AMn₂Al₂₀ ALLOYS (WHERE A=LANTHANIDE/ACTINIDE/RARE EARTH ELEMENTS)

A-T-Al aluminides (where A = actinide/lanthanide/rare earth elements and T=transition metal) were intensively studied due to potential heavy fermion properties. Normally, these researches are performed using “trial and error” approach, which could be time consuming. It would be of clear benefit to formulate a rule that could predict the relative stability of the structures that may form in the ternary Al-richest phases in the A-T-Al systems. Although A-T-Al family contains hundreds of phases, they can be classified into few series of phases with isotypical structures. Al richest are: tetragonal AT_xAl_12-x (ThMn₁₂ type), tetragonal AT₂Al₁₀ (CaCr₂Al₁₀ type), orthorhombic AT₂Al₁₀ (YbFe₂Al₁₀ type) and cubic AT₂Al₂₀ (CeCr₂Al₂₀ type). Cubic AT₂Al₂₀ phases were chosen to be a focus of current research since it was documented that in some systems these phases exhibited superconducting properties. Following study of the information found in the crystallographic database, it can be concluded that the cubic structure is formed with T=Ti-Cr for almost all A atom types. In T=Mn, breakage of the symmetry occurs, and different structures crystallize with types listed above. We aim to understand the influence of A atom type on the formation of the stable structures. The work was performed systematically, investigating several AMn₂Al₂₀ alloys, with different electronic structures, both experimentally and by Density Functional theory (DFT) calculations. It was found that in the AMn₂Al₂₀ alloys - tetragonal CaCr₂Al₁₀-type structure is the stable one when A=Y, Gd and Th. However, when A=U cubic CeCr₂Al₂₀-type structure crystallizes. Theoretical and experimental results were found to be in perfect agreement. It was concluded that different behavior of the 4f and 5f-shell electrons of the heavy atoms determines the energetically favorable structure.