Half-Heusler compounds

The Zintl compound NaTl feamres a diamond lattice of Tl anions with Na cations in tetrahedral interstitial sites. However, unlike most other Zintl compounds that are insulators, NaTl does not have a bandgap and exhibits metallic conductivity. " Insulating half-Heusler compounds of form AML (where A = Grp. 3 such as Sc, Y M = late transition metal such as Ni, Pd, Au L = heavy main group atom of Grp. 14/15 such as As, Sn, Sb, Pb, Bi) are structurally related to NaTl, with M and L forming a zincblende lattice and A atoms occupying 1/2 of the 10-coordinate sites defined by a M4 tetrahedron and Lg octahedron (Figure 3.32). ... 

Fig. 1.14 a The crystal stmcture of half-Heusler compounds b the possible candidate atoms for A, B, and X, Reprinted from ref. [150], copyright 2012, with permission from Elsevier... 

If half of the nickel atoms in Figure 3.26a are removed, the half-Heusler ABC structure of Figure 3.26b is obtained. In the half-Heusler strucmre, each atom still resides on a cesium-chloride lattice site. The rock-salt component (B and C) remains intact, but the A atoms form a zinc blende lattice with B and C. Examples of compounds with this structure include MnNiSb, AuMgSn, BiMgNi, and RhSnTi. 

Sb, and locate in 4a (0, 0, 0) site. The crystal framework can be regarded as four mutual-linked FCC lattices A and X atoms constitute the NaCl stmcture the B atoms occupy half of the cubic voids, and leave the other half voids empty. It is exacdy the unoccupied voids in the structure that result in the difference between half-Heusler and full-Heusler alloys half-Heusler is semiconductor with band gap while full-Heusler is a metalloid compound without gap. The reason lies in the decreased B-A and B-X bondings, which enlarge the distance between these atoms and weaken the overlap degree of d state wave function, and finally produce the band gap. A half-Heusler unit cell usually has high-symmetry stmcture with many heavy atoms, so that it has excellent electrical performance, large Seebeck coefficient, and high electrical conductivity. 

The structural similarity of MgAgAs-type compounds with Heusler alloys and with the transition-metal based half-metallic ferromagnets (de Groot et al. 1983) has provoked band-structure calculations of UNiSn (Mueller et al. 1987, Albers et al. 1987). Self-consistent-field scalar relativistic spin-polarized calculations neglecting the spin-orbit coupling revealed the following features of the valence band ... 

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