On the Effects of Aliovalent Substitutions in Thermoelectric Zintl Pnictides. Varied Polyanionic Dimensionality and Complex Structural Transformations─The Case of Sr₃ZnP₃ vs Sr₃Al_xZn_1–xP₃

The structures and the transport properties of a novel family of Zintl phosphides and arsenides with the formula AE3ZnPn3 and the solid solutions AE3AlxZn1–xPn3, AE3ZnAsyP1–y (AE = Sr, Eu; Pn = P, As) are reported. Crystals of nine new phases have been obtained via Pb-flux reactions and used for structural work by means of single-crystal X-ray diffraction methods. The derived orthorhombic structure is without a direct analogue and features unusual structural units, where the Zn atoms are in both distorted tetrahedral and trigonal-planar coordination of pnictogens. Electronic structure calculations reveal moderately wide band gaps for Sr3ZnP3 and Sr3ZnAs3, on the order of 0.70 and 0.63 eV, respectively. Electrical transport measurements above room temperature indicate relatively high resistivity values above 500 K (ρ ≈ 4.8 Ω cm and above), but some of the samples exhibit very high Seebeck coefficients, as large as 300 μV/K at 560 K for Sr3ZnAs3. Aliovalent substitutions in AE3ZnPn3, achieved by the partial replacement of Zn2+ with Al3+ cations, promote occupational and positional disorders, which cause structural transformation toward the disordered variant of the Sr5Al2Sb6 structure type. Such substitutions also change the dimensionality of the polyanionic sublattice in the resulting quaternary AE3AlxZn1–xPn3 phases. Preliminary transport property data on the latter reveal nine times lower electrical resistivity (ρ500 ≈ 0.5 Ω cm) together with a significantly enhanced Seebeck coefficient, αmax ≈ 430 μV/K at 560 K.