Half-Heusler Compounds With 18 Electrons:Thermoelectric Properties And Novel Material Predictions

Thermoelectric materials have long been an attractive solution in pursuing sustainable energy generation by recovering the waste heat to electric power.Among all of the state-of-the-art TE materials,the 18-electron half-Heusler(HH)compounds with semiconducting properties are promising candidates for high-temperature applications,which is attributed to the excellent power factors,mechanical robustness,thermal stability,tunable properties,and reasonable cost.However,the high intrinsic thermal conductivity(>15 W m-1 K-1)in HH compounds hinders their widespread application.Therefore,understanding the electrical and thermal mechanisms of currently well-studied 18-electron HH thermoelectric materials is highly demanded to further improve its thermoelectric performance.However,the number of 18-electron HH compounds is rare.Thus,we can use high-throughput method to screen out novel 18-electrcon candidates with high thermoelectric performance,or mix 17-and 19-electron systems to form nominally 18-electron double half-Heusler systems,providing promising candidates for further experimental investigation.The followings are the descriptions of our studies:1.Defect engineering of NbFeSb-based HH materials:18-electron NbFeSb is an important p-type thermoelectric material.In experiment,doping to form solid solutions can scatter phonon and lower thermal conductivity,which is an effective way to improve its thermoelectric performance(its maximum zT can reach 1.5 at 1200 K).We have studied the defect(V and Ti)distributions in NbFeSb-based systems.Through theoretical predicted phase diagrams,we found the different defect distribution behaviors in(Nb,V)FeSb and(Nb,Ti)FeSb systems at low temperature:forming the NbFeSb-VFeSb phase separations in the(Nb,V)FeSb system but two thermodynamically stable phases in(Nb,Ti)FeSb.At high temperature,they both form solid solutions.Around phase boundary,the solid solution would coexist with phase seperations.Therefore,we propose startegies to further decrease thermal conductivity:lowering the experimental preparation temperature to around the phase boundary to form a mixture of the solid solution and phase separation.The point defects in the solid solution effectively scatter the short-wavelength phonons and the(coherent or incoherent)interfaces introduced by the phase separation can additionally scatter the middle-wavelength phonons.Our results give insight into the understanding of the impact of the defect distribution on the thermoelectric performance of materials and give insight into further lower thermal conductivity and improve thermoelectric performance of NbFeSb-based materials.2.Promising 18-electron HH thermoelectrics through high-throughput material computations:The number of experimentally well-studied 18-electron HH compounds is rare,leaving a large number of unstudied 18-electrcon HH systems.Therefore,we have carried out the high-throughput methods and selected high performance HH thermoelectrics within 95 HH compounds.Taking band-gap,abundance of elements into consideration,and using the thermoelectric properties of experimentally well-studied NbFeSb and ZrNiSn compounds as the screening criterion,we filtered out nine p-type and six n-type promising candidates with high electrical properties,ascribing to the high band degeneracy,small deformation potential,light band and large phonon velocity.(Quasi)harmonic phonon calculations together with the first-principles Debye-Callaway approach are further performed to study their thermal properties.Considering the excellent electrical properties and relatively low thermal conductivities,three HH compounds(VCoGe,NbCoSi,and TiNiGe)are predicted as promising thermoelectric candidates.Our work not only provides novel promising materials for future experimental investigation but also offers insights into understanding the underlying physical nature of high thermoelectric performance.3.Vibirational entropy influence on the phase stability of 18-electron HH compounds:High-throughput methods only focus on the cubic phase of HH compounds.However,experiments are observed to be noncubic low-symmetry phases(hexagonal and orthorhombic)for several HH compounds.Different phases will obviously lead to significantly different thermoelectric performances(the cubic phase usually exhibites good thermoelectric properties).Considering the puzzle of the phase stability of the HH compounds,we have studied the influence of vibrational entropy on the phase stability of 17 half-Heusler systems.We found that,in general,the lower symmetry phases have larger vibrational entropies,favoring their stabilities at higher temperatures.The high vibrational entropy of the distorted phase possibly comes from the weak bonding associated with larger atom motion,which leads to a large phonon density of states at the low frequency region.Our work explains the discrepancy between first-principles predictions and experimental observations and emphasizes the important effect of including vibrational entropy on the phase stability.4.Design novel 18-electron HH compound:18-electron HH thermoelectric materials has been extensively studied.But the number of such 18-electron systesms is rare.Therefore,we have mixed 17-and 19-electron HH systems(TiFe1-xNixSb,ZrFe1-xNixBi and VFex-xNixGe)to form norminal 18-electron double half-Heusler.Two stable ground state ordered structures(Ti4Fe2Ni2Sb4 and V4Fe2Ni2Ge4)are identified.Based on the calculations of electronic and phonon structures,we found that the two ordered structures can maintain the excellent electrical properties of pristine half-Heusler compounds compared to experimentally used solid solutions,owing to high weighted carrier mobility.Phonon calculation shows that ground states own relatively low thermal conductivities.Combing the high electrical properties and low thermal conductivities,the p-type(n-type)zT values of Ti4Fe2Ni2Sb4 and V4Fe2Ni2Ge4 are predicted to reach to 1.75(0.64)and 1.33(0.95),respectively.Our work not only provides promising double half-Heusler candidates for further experimental investigation but also suggests that forming ordered structure instead of solid solution is an efficient way to achieve excellent thermoelectric properties in the double half-Heusler systems.