Influence of the triel elements (M = Al, Ga, In) on the transport properties of Ca ₅ M ₂Sb ₆ zintl compounds

The Zintl compound Ca ₅Al ₂Sb ₆ has extremely low lattice thermal conductivity (<0.6 W/mK at 1000 K) and tunable electronic properties, making it a promising thermoelectric material for high temperature waste-heat recovery. The current study investigates trends in the chemical and transport properties of the Ca ₅M ₂Sb ₆ compounds (M = Al, Ga, or In), revealing potential routes toward improved thermoelectric properties in this system. Here, we show that isoelectronic M-site substitutions can be used to "fine-tune" the electronic properties of the Ca ₅M ₂Sb ₆ system, without inducing electronic doping effects. Electronic structure calculations reveal that the electronegativity of the M element is a good indicator for the energy level of M electronic states. The effects of M-site substitutions on the effective mass and band gap are reflected in measurements of the high temperature electronic properties of Ca ₅M ₂Sb ₆ samples (M = Al, Ga, and In) which reveal increased hole mobility as well as a smaller thermal band gap in the Ga analogue, relative to Ca ₅Al ₂Sb ₆ and Ca ₅In ₂Sb ₆. Optical absorption measurements reveal a trend in the direct band gaps consistent with both calculations and transport measurements. Additionally, a direct benefit of substituting heavier elements on the Al site arises from the increased density and softer lattice, which leads to reduced sound velocity and lattice thermal conductivity.