(Color online) (a) Ca5Al2Sb6 orthorhombic unit cell (Pbam) viewed along the  direction. (b) Covalent substructure formed from infinite chains of corner sharing AlSb4 tetrahedra connected via Sb–Sb bonds. Sb atoms are orange, Ca are green, and Al are blue (Ref. 12 ).
Ca5Al2− x Zn x Sb6 x-ray diffraction patterns (x = 0, 0.20), Reitveld fit to the x = 0 sample, and associated difference profile. No secondary phases are observed for the x range investigated (0 < x < 0.20).
With zinc doping, the carrier concentration at 300 K is directly proportional to the level predicted using simple electron counting (dashed line). This is in contrast to the lower doping efficiency with sodium (Ref. 13 ).
(a) High temperature Hall coefficient measurements of Ca5Al2− x Zn x Sb6 yield the carrier concentration. Doped samples exhibit extrinsic behavior to 800 K. (b) Undoped Ca5Al2Sb6 exhibits a mobility dominated by phonon scattering across the entire temperature range investigated, whereas Zn-doped samples show evidence of additional scattering mechanisms at low temperature. (c) The high temperature resistivity decreases with increasing Zn doping level.
Similar Seebeck coefficient values for Na- and Zn-doped Ca5Al2Sb6 at equivalent carrier concentrations suggest similar band mass. The curve was generated using a single parabolic band approximation and an effective mass of 1.8 m e (Ref. 13 ).
High temperature Seebeck coefficients of Ca5Al2− x Zn x Sb6 show degenerate behavior for the extrinsically doped, p-type compositions. Similar behavior is seen in the Na-doped analog (Ref. 13 ).
(Color online) (a) The calculated Ca5Al2Sb6 band structure shows a direct gap at X. The valence bandedge consists of two nested hole pockets with band masses between 0.25 and 2 m e, depending on the direction. The dashed lines at −0.08 and −0.13 eV correspond to 1 and 5 × 1020 h + cm−3, respectively. (b) The density of states of Ca5Al2Sb6 shows that the material is a semiconductor with Sb p states as the dominant influence near the valence bandedge.
(Color online) (a) The total thermal conductivity of Ca5Al2− x Zn x Sb6 increases with increasing doping level and thus electronic conductivity. (b) The lattice contribution (κL ) decays with T −1 and approaches a minimum value at high temperature, consistent with prior Na-doped compositions (Ref. 13 ). To calculate κL, the Lorenz coefficients (inset) were calculated using the single parabolic band approximation.
As compared to sodium doped Ca5Al2Sb6, the reduced mobility of the samples in this study lead to a lower zT. Within the Zn series, the highest doped composition exhibits the largest zT.
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