Structural diversity of the Zintl pnictides with rare-earth metals

Abstract

Zintl phases have been purely a laboratory curiosity for decades, recently becoming a familiar subject in many research topics of practical importance, such as thermoelectrics for example. This distinctive group of compounds holds a special place among the broader intermetallic family because its members are commonly composed of the electropositive alkali or alkaline-earth metals and the much more electronegative post-transition metalloid elements. The structures of the Zintl phases are often very complex, but they can be conceptually understood by assuming an electron transfer from the atoms of the less electronegative to the atoms of the more electronegative elements in a way that the classic valence rules are satisfied. In this manner, the donated electrons are used by the electronegative constituents to form bonds, usually through relatively strong covalent interactions, whereas the cation-anion interactions ought to be considered to be more ionic than covalent in character. Because of the peculiar nature of the chemical bonding, Zintl phases are located at the border between the valence compounds, which are typical insulators, and the intermetallic compounds, which are typical metals, often exhibiting intermediate properties, that is, Zintl phases are by nature narrow bandgap semiconductors. A wide variety of main-group based Zintl phases have already been synthesized and structurally characterized. Their structures are truly diverse and many are remarkable on their own. In the past two to three decades, the chemistry of the Zintl phases has been extended to include lanthanides and some of the transition metals. This chapter provides a comprehensive overview of the crystal chemistry and some properties of the Zintl pnictides containing rare-earth metals, recounting many structures of binary, ternary and multinary compounds.