Half-Heusler alloys

S. Joseph Poon, Electronic and Thermoelectric Properties of Half-Heusler Alloys Terry M. Tritt, A. L. Pope, and J. W. Kolis, Overview of the Thermoelectric Properties of Quasicrystalline Materials and Their Potential for Thermoelectric Applications Alexander C. Ehrlich and Stuart A. Wolf, Military Applications of Enhanced Thermoelectrics David J. Singh, Theoretical and Computational Approaches for Identifying and Optimizing Novel Thermoelectric Materials... 

Kohler, J. Deng, S. Lee, C. Whangbo, M. -H. Inorg. Chem. 2007,46, 1957, and references therein. Fot a discussion regarding the bandgap of half-Heusler alloys, see Kohler, J. Deng, S. Inorg. Chem. 2007, 46, 1957, and references therein. 

Heusler alloys are intermetallics with the general formula M2NiSn (M = Zr, Ti), with the unit cell being a NaCl lattice of two metals, with the third metal occupying tetrahedral interstitial sites. In contrast, half-Heusler alloys have the structure MNiSn, with the third metal occupying only one-half of the interstitial sites. With this in mind, would Heusler or half-Heusler alloys be a mrae efficient thermoelectric material Explain your choice. 

Makongo, J. P. A., Misra, D. K., Zhou, X. Y., Pant, A., Shabetai, M. R., Su, X. L., et al. (2011). Simultaneous large enhancements in thermopower and electrical conductivity of bulk nanostractured half-Heusler alloys. Journal of the American Chemical Society, 133, 18843-18852. 

Chemical stability indicates that in the cubic, metallic perovskites the interstitial C and N are probably neutral. They represent, therefore, an M atom with half filled p or s-p orbitals, and in the cubic structure the metal-M-metal interaction is defined by Figure 86. [This is to be contrasted with low-temperature CrN, which has considerable ionic character and an ordering of its covalent character along a given axis.] In contrast to the M atoms of the Heusler alloys, the p electrons of C and N correlate with, and therefore spin pair, the near-neighbor eg electrons. The metal- -metal interactions are determined by the Ug electron-spin correlations since Hu 2.76 A < Rc. 

Heusler alloys have a rich variety of apphcations, owing to some of their unique properties. Some of these phases are half-metallic ferromagnets, exhibiting semiconductor properties for the majority-spin electrons and normal metallic behavior for the minority-spin electrons. Therefore, the conduction electrons are completely polarized. The Ni2MnGa phase is used as a magnetic shape memory alloy and single crystals of Cu2MnAl are used to produce monochromatic beams of polarized neutrons. 

The surface of a half-metallic semi-Heusler alloy NiMnSb 001 ... 

Despite their long pedigree, however, the ewent interest in the Heusler and semi-Heusler alloys was only sparked in 1983 upon the publication of a theoretical paper by de Groot et al [125] in which the prototypical semi-Heusler alloy NiMnSb was predicted to display half-metallic properties. The concept of a half-metal needs to be carefully distinguished from the similar-sounding but entirely different conc ts of Ae semi-metal and the semiconductor. The property of half-metallicity is inextricably linked with the permanent magnetic polarisation of a material, and refers to the case when the electrons at the Fermi level display 100% spin-polarisation. In other words, while the electronic bands of one spin-species cross the Fermi level, those of the other spin-species do not. In the case of half-metallic semi-Heusler alloys, the majority-spin electrons behave as if the material were a good metal, while the minority-spin electrons behave as if the material were an insulator or semiconductor. 

Subsequent to the initial work of de Groot et al [125] on NiMnSb, a number of other semi-Heusler alloys have also been predicted to be half-metallic. Their potential in spintronic applications cannot be overstressed. In effect, semi-Heusler alloys could one day be employed as perfect spin-valves, for injection of polarised clarge carriers into semiconductor devices. Before such potential can be realised, however, it will be necessary to understand in detail die electronic... 

It is fair to say that the surface science of the half-metal semi-Heusler alloys is in its infancy. The majority of studies on these materials have focussed upon the bulk properties, and surface effects have been viewed largely as unwanted complications. Nevertheless, a small amount of experimental data on the electronic nature of the NiMnSb surface is now available, and DFT calculations have a considerable role to play, both in interpreting the results and in suggesting routes for further investigation. 

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 ... 

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. 

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