Heusler compound

Heusler compounds are magnetic intermetallics with face-centered cubic or body-centered tetragonal 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. The term derives from the name of German mining engineer and chemist Friedrich Heusler, who studied such a compound (Cu2MnAl) in 1903. 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. Their magnetism results from a double-exchange mechanism between neighboring magnetic ions. Manganese, which sits at the body centers of the cubic structure, was the magnetic ion in the first Heusler compound discovered. (See the Bethe–Slater curve for details of why this happens.)

Styles of writing chemical formula Depending on the field of literature being surveyed, one might encounter the same compound referred to with different chemical formulas. An example of the most common difference is X2YZ versus XY2Z, where the labels of the two transition metals X and Y in the compound are swapped. The traditional convention X2YZ arises from the interpretation of Heuslers as intermetallics and is used predominantly in literature studying magnetic applications of Heuslers compounds. The XY2Z convention on the other hand is used mostly in thermoelectric materials and transparent conducting applications 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 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 one might find both styles being used. In the present article semiconducting compounds might sometimes be mentioned in the XY2Z style.

"Off-stoichiometric" Heuslers

Although traditionally thought to form at compositions XYZ and X2YZ, studies published after 2015 have discovered and reliably predicted Heusler compounds with atypical compositions such as XY0.8Z and X1.5YZ. Besides these ternary compositions, quaternary Heusler compositions called the double Half-Heusler X2YY'Z2 (e.g. Ti2FeNiSb2) and triple Half-Heusler X2X'Y3Z3 (for e.g. Mg2VNi3Sb3) have also been discovered. These "off-stoichiometric" (that is, differing from the well-known XYZ and X2YZ compositions) Heuslers are mostly semiconductors in the low temperature T = 0 K limit. The stable compositions and corresponding electrical properties for these compounds can be quite sensitive to temperature and their order-disorder transition temperatures often occur below room-temperatures. 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. 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.

Half-Heusler thermoelectrics

The half-Heusler compounds have distinctive properties and high tunability which makes the class very promising as thermoelectric materials. 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. 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). This flexible range of element selection allows many different combinations to form a half-Heusler phase and enables a diverse range of material properties. Half-Heusler thermoelectric materials have distinct advantages over many other thermoelectric materials; low toxicity, inexpensive element, robust mechanical properties, and high thermal stability make half-Heusler thermoelectrics an excellent option for mid-high temperature application. However, the high thermal conductivity, which is intrinsic to highly symmetric HH structure, has made HH thermoelectric generally less efficient than other classes of TE materials. Many studies have focused on improving HH thermoelectric by reducing the lattice thermal conductivity and zT > 1 has been repeatedly recorded.

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