Unravelling Local Atomic Order of the Anionic Sublattice in M(Al_1−xGa_x)₄ with M=Sr and Ba by Using NMR Spectroscopy and Quantum Mechanical Modelling

Abstract

The quasibinary section of the intermetallic phases MAl and MGa with M=Sr and Ba have been characterised by means of X-ray diffraction (XRD) studies and differential thermal analysis. The binary phases show complete miscibility and form solid solutions M(Al Ga ) with M=Sr and Ba. These structures crystallise in the BaAl structure type with four- and five-bonded Al and/or Ga atoms (denoted as Al(4b), Al(5b), Ga(4b), and Ga(5b), respectively) that form a polyanionic Al/Ga sublattice. Solid state Al NMR spectroscopic analysis and quantum mechanical (QM) calculations were applied to study the bonding of the Al centres and the influence of Al/Ga substitution, especially in the regimes with low degrees of substitution. M(Al Ga ) with M=Sr and Ba and 0.925≤x≤0.975 can be described as a matrix of the binary majority compound in which a low amount of the Ga atoms has been substituted by Al atoms. In good agreement with the QM calculations, Al NMR investigations and single crystal XRD studies prove a preferred occupancy of Al(4b) for these substitution regimes. Furthermore, two different local Al environments were found, namely isolated Al(4b1) atoms and Al(4b2), due to the formation of Al(4b)-Al(4b) pairs besides isolated Al(4b) atoms within the polyanionic sublattice. QM calculations of the electric field gradient (EFG) using superlattice structures under periodic boundary conditions are in good agreement with the NMR spectroscopic results.

Making and Breaking Bonds in Superconducting SrAl4–xSix(0 ≤x≤ 2)

“…To provide insight into the ordering and site preference of the network atoms, 27 Al and 29 Si NMR spectroscopy of SrAl 4−x Si x samples was performed using the magic-angle spinning (MAS) technique, as shown in Figure 3a. The 27 Al results for SrAl 4 are consistent with a previous report by Pecher et al, 33,34 in which the lowerfrequency signal was identified as belonging to five-bond Al2 (4e site) and the high-frequency signal to four-bond Al1 (4d site). In contrast, the spectrum of tI10-SrAl 2 Si 2 exhibits a single, broader signal with a nonspecific bell-shaped satellite background.…”

Structural Characterization of Intermetallic Compounds by 27Al Solid State NMR Spectroscopy

“…Table illustrates excellent agreement between experimental and theoretical values of C Q and η Q. Site differentiation and characterization based on 27 Al quadrupolar coupling parameters has also been used in earlier work. ,,,,,, …”

“…Site differentiation and characterization based on 27 Al quadrupolar coupling parameters has also been used in earlier work. 21,23,24,26,29,31,58 ■ STRUCTURAL DISORDER AND MIXED SITE OCCUPANCIES Order−disorder phenomena have been investigated for compounds of the ScTAl series (T being a transition metal). 38 In ScPtAl (TiNiSi type) and ScAuAl (HfRhSn type), the Al atoms are exclusively positioned on the 4c/6g Wyckoff sites, giving rise to well-defined 27 Al NMR spectra.…”

Equiatomic AEAuX (AE=Ca−Ba, X=Al−In) Intermetallics: A Systematic Study of their Electronic Structure and Spectroscopic Properties

“…The larger anisotropy of the charge distributions, observed for the gallium compounds compared to the aluminium ones in turn is due to the higher polarizability of the heavier element. A similar trend was also observed for the earth metal elements Al, Ga and In in various compounds crystallising in the BaAl 4 ‐type structure …”