Thermoelectric properties of Sr3GaSb3 – a chain-forming Zintl compound

Abstract

Inspired by the promising thermoelectric properties in the Zintl compounds Ca 3 AlSb 3 and Ca 5 Al 2 Sb 6 , we investigate here the closely related compound Sr 3 GaSb 3 . Although the crystal structure of Sr 3 GaSb 3 contains infinite chains of corner-linked tetrahedra, in common with Ca 3 AlSb 3 and Ca 5 Al 2 Sb 6 , it has twice as many atoms per unit cell (N ¼ 56). This contributes to the exceptionally low lattice thermal conductivity (k L ¼ 0.45 W m À1 K À1 at 1000 K) observed in Sr 3 GaSb 3 samples synthesized for this study by ball milling followed by hot pressing. High temperature transport measurements reveal that Sr 3 GaSb 3 is a nondegenerate semiconductor (consistent with Zintl charge-counting conventions) with relatively high p-type electronic mobility ($30 cm 2 V À1 s À1 at 300 K). Density functional calculations yield a band gap of $0.75 eV and predict a light valence band edge ($0.5 m e ), in qualitative agreement with experiment. To rationally optimize the electronic transport properties of Sr 3 GaSb 3 in accordance with a single band model, doping with Zn 2+ on the Ga 3+ site was used to increase the p-type carrier concentration. In optimally hole-doped Sr 3 Ga 1Àx Zn x Sb 3 (x ¼ 0.0 to 0.1), we demonstrate a maximum figure of merit of greater than 0.9 at 1000 K.

Thermoelectric Properties and Electronic Structure of the Zintl‐Phase Sr3AlSb3

“…6 We have previously reported exceptionally low κ L in several other chain-forming A 5 M 2 Sb 6 and A 3 MSb 3 compounds (A = Ca, Sr and M = Al, Ga, In). [15][16][17][18] The temperature dependence of κ L in Sr 5 Al 2 Sb 6 is very similar to these compounds, decreasing as 1/T with increasing temperature due to Umklapp scattering, and approaching the predicted minimum at high temperature. Because the unit cell size and average speed of sound of Sr 5 Al 2 Sb 6 (52 atoms, 2610 m s −1 ) are comparable to that of Sr 3 AlSb 3 (56 atoms, 2590 m s −1 ), κ L should be very similar in these two materials, despite their different structure types.…”

“…Similar characteristics of the electronic structure were also observed in the related A 5 M 2 Sb 6 and A 3 MSb 3 Zintl phases (A = Ca, Sr and M = Al, Ga, In). 15,16,27 Semi-classical Boltzmann transport theory was employed to calculate the Seebeck coefficient, α, as a function of temperature in the three principle directions, as shown in the inset of Fig. 2.…”

“…By doping with Zn 2+ on the M 3+ site or Na 1+ on the A 2+ site, the charge carrier concentration of these compounds can be optimized, leading to figures of merit as high as zT = 0.9. [15][16][17][18] Among A 5 M 2 Sb 6 compounds, most form either the Ca 5 Al 2 As 6 or Ca 5 Al 2 Bi 6 structure type. 19,20 These structure types both contain infinite chains of corner-linked MSb 4 tetrahedra in which neighboring chains are linked by Sb-Sb bonds to form ladder-like anionic structures.…”

Enhanced thermoelectric properties of Sr5In2Sb6via Zn-doping

“…5. 9 -the latter of which might be explained by impurity oxides or other secondary phases at the grain boundaries. Above 450 K for undoped and 550 K for Zn-doped samples, the mobility values start to decrease due to acoustic phonon scattering (m f T Àv , 1.0 # v # 1.5).…”

“…[5][6][7][8][9][10] While the complex primitive cells of Zintl compounds reduce the velocity of the optical phonon modes, which lowers the lattice thermal conductivity, 11 their covalently-bonded polyanionic structures may also contribute to the high electronic conductivity. 4 Through doping, these phases become extrinsic semiconductors with carrier concentrations in the range of 10 19 to 10 21 carriers per cm 3 and exhibit signicant improvements to their electronic properties and zT.…”