Theoretical and ²⁷Al NMR Spectroscopic Investigations of Binary Intermetallic Alkaline-Earth Aluminides

Abstract

The binary alkaline-earth aluminides AEAl2 (AE = Ca and Sr) and AEAl4 (AE = Ca–Ba) have been synthesized from the elements and investigated via powder X-ray diffraction experiments. CaAl2 adopts the cubic MgCu2-type structure (Fd3̅m), while SrAl2 crystallizes in the orthorhombic KHg2-type (Imma). LT-CaAl4 crystallizes with the monoclinic CaGa4-type (C2/m), while HT-CaAl4, SrAl4, and BaAl4 adopt the tetragonal BaAl4-type structure (I4/mmm). The close structural relation of the two CaAl4 polymorphs was established using a group–subgroup relation in the Bärnighausen formalism. In addition to the room-temperature and normal pressure phase of SrAl2, a high-pressure/high-temperature phase has been prepared using multianvil techniques, and its structural and spectroscopic parameters were determined. Elemental analysis by inductively coupled plasma mass spectrometry showed that no significant impurities with other elements besides the weighed ones are present and the chemical compositions match the synthesized ones. The title compounds have been furthermore investigated by 27Al solid-state magic angle spinning NMR experiments to validate the crystal structure and to gain information about the influence of the composition on the electron transfer and the NMR characteristics. This has also been investigated from a quantum chemical point of view using Bader charges, while the stabilities of the binary compounds in the three phase diagrams (Ca−Al, Sr−Al and Ba−Al) have been studied by calculations of formation energies per atom.

Raman and NMR spectroscopic and theoretical investigations of the cubic laves-phases REAl2 (RE = Sc, Y, La, Yb, Lu)

“…From the resonance shifts δ gained by fitting the data, the Knight shift contribution δ K can be calculated using the NMR parameters from the DFT calculations as described in the literature. 27,65 Since all other contributions are small, they can be neglected as the Knight part dominates.…”

Formation of the Sub-oxide Sc4Au2O1–x and the Drastically Negative 27Al NMR Shift in Sc2Al

“…The main resonance is observed at δ = −673 ppm, which is significantly different compared to the majority of investigated intermetallic aluminum compounds. For example, the binary alkaline-earth and rare-earth dialuminides exhibit positive resonance shifts of δ = +1091 and +1152 ppm for AE Al 2 ( AE = Ca and Sr) and δ = +213 to +924 ppm for RE Al 2 ( RE = Sc, Y, La, Yb, Lu) . Ternary aluminum intermetallics also exhibit positive shifts, e.g., δ = 440–851 (Sc T 2 Al and Sc­( T , T ′) 2 Al; T = T ’ = Ni, Pd, Pt, Cu, Ag, Au), 762 and 1031 (Ba 3 Pt 4 Al 4 ), 273–636 ppm ( RET 5 Al 2 ; RE = Y, Lu; T = Pd, Pt), or 419–767 ( MT Al 2 ; M = Ca-Ba, Sc, Y, La, Lu; T = Ni, Pd, Pt). , Negative 27 Al NMR shifts, in contrast, are scarce and have, to the best of our knowledge, only been observed for the binary compounds VAl 3 (δ = −170 and −100/–97 and −130 ppm), Fe 4 Al 13 (δ = −175 ppm) and TiAl (δ = −170 ppm).…”

“…The main resonance is observed at δ = −673 ppm, which is significantly different compared to the majority of investigated intermetallic aluminum compounds. For example, the binary alkaline-earth and rare-earth dialuminides exhibit positive resonance shifts of δ = +1091 and +1152 ppm for AEAl 2 (AE = Ca and Sr) 110 and δ = +213 to +924 ppm for REAl 2 (RE = Sc, Y, La, Yb, Lu). 111 Ternary aluminum intermetallics also exhibit positive shifts, e.g., δ = 440−851 (ScT 2 Al and Sc(T,T′) 2 Al; T = T' = Ni, Pd, Pt, Cu, Ag, Au), 103 762 and 1031 (Ba 3 Pt 4 Al 4 ), 105 273−636 ppm Two competing magnetic screening mechanisms were described by Ferreira: the so-called Knight shift that originates from the hyperfine interaction between the nucleus with the spins of itinerant (conduction) electrons and the hyperfine interaction with local magnetic moments at the transition metal site.…”

Laves phases: superstructures induced by coloring and distortions

“…[78] However, SrAl 2 , Ba 7 A 13 and Ba 21 Al 40 can be transformed into the cubic MgCu 2 type structure using highpressure / high-temperature techniques. [79][80][81][82]…”

“…Reinvestigations of Ba 7 Al 13 by selected area and convergent beam electron diffraction in a TEM revealed a superstructure with space group P 31 m and a composition of Ba 21 Al 40 [78] . However, SrAl 2 , Ba 7 A 13 and Ba 21 Al 40 can be transformed into the cubic MgCu 2 type structure using high‐pressure / high‐temperature techniques [79–82] …”