Reimagining Thermoelectrics: The Rubik’s Cube Structure Unlocks Heusler Potential

Researchers have developed Slater-Pauling (S-P) Heusler materials, resembling a Rubik’s cube structure, with promising thermoelectric applications. These materials defy conventional semiconductor rules while maintaining semiconductor behavior. Credit: Chinese Academy of Sciences

Scientists have created unique Slater-Pauling Heusler materials with semiconductor properties, offering significant potential in thermoelectric applications. Their research reveals these materials’ unique electron redistribution and thermal properties.

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.

Unique Semiconductor Behavior

“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 Zhuoyang Ti, 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.

Figure 1. Theoretically predicted TiFe1.5Sb and MCo1.33Sn crystal structures and the arrangement of substructures. Credit: Zhuoyang Ti

Exploration of Off-Stoichiometry Heusler Compounds

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-Depth Research on Heusler Systems

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.

Figure 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. Credit: Zhuoyang Ti

Understanding the Unique Properties

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 TiFe_1.5Sb and MCo_1.33Sn, 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 ZrCo_1.33Sn reaches 0.54 at 1000K, thanks to its high-power factor and low thermal conductivity.

Conclusion and Potential Impact

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

Reference: “Bonding properties of Rubik’s-cube-like Slater-Pauling Heusler semiconductors for thermoelectrics” by Zhuoyang Ti, Jianbo Zhu, Shuping Guo, Jingyu Li, Xiaobing Liu and Yongsheng Zhang, 15 November 2023, Physical Review B.
DOI: 10.1103/PhysRevB.108.195203