Interplay of Synthesis, Structure and Thermoelectric Properties in Two Families of Silicide Zintl Phases Grown in Magnesium-Rich Flux

Abstract

Large crystals of metal silicide Zintl phases with general formula (Ca/Eu/Mg)2Si were grown from reactions of silicon with varying amounts of europium and calcium in magnesium/zinc flux. The Eu/Ca reactant ratio determined the structure type of the products. Calcium-rich reactions yielded Ca1−xEuxMgSi (0 < x < 0.7) compounds adopting the orthorhombic TiNiSi structure type in space group Pnma. Increasing europium content led to the prevalence of Eu8−yCayMg16Si12 (0 < y < 3.4) products with the hexagonal Ho5Ni19P12 structure type in space group P6̅2m. Single crystal X-ray diffraction studies indicate mixing of Zn on one Mg site in the hexagonal structures; thus, the formula can be expressed as Eu8−yCayMg16−αZnαSi12. Magnetic susceptibility data indicate europium is divalent in all of the compounds, with low-temperature magnetic ordering transitions. Density of states calculations show a pseudogap at the Fermi level for both structures, indicating semimetallic character; transport measurements indicate p-type behavior. Ca/Eu mixing and higher ratios of Eu2+ suppress lattice thermal conductivity in both structure types. In Ca1−xEuxMgSi, Eu-substitution lowers resistivity but yields minimal Seebeck enhancement; in Eu8−yCayMg16Si12 (Eu8−yCayMg16−αZnαSi12), Ca-substitution increases resistivity and markedly improves the Seebeck coefficient, with Eu7.59Ca0.41Mg15.62Zn0.36Si12 reaching S ≈ 150 μV K−1 at 300 K.