMR spectra of monocrystalline samples are strongly inhomogeneously broadened by the electric quadrupole interaction and the line shapes are featureless and powderlike, but still exhibit significant orientation-dependent variation of the intensity on the satellite part of the spectrum in the magnetic field. Measuring orientation-dependent satellite intensity in appropriate frequency windows yields rotation patterns that can be related to the structure and symmetry of the giant unit cells. For a theoretical reproduction of the rotation patterns, we derived a distribution of the electric-field-gradient (EFG) tensors for each of the investigated compounds from existing structural models using point-charge and ab initio calculations. The EFG distribution yields important structural information on the manifold of different local atomic environments in the unit cell and distinguishes crystallographically inequivalent lattice sites from the equivalent ones. The distribution of the EFGs for the 1168-atom unit cell of $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Al}}_{3}{\mathrm{Mg}}_{2}$ was successfully determined by a point-charge calculation, whereas the ab initio approach was successful for the 160-atom unit cell of the ${\mathrm{Mg}}_{32}{(\mathrm{Al},\mathrm{Zn})}_{49}$ Bergman phase. For the 258-atom unit cell of ${\ensuremath{\xi}}^{\ensuremath{'}}\text{\ensuremath{-}}\mathrm{Al}\text{\ensuremath{-}}\mathrm{Pd}\text{\ensuremath{-}}\mathrm{Mn}$, the experimental rotation patterns revealed a pseudotenfold symmetry, whereas the theoretical point-charge calculation revealed predominant twofold symmetry with traces of tenfold symmetry, so that no quantitative matching between the theory and experiment could be obtained.