Titanium embedded cage structure formation in Al(n)Ti+ clusters and their interaction with Ar.

Area of Science:

• Physical Chemistry
• Computational Materials Science
• Atomic and Molecular Physics

Background:

• Argon (Ar) physisorption is a key experimental technique for probing the structural location of dopant atoms within aluminum cluster cations (Al(n)Ti(+)).
• The transition from exohedral (dopant on surface) to endohedral (dopant inside cage) structures is crucial for understanding cluster properties.
• Previous experimental results indicated a critical size (nc) beyond which Ar complexes were not observed, suggesting internal dopant location.

Purpose of the Study:

• To computationally investigate the structural and electronic properties of titanium-doped aluminum cluster cations (Al(n)Ti(+)) for n=16-21.
• To elucidate the factors governing the formation of endohedrally doped Al(n)Ti(+) clusters.
• To study the adsorption behavior of Ar atoms on these clusters and validate experimental findings.

Main Methods:

• Density Functional Theory (DFT) calculations were employed to determine structural isomers, relative energies, and electronic properties.
• The adsorption energies and preferred sites of Ar atoms on Al(n)Ti(+) clusters were calculated.
• Comparison of calculated Ar adsorption energies with experimental values was performed.

Main Results:

• The critical size for endohedral encapsulation of Ti in Al(n)Ti(+) was found to be nc=21, consistent with experimental observations of an exohedral-endohedral transition.
• At nc=21, the cluster structure shifts from a non-crystalline icosahedral pattern to a face-centered cubic (fcc)-like endohedral structure.
• Argon adsorption occurs on the exohedral Ti atom for n<21, but shifts to an Al atom with low adsorption energy at n=21, indicating the Ti atom is encapsulated.

Conclusions:

• The critical size for endohedral Ti doping in cationic Al(n)Ti(+) clusters is nc=21, differing from nc=20 for neutral clusters, highlighting the role of net charge.
• The drastic decrease in Ar binding energy and increase in Ar-cluster distance at nc=21 strongly support Ar physisorption on the exterior, consistent with experimental assumptions.
• The stability of Al(n)Ti(+) cluster geometries upon Ar adsorption validates the experimental technique for structural determination.