Structural variants and the modified Slater-Pauling curve for transition-metal-based half-Heusler alloys

We compare the physical differences between two atomic configurations, found in the literature, of the half-Heusler alloys-XMnY, where X is a transition-metal element and Y is a nonmetallic element. The structural differences arise from the placement of the X and Y atoms and the vacancy within the full-Heusler (L2(1)) structure. In one configuration, Y and Mn are nearest neighbors and the vacancy is at (1/4,3/4,1/4)a (4d) while in the other configuration, X and Mn are nearest neighbors and the vacancy is located at (0,0,1/2)a (4b), where a is the lattice constant of face-centered cube. We suggest that the important difference between the two configurations is the identity of the transition-metal element nearest to the non-metal element. Physical properties, in particular the bonding features, reflect this difference. The general validity of the modified Slater-Pauling curve, which gives successful zeroth-order prediction of the magnetic moments of many half-Heusler alloys including CrMnSb in the second configuration, is tested in the six XMnY alloys studied here. Their calculated moments obey the predictions by the modified Slater-Pauling curve for positive moments only. Exceptions appear for predictions with negative moments. A simple and plausible physical reason is provided. Furthermore, the possibility of a non-ferromagnetic phase is examined and the energetics between the ferromagnetic and non-ferromagnetic phases are compared. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4788825]