Recently, the research group led by professor ZHANG Yongsheng from the Institute of Solid State Physics, Hefei Institutes of Physical Science, revealed that thermoelectric performance could be improved by ordering in double Half-Heusler (DHH) materials.

The researchers studied the phase stability of three Cobalt-based DHH systems (TiFe_1-xNi_xSb, ZrFe_1-xNi_xBi and VFe_1-xNi_xGe) through closely entangled combination of density-functional theory and a cluster expansion method. Two stable ground state ordered structures (Ti₄Fe₂Ni₂Sb₄ and V₄Fe₂Ni₂Ge₄) were identified.

After the calculations of electronic and phonon structures, they found that the two ordered structures could maintain the excellent electrical properties of pristine Half-Heusler (HH) compounds but with low thermal conductivity as found experimentally. The p-type (n-type) zT values of Ti₄Fe₂Ni₂Sb₄ and V₄Fe₂Ni₂Ge₄ were predicted to reach 1.75 (0.64) and 1.33 (0.95), respectively.

18-electron HH thermoelectric compounds with semiconducting properties, though extensively studied, remains low in quantity. A way to get the compounds is by mixing 17- and 19-electron HH systems. The mixture could form a solid solution at high temperature which can be used to decrease thermal conductivity but may deteriorate carrier mobility, and an ordered structure at low temperature which could maintain high weighted mobility.

Therefore, it is desirable to discover ordered structures with better thermoelectric properties than solid solutions. Theoretical prediction of the structures and analysis of their thermoelectric properties will greatly speed up the search for new materials.

This work not only suggests that forming ordered structures is an efficient method to maintain the excellent electrical properties of pristine HH compounds, but also provides insights into optimizing and designing promising thermoelectric HH materials.

Fig. 1. DFT + CE predicted ground state lines of the (a) TiFe_1-xNi_xSb, (b) ZrFe_1-xNi_xBi and (c) VFe_1-xNi_xGe systems. (Image by GUO Shuping) 

Fig. 2. Predicted zT values of Ti₄Fe₂Ni₂Sb₄ (lines with circles) and V₄Fe₂Ni₂Ge₄ (lines with triangles). (Image by GUO Shuping)