M-Al-Mgroups trapped in cages ofAl13M4(M=Co, Fe, Ni, Ru)complex intermetallic phases as seen via NMR

Abstract

The crystallographic structures of decagonal quasicrystals and their periodic approximants are traditionally described as a periodic stacking of atomic planes. By performing a $^{27}\text{A}\text{l}$ NMR spectroscopic study of the ${\text{Al}}_{13}{M}_{4}$ ($M=\text{transition}$ metal) family of four-layer decagonal approximants, including the orthorhombic ${\text{o-Al}}_{13}{\text{Co}}_{4}$, the monoclinic ${\text{Al}}_{13}{\text{Fe}}_{4}$, its ternary derivative ${\text{Al}}_{13}{(\text{Fe},\text{Ni})}_{4}$, and the monoclinic ${\text{Al}}_{13}{\text{Ru}}_{4}$, we show that all these phases contain structural detail of a nearly linear $M\text{-Al-}M$ atomic group trapped inside an elongated cage, resembling the three-dimensional (3D) ``cage-compound'' structure of the intermetallic clathrates. We determined the electric-field-gradient- (EFG) and the magnetic-shielding tensors at the Al site of the $M\text{-Al-}M$ groups. The asymmetry parameter of the EFG tensor was estimated theoretically by a point-charge model, taking into account the charges of both the $M\text{-Al-}M$ atoms and the surrounding cage atoms. The calculations support ionic bonding of the $M\text{-Al-}M$ group to the cage atoms and the existence of a 3D chemical bonding network in the ${\text{Al}}_{13}{M}_{4}$ phases. The above results show that the traditional description of the ${\text{Al}}_{13}{M}_{4}$ decagonal approximant phases in terms of two-dimensional (2D) atomic layers stacked along the pseudotenfold crystallographic direction is a convenient geometrical approach to describe their complex structures but is not appropriate for the description of their physical properties, which should be analyzed by taking into account the full 3D nature of the chemical bonding framework. This favors the 3D cage-compound structural description of the ${\text{Al}}_{13}{M}_{4}$ phases over the pseudo-2D stacked-layer description.