Heusler compound

Heusler compounds are magnetic intermetallics with face-centered cubic crystal structure and a composition of XYZ (half-Heuslers) or X2YZ (full-Heuslers), where X and Y are transition metals and Z is in the p-block.

[1] Many of these compounds exhibit properties relevant to spintronics, such as magnetoresistance, variations of the Hall effect, ferro-, antiferro-, and ferrimagnetism, half- and semimetallicity, semiconductivity with spin filter ability, superconductivity, topological band structure and are actively studied as thermoelectric materials.

The XY2Z convention on the other hand is used mostly in thermoelectric materials[3] and transparent conducting applications [4] literature where semiconducting Heuslers (most half-Heuslers are semiconductors) are used.

This convention, in which the left-most element on the periodic table comes first, uses the Zintl interpretation[5] of semiconducting compounds where the chemical formula XY2Z is written in order of increasing electronegativity.

In well-known compounds such as Fe2VAl which were historically thought of as metallic (semi-metallic) but were more recently shown to be small-gap semiconductors[6] one might find both styles being used.

[10] Large amounts of defects at the atomic scale in off-stoichiometric Heuslers helps them achieve very low thermal conductivities and make them favorable for thermoelectric applications.

[13][14] The X1.5YZ semiconducting composition is stabilized by the transition metal X playing a dual role (electron donor as well as acceptor) in the structure.

A study has predicted that there can be as many as 481 stable half-Heusler compounds using high-throughput ab initio calculation combine with machine learning techniques.

[16] The particular half-Heusler compounds of interest as thermoelectric materials (space group ) are the semiconducting ternary compounds with a general formula XYZ where X is a more electropositive transition metal (such as Ti or Zr), Y is a less electropositive transition metal (such Ni or Co), and Z is heavy main group element (such as Sn or Sb).

[29] Neutron diffraction and other techniques have shown that a magnetic moment of around 3.7 Bohr magnetons resides almost solely on the manganese atoms.

[32] These antiferromagnetic layers completely supersede the normal magnetic domain structure and stay with the APBs if they are grown by annealing the alloy.

[34] In fact, the commercialization of these compounds is limited by the material's ability to undergo intense, repetitive thermal cycling and resist cracking from vibrations.

[34] A collection of various density functional theory (DFT) calculations show that half-Heusler compounds are predicted to have a lower elastic, shear, and bulk modulus than in quaternary-, full-, and inverse-Hausler alloys.

[38] However, the addition of Indium to the Ni-Mn-Sn ternary alloy not only increases the porosity of the samples, but it also reduces the compressive strength to 500 MPa.

Note that this is opposite to the outcome expected from solid solution strengthening, where adding indium to the ternary system slows dislocation movement through dislocation-solute interaction and subsequently increases the material's strength.

In the case of the full Heusler compounds with formula X 2 YZ (e.g., Co 2 MnSi) two of them are occupied by X atoms (L2 1 structure); for the half-Heusler compounds XYZ one fcc sublattice remains unoccupied (C1 b structure).
Phase diagrams sketches demonstrating how Double and Triple Half-Heusler compositions are different from traditional alloy compositions. [ 7 ]
A schematic of a HH thermoelectric. X and Z have a larger electronegativity difference between them and form an NaCl-type ionic sublattice while Y and Z form a ZnS-type covalent sublattice
Electron microscope images of Cu-Mn-Al Heusler compound showing magnetic domain walls tied to APB's (a) L2 1 antiphase boundaries by <111> dark-field imaging - the remaining micrographs are in bright-field so that the APB's are not in contrast (b) magnetic domains by Foucault (displaced aperture) imaging, and (c) magnetic domain walls by Fresnel (defocus) imaging.