Electronic Structure And Thermoelectric Performance Of Zintl Compound Sr3GaSb3,A3AlSb3(A=Ca, Sr) And Sr5Al2Sb6: A First-principles Study

The thermoelectric energy conversion technology, which can be used to convert wasted heat intoelectricity, has received much attention in the past decade. Zintl compounds have emerged as a promisingclass of materials for thermoelectric materials due to their complex structures. Zintl compounds are madeup of electropositive cations (typically, Groups1and2) which donate their electrons to electronegativeanions; which, in turn, use them to form bonds in order to satisfy valence. The resulting mix of ionic andcovalent bonds frequently leads to complex crystal structures with large unit cells, leading to low thermalconductivity. Additionally, it is important that we can control the carrier concentration via doping toachieve the optimal thermoelectric properties. This has been demonstrated in several Zintl compoundsincluding Ca5Ga2As6, Ca5Al2Sb6, Sr3GaSb3and Ca3AlSb3.We calculated the thermoelectric properties of Sr3GaSb3, A3AlSb3（A=Ca, Sr）and Sr5Al2Sb6by usingfirst-principles calculation and semiclassical BoltzTrap theory. The expermental structures are taken as theinitial bulk model to relax to find the minimum energy structure by using the vienna ab initio simulationpackage (VASP) based on DFT. The electronic structure was calculated by using the full potentiallinearized augmented planewaves (FLAPW) method implemented in the WIEN2K. The transportcoefficients were derived from the electronic structure by using the semiclassical Boltzmann theory asimplemented in the Boltzmann code.The structural, thermoelectric properties, and electronic structures of Sr3GaSb3are studied by thefirst-principles calculations and the semiclassical Boltzmann theory. It is found that the transport propertiesof n-type Sr3GaSb3are probably better than that of the p-type, which is mainly due to the higher Seebeck coefficient of n-type doping. The large band degeneracy of the conduction bands along the Γ–Y directioninduced a high Seebeck coefficient of n-type Sr3GaSb3along the yy direction. Moreover, n-type Sr3GaSb3along the yy direction can achieve a ZT value of1.74at850K, corresponding to the carrier concentration3.5×1020e cm-3. For p-type doping, the increased number of band valley on the top of valence bands maylead to the high carrier concentration of p-type Sr3GaSb3. Such high carrier concentration is helpful toachieve high electrical conductivity of p-type Sr3GaSb3.By analysing the properties of A3AlSb3（A=Ca, Sr）, we find that Ca3AlSb3and Sr3AlSb3will bepromising thermoelectric compounds.On the one hand, One-dimensional chain structure of Ca3AlSb3,leading to high electrical conductivity, especially along the yy direction (i.e. chain direction). On the otherhand,The small band mass along three direction and large band degeneracy (Nv=2) of p-type Ca3AlSb3,result in large Seebeck coefficients. As a result, Ca3AlSb3will have good thermoelectric properties alongthe chain direction, this requires further research in the experiment. By calculating the formation energiesand electron localization function of A3AlSb3, Sr3AlSb3has stronger Al-Sb covalent bond than that ofCa3AlSb3, this is why Sr3AlSb3has low carrier concentration in the experimental results. By appositedoping, p-type Sr3AlSb3will also have good thermoelectric properties, when the carrier concentrationabove above1×1020e+cm-3. Additionally, the complex structure of Sr3AlSb3(56atoms per unit cell) leadsto low thermal conductivity. By researching the transport properties of Sr3AlSb3, we think that the maximaZT of n-type Sr3AlSb3can be achieved0.77, corresponding to the carrier concentration4.5×1020e cm-3By analysing the properties of Sr5Al2Sb6, we find that the transport properties of n-type Sr5Al2Sb6maybetter than that of p-ype Sr5Al2Sb6, owing to its larger band degeneracy which leading to larger Seebeckcoefficients. Sr5Al2Sb6will have good thermoelectric performance on the wide range of carrierconcentration. p-type Sr5Al2Sb6has high electrical conductivity, due to its larger band width,(i.e. bandIV dispersions), samll band mass which is helpful to the electron transporting. We think that Sr5Al2Sb6will bepromising thermoelectric materials.