Tunable bandgap and ultra‐low thermal conductivity of the Sr/Zn co‐substituted Ca₃AlAs₃‐type Zintl phase

Nine new Zintl phase thermoelectric (TE) materials in the Ca 3− x Sr x Al 1−Y Zn Y Sb 3 (0 ≤ x ≤ 1.55(4); 0 ≤ Y ≤ 0.06(1)) system were successfully synthesized by arc‐melting and the Pb‐flux methods. All the title compounds adopted the orthorhombic Ca 3 AlAs 3 ‐type phase (space group Pnma , Pearson code oP 28), where three distinct cationic sites and one tetrahedral anionic site exhibited the mixed occupancies of Ca/Sr and Al/Zn. Interestingly, the Sr‐substituent preferred to fill the largest M2 site, then the M3 site, reaching the M1 site only at higher overall loading, which was consistent with the ionic size‐factor criterion. A series of DFT calculations by the TB‐LMTOASA method on the three model structures: Ca 3 AlSb 3 , Ca 1.5 Sr 1.5 AlSb 3 , and Ca 1.5 Sr 1.5 Al 0.75 Zn 0.25 Sb 3 , indicated (1) the Sr substitution widened the small bandgap, (2) the Zn substitution pushed E F through the valence‐band region yielding p ‐type character while maintaining a narrow direct gap at Γ, and (3) the valence‐band flattening between Γ and Z, and T and Y increased effective mass. Optical reflectance study using the Kubelka–Munk function gave the bandgap of 0.28 and 0.30 eV for Ca 3 AlSb 3 and Ca 2.00(3) Sr 1.00 AlSb 3 , respectively, which corroborated the calculated results. TE property measurements showed an increase of electrical conductivity σ with temperature and the positive Seebeck coefficients S between 303 and 850 K. Atomic disordering through the Sr and Zn substitutions strongly suppressed total thermal conductivity κ tot down to 0.46 W/mK at 873 K for Ca 2.48(2) Sr 0.52 AlSb 3 . The maximum ZT value was obtained for the Zn‐doped Ca 3 Al 0.96(1) Zn 0.04 Sb 3 , and this enhanced result should be attributed to the synergy of enhanced σ , moderated S , and reduced κ tot .