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

Li-based half-Heusler alloys have attracted much attention due to their potential applications in optoelectronics and because they carry the possibility of exhibiting large magnetic moments for spintronic applications. Due to their similarities to metastable zinc blende half-metals, the halfHeusler alloys β-LiMnZ (Z = N, P and Si) were systematically examined for their electric, magnetic and stability properties at optimized lattice constants and strained lattice constants that exhibit half-metallic properties. Other phases of the half-Heusler structure (α and γ) are also reported here, but they are unlikely to be grown. The magnetic moments of these stable Li-based alloys are expected to reach as high as 4 μ B per unit cell when Z = Si and 5 μ B per unit cell when Z = N and P, however the antiferromagnetic spin configuration is energetically favored when Z is a pnictogen. β-LiMnSi at a lattice constant 14% larger than its equilibrium lattice constant is a promising half-metal for spintronic applications due to its large magnetic moment and vibrational stability. The modified Slater-Pauling rule for these alloys is determined. Finally, a plausible method for developing halfmetallic LixMnZ at equilibrium, by tuning x, is investigated, but, unlike tetragonalization, this type of alloying introduces local structural changes that destroy the half-metallicity.

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