Recently, researchers from 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 TiFe1.5Sb and MCo1.33Sn S-P semiconductors.
The researchers further explained the reason behind the unique properties of these compounds.
Delving deeper, the researchers explained the unique properties of these compounds. Beyond the recognized HH and FH local geometries, these S-P structures incorporate 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. An intriguing consequence of this is the formation of second- and third-order Rubik’s cube patterns in TiFe1.5Sb and MCo1.33Sn, attributed to the regular stacking of these substructures. This unique arrangement is key in redistributing 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 lattice thermal conductivity. As a result, these materials exhibit lower thermal conductivities compared to traditional HH and FH compounds. Notably, the calculated zT value of p-type ZrCo1.33Sn reaches 0.54 at 1000K, thanks to its high-power factor and low thermal conductivity.
"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 TiFe1.5Sb and MCo1.33Sn crystal structures and the arrangement of substructures. (Image by TI Zhuoyang)
Fig 2. (a, b) Atom-resolved density of states (DOS) and crystal orbital Hamiltonian population (COHP) of TiFe1.5Sb. (c, d) Schematic illustration of molecular orbital (MO) diagram in forming TiFe1.5Sb. (Image by TI Zhuoyang)
