Phase Characterization, Thermal Stability, High-Temperature Transport Properties, and Electronic Structure of Rare-Earth Zintl Phosphides Eu3M2P4 (M = Ga, In)

Two rare-earth-containing ternary phosphides, Eu3Ga2P4 and Eu3In2P4, were synthesized by a two-step solid-state method with stoichiometric amounts of the constitutional elements. Refinements of the powder X-ray diffraction are consistent with the reported single-crystal structure with space group C2/c for Eu3Ga2P4 and Pnnm for Eu3In2P4. Thermal gravimetry and differential scanning calorimetry (TG-DSC) measurements reveal high thermal stability up to 1273 K. Thermal diffusivity measurements from room temperature to 800 K demonstrate thermal conductivity as low as 0.6 W/m.K for both compounds. Seebeck coefficient measurements from room temperature to 800 K indicate that both compounds are small band gap semiconductors. Eu3Ga2P4 shows p-type conductivity and Eu3In2P4 p-type conductivity in the temperature range 300-700 K and n-type conductivity above 700 K. Electronic structure calculations result in band gaps of 0.60 and 0.29 eV for Eu3Ga2P4 and Eu3In2P4, respectively. As expected for a valence precise Zintl phase, electrical resistivity is large, approximately 2600 and 560 m Omega.cm for Eu3Ga2P4 and Eu3In2P4 at room temperature, respectively. Measurements of transport properties suggest that these Zintl phosphides have potential for being good high-temperature thermoelectric materials with optimization of the charge carrier concentration by appropriate extrinsic dopants.