ИСТИНА

Zintl clathrates are polar intermetallic compounds, in which the charge of an anionic framework is compensated by large cations sitting in cage-like polyhedral voids. Although tetrahedrally-bonded frameworks require sp3-hybridized p-elements, some d-metals are also known to enter such frameworks, copper being notable for its presence in various types of clathrate compounds. In this work, we focus on the role of copper in Zintl clathrates. We discuss two types of compounds, A8Cu16P40 [1] and A7Cu44E23 [2]. The former type (A = Ba, Eu) displays an orthorhombic superstructure of the type-I clathrate. In its crystal structure, copper atoms are distributed over 8 crystallographic positions in such a way that Cu-Cu contacts are avoided, and P12 and P18 clusters form instead. The crystal structure of A7Cu44E23 (A = Eu, Sr; E = As, Sb) is of clathrate-like type, with 6/7 of large A-cations residing in oversized 20-vertex cages. Unlike A8Cu16P40, these compounds show segregation of E atoms, whereas copper forms from 4 to 9 Cu-Cu bonds depending on its position in the crystal structure. We will compare crystal and electronic structures, and transport and thermal properties of these compounds, paying special attention to the reasons underlying the difference in their properties