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

초록

Nine new Zintl phase thermoelectric (TE) materials in the Ca3-xSrxAl1-YZnYSb3 (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 Ca3AlAs3-type phase (space group Pnma, Pearson code oP28), 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: Ca3AlSb3, Ca1.5Sr1.5AlSb3, and Ca1.5Sr1.5Al0.75Zn0.25Sb3, 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 Gamma, and (3) the valence-band flattening between Gamma 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 Ca3AlSb3 and Ca2.00(3)Sr1.00AlSb3, respectively, which corroborated the calculated results. TE property measurements showed an increase of electrical conductivity sigma 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 kappa(tot) down to 0.46 W/mK at 873 K for Ca2.48(2)Sr0.52AlSb3. The maximum ZT value was obtained for the Zn-doped Ca3Al0.96(1)Zn0.04Sb3, and this enhanced result should be attributed to the synergy of enhanced sigma, moderated S, and reduced kappa(tot).

키워드