Research unveils Rubik's cube-like Heusler materials with potential for thermoelectric applications

Researchers from the Hefei Institutes of Physical Science of the Chinese Academy of Sciences have designed Slater-Pauling (S-P) Heusler materials with a unique structure resembling a Rubik's cube. These materials exhibit semiconductor-like properties and have potential in thermoelectric applications. In traditional semiconductor Heusler alloys, the number of valence electrons follows a specific rule. However, these S-P Heusler compounds defy this rule while still displaying semiconductor behavior. In this study, the team focused on two Heusler systems: Ti-Fe-Sb and M-Co-Sn (M = Ti, Zr, Hf). Within these two systems, they predicted the thermodynamically stable TiFe1.5Sb and MCo1.33Sn S-P semiconductors. he researchers explained the unique properties of these compounds. In addition to the known half-Heusler (HH) and full-Heusler (FH) local geometries, these S-P structures contain defective-HH (DH) and defective-FH (DF) substructures. This is due to the partial occupation of Y atoms (Fe or Co) at the 4d Wyckoff site. Some off-stoichiometry Heusler compounds have been predicted to exhibit semiconductor characteristics. However, the bonding behavior in these S-P semiconductors and the relationship between their crystal structure and thermoelectric performance have remained unclear. This unique arrangement is key to the redistribution of electrons within the lattice, leading to the formation of a bandgap. It also reduces the phonon Debye temperature and enhances anharmonic vibrations, which in turn suppress the lattice thermal conductivity. As a result, these materials exhibit lower thermal conductivities compared to conventional HH and FH compounds. In particular, the calculated zT value of p-type ZrCo1.33Sn reaches 0.54 at 1,000 K, thanks to its high-power factor and low thermal conductivity.

        Heusler and half-Heusler compounds are ternary intermetallic compounds with highly tunable magnetic, topological, multiferroic, and electronic properties. They share a common crystal structure with more than 1000 members and compositions spanning most of the periodic table. 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. 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. 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.

        In condensed matter physics, the Slater–Pauling rule states that adding a element to a metal alloy will be reduce the alloy's saturation magnetization by an amount proportional to the number of valence electrons outside of the added element's d shell. Conversely, elements with a partially filled d shell will increase the magnetic moment by an amount proportional to number of missing electrons. Investigated by the physicists John C. Slater and Linus Pauling in the 1930s, the rule is a useful approximation for the magnetic properties of many transition metals.