Abstract
The solid-state structures of Aluminum-Transition Metal (Al<sub>3</sub>TM) Alloys (TM = Ti, Sc, Mn, Fe), have been explored in detail using computational electron density methods. Topological analysis of the electron density in Al<sub>3</sub>Ti alloys revealed two interesting types of interactions between the atoms in the layered Al<sub>3</sub>Ti alloy structure lattice; viz. Al-Al and Al-Ti interactions. Two types of both Al-Al interactions and Al-Ti interactions were observed between the atoms in the same plane (2.725 Å apart), and those on adjacent planes or layers (2.884 Å apart). All interactions in the same plane of the Al<sub>3</sub>Ti alloy are stronger than those on adjacent planes, owing to the high values of the electron density and the bond distances between the interacting atoms (Table 1). The Laplacian of the electron density in a plane of the alloy indicates that Ti atoms are polarized towards the Al-atoms thereby transferring most of their valence electron density to Al (Figure 2b). Similar interactions were observed in Al<sub>3</sub>Sc alloy but these interactions were weaker than those in Al<sub>3</sub>Ti alloy. This signifies that Ti and Al atoms in the alloy are interchangeable or completely miscible, and indicates an increased stability of the alloy compared to Al<sub>3</sub>Sc alloy. In addition, analysis of Al<sub>3</sub>Mn and Al<sub>3</sub>Fe alloys indicate that these alloys were more stable compared to Al<sub>3</sub>Ti and Al<sub>3</sub>Sc alloys. Furthermore, the thermodynamic studies of Al<sub>3</sub>TM alloys (TM = Ti, Sc, Mn, Fe) were investigated. The results confirm the conclusion that Al<sub>3</sub>Ti and other Al-based transition metal alloys play little or no active role in the reversible re/dehydrogenation of Ti-doped NaAlH<sub>4</sub>. These alloys are thermodynamically stable.
Highlights
Atomic packing is of great importance in classifying the different types of interactions within crystals and alloys [1] These crystals may be finite or extend indefinitely in one, two, or three dimensions and stabilized by either ionic, covalent, metallic, Van der Waals or hydrogen bonds
Al-Mn and Al-Fe interactions in Al3Mn and Al3Fe are stronger than the Al-Ti and Al-Sc interactions in the Al3Ti and Al3Sc alloys
The Laplacian of the electron density in a plane of the alloy indicates that the Ti atoms were polarized towards the Al-atoms, thereby transferring most of their valence electron density to Al
Summary
Atomic packing is of great importance in classifying the different types of interactions (bonding) within crystals and alloys (complexes) [1] These crystals may be finite or extend indefinitely in one, two, or three dimensions and stabilized by either ionic, covalent, metallic, Van der Waals or hydrogen bonds. Numerous intermediates have to be recognized, and this makes classification of crystals based on bond types to be complicated and incomprehensive. It is necessary investigate, analyze and discuss the nature and types of bonds in crystals and alloys without having prejudged the issue by classifying them as ionic, for example [1]. The ways in which atoms are arranged and interact with one another determines the
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