Half-metallicity versus symmetry in half-Heusler alloys based on Pt, Ni, and Co: An ab initio study

Using first-principles calculations based on density functional theory, we study the geometric, electronic, and magnetic properties of Pt-, Ni-, and Co-based half-Heusler alloys, namely, PtBC, NiBC, and CoBC (B = Cr, Mn, and Fe; C = Al, Si, P, S, Ga, Ge, As, Se, In, Sn, Sb, and Te). We calculate the formation energy of these alloys in various crystal symmetries, which include the cubic C1(b) (F (4) over bar 3m), orthorhombic (Pnma), as well as hexagonal (P (6) over bar 2m and P6(3)/mmc) structures. It has been observed that, out of the alloys studied, only 18 are energetically stable in the cubic lowest energy structure. These alloys primarily have either a C atom or an A atom (Pt, Ni, or Co) with a high atomic number. We also observe that, along with the alloys with C atoms from groups IIIA, IVA, and VA, alloys with C atoms from group VIA are also found to be, by and large, energetically stable. Furthermore, among the energetically stable cubic alloys, only CoMnSb, CoMnTe, and NiMnSb show the half-metallic property. Under volume-conserving tetragonal distortion, the half-metallic property is completely destroyed in CoMnSb, whereas CoMnTe and NiMnSb maintain half-metallicity for a c/a value from 0.8 to 1.2 for CoMnTe and from 1.0 to 1.2 for NiMnSb, respectively. From the theoretical calculations, many of the half-Heusler alloys are reported to be half-metals in the cubic phase, but these are synthesized in noncubic structures. We analyze the magnetic moment, the electronic density of states, and the spin polarization at the Fermi level, in detail, to find whether a material in the noncubic lowest energy structure exhibits half-metallicity or not. Based on these analyses, the possibility of existence of any one-to-one relationship between the cubic symmetry and the half-metallicity in these half-Heusler alloys is explored. We predict about the existence of a new noncubic half-Heusler alloy with a substantially low density of states at one of the spin channels and subsequently a reasonably high spin polarization at the Fermi level. However, it is found that for alloys in the lowest energy structure, cubic symmetry is necessary for 100% spin polarization. Furthermore, for Pt-based alloys, the effects of spin-orbit coupling (SOC) have been explored. It is observed that inclusion of SOC in the Pt-based alloys does not play a crucial role in either the geometric or the electronic structure consideration.