Crystal Chemistry of the Europium Pnictides

Abstract

Europium occurs in the pnictides with oxidation number two and three, depending on concentration and electronegativity of the anions. The crystal chemistry of trivalent Eu resembles that of the normal rare-earth elements, whereas the divalent Eu compounds are closely related to their alkaline-earth analogs. The alkaline earths as well as divalent Eu have a relatively low electronegativity. Nonmetallic properties are therefore most likely to occur in the phosphides and arsenides, possibly also in the antimonides. The discussion of the crystal chemistry of these pnictides thus will turn out to be an illustration of the Mooser — Pearson rule (1,2). This rule correlates the electronic properties of a compound to its crystal structure or short-range order. Nonmetallic properties require that in an idealized crystal (i.e. a crystal without the unavoidable imperfections) at T=0 the valence electrons occupy all states in some low-lying energy bands (valence band) while the next higher available states (in the conduction band) are well separated energetically. The generation of free charge carriers therefore requires an activation energy. From a chemical point of view this means that in a nonmetallic compound all the chemical bonds have to be saturated while non-bonding electrons (such as 4f-electrons) have to be localized on the ions.