Recently, researchers from Institute of Solid State Physics, Hefei Institutes of Physical Science (HFIPS) of Chinese Academy of Sciences (CAS) designed Slater-Pauling (S-P) Heusler materials with a unique structure resembling a Rubik's cube. These materials showed potential in thermoelectric applications due to their semiconductor-like properties.

"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," said TI Zhuoyang, first author of the paper, "we successfully explained the underlying reasons for this phenomena in this study."

These findings have been published in Physical Review B.

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.

In this research, 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 TiFe_1.5Sb and MCo_1.33Sn S-P semiconductors.

The researchers further explained the reason behind the unique properties of these compounds.

In addition to the well-known HH and FH local geometries, these S-P structures contain the defective-HH (DH) and defective-FH (DF) substructures, due to the partial occupation of Y atoms (Fe or Co) at the 4d Wyckoff site. The regular stacking of these substructures gives rise to the intriguing formation of second- and third-order Rubik's cubes in TiFe_1.5Sb and MCo_1.33Sn. This unique stacking plays a crucial role in redistributing electrons in the lattice to form a bandgap, lowering the phonon Debye temperature, and enhancing anharmonic vibrations to suppress lattice thermal conductivity, resulting in lower thermal conductivities than traditional HH and FH compounds. Owing to the high-power factor and low thermal conductivity, the calculated zT value of p-type ZrCo_1.33Sn can reach 0.54 at 1000K.

"Our research foresees unique S-P Heusler semiconductors with exceptional thermoelectric capabilities and clarifies the physical mechanism driving their emergence," said TI Zhuoyang.

Fig 1. Theoretically predicted TiFe_1.5Sb and MCo_1.33Sn crystal structures and the arrangement of substructures. (Image by TI Zhuoyang)