Alloying-Induced Structural Transition in The Promising Thermoelectric Compound CaAgSb

<i>AMX</i> Zintl compounds, crystallizing in several closely related layered structures, have recently garnered attention due to their exciting thermoelectric properties. In this study, we show that the orthorhombic CaAgSb can be alloyed with hexagonal CaAgBi to achieve a solid solution with a structural transformation at <i>x</i> ~ 0.8. This transition can be seen as a switch from 3D to 2D covalent bonding, in which the interlayer <i>M-X</i> bond distances expand while the in-plane <i>M-X</i> distances contract. Measurements of the elastic moduli reveal that CaAgSb_1-xBi_x becomes softer with increasing Bi content, with the exception of a step-like 10% stiffening observed at the 3D-to-2D phase transition. Thermoelectric transport measurements reveal promising Hall mobility and a peak <i>zT</i> of 0.47 at 620 K for intrinsic CaAgSb, which is higher than previous reports for unmodified CaAgSb. However, alloying with Bi was found to increase the hole concentration beyond the optimal value, effectively lowering the <i>zT</i>. Interestingly, analysis of the thermal conductivity and electrical conductivity suggests that the Bi-rich alloys are low Lorenz-number (<i>L</i>) materials, with estimated values of <i>L</i> well below the non-degenerate limit of <i>L</i> = 1.5 x 10^-8 W.Ω.K^-2, in spite of the metallic-like transport properties. A low Lorenz number decouples electrical and electronic thermal conductivity, providing greater flexibility for enhancing thermoelectric properties.