Structure and peculiarities of bonding in the Al-rich A-Mn-Al alloys (where A=Y, Gd, Th and U)

A-T-Al family of alloys (where A = actinides, lanthanides or rare earth elements; T = transitional metals) has unique physical properties. AT₂Al₂₀ phases are of particular importance since they have exhibited superconductivity properties. Understanding the reasons for the formation of the AT₂Al₂₀ structures in various A-T-Al alloys is the objective of current study, since it will allow formulation of rule for prediction of stable structures in these alloys. Such rule will shorten the research time because usually the search for these phases (although is based on some criteria) performed by serendipity. Formerly we proposed that type of transition metal dictates the symmetry of the ternary phases’ structure formed in the AT₂Al₂₀ systems, while symmetry breakage point was found to be in T = Mn. Here, AMn₂Al₂₀ alloys were studied with a purpose to understand the influence of A type atoms on the stable ternary aluminide formed. Despite adequate amount of Al provided in these alloys, cubic AMn₂Al₂₀ phase did not formed in any of the studied compounds besides A = U. The results of Density Functional Theory calculations were in excellent agreement with the experimental ones. It was found that proposed earlier rule for the prediction of the stable structure in these systems using the minimal magnetic moment of the transition metal works as long as A type atoms do not have 5f-electrons. Otherwise, the stable structure in these systems can be assessed by the minimal average energy of the transitional metal 3d-electrons participating in bonding.