Atomic structure of the (Al,Si)CuFe cubic approximant phase

Area of Science:

• Materials Science
• Crystallography
• Solid-State Physics

Background:

• Icosahedral quasicrystals and their approximant phases are complex metallic alloys with unique atomic structures.
• Understanding the relationship between ideal quasicrystalline structures and their crystalline approximants is crucial for materials design.

Purpose of the Study:

• To determine the crystal structure of the alpha-(Al,Si)CuFe approximant phase using single-crystal X-ray diffraction.
• To compare the local atomic environments and structural features of the alpha-(Al,Si)CuFe approximant phase with the parent icosahedral phase.

Main Methods:

• Single-crystal X-ray diffraction was employed to elucidate the precise atomic arrangement within the alpha-(Al,Si)CuFe approximant phase.
• The determined structure was compared with the ideal structure derived from the perpendicular shear method of the icosahedral phase.

Main Results:

• The local atomic environments, characterized by atomic clusters, were found to be similar between the approximant and icosahedral phases.
• These local environments exhibited significant expansion beyond the unit cell dimensions of the approximant.
• A specific Al(2) orbit, characteristic of the icosahedral phase's Mackay-type cluster, was not identified in the approximant. Instead, a partially occupied inner icosahedron with a large Debye-Waller factor was observed.

Conclusions:

• The alpha-(Al,Si)CuFe approximant phase shares similarities in local atomic clusters with the parent icosahedral phase.
• Structural differences, particularly the absence of the Al(2) orbit and the presence of a disordered inner icosahedron, distinguish the approximant structure.