Structure and stability of Ba–Cu–Ge type-I clathrates

Abstract

We have prepared samples of nominal type Ba8CuxGe46−x by induction melting and solid state reaction. Analysis shows that these materials form type-I clathrates, with copper content between x = 4.9 and 5.3, nearly independent of the starting composition. We used x-ray powder diffraction and single-crystal electron diffraction to confirm the cubic type-I clathrate structure, while electron microprobe measurements confirmed the stability of the x ≈ 5 composition. This result differs from the corresponding Ag and Au clathrates, and was not known previously due perhaps to the similar Cu and Ge form factors in x-ray diffraction. The observed composition adheres very tightly to a valence-counting scheme, in agreement with a Zintl-type stability mechanism. This implies a gap in the electronic density of states, also in contrast to the metallic behavior of the Au and Ag analogs. Magnetization measurements showed a large diamagnetic response in the Ba-Cu-Ge clathrate. This behavior is consistent with semiconducting or semimetallic behavior, and is similar to that of a number of intermetallic semiconductors.

Preparation, Crystal Structure and Physical Properties of the Superconducting Cage Compound Ba3Ge16Ir4

“…[1] In intermetallic compounds of elements with strongly different electronegativity, structural motifs can often be understood applying the Zintl-Klemm concept. [1,2] Striking examples are intermetallic clathrates with frameworks based on p-block elements such as K 8 Ge 44 ᮀ 2 [3] or Ba 8 Ge 30 Ga 16 . [4] In these compounds the connection of the four-bonded framework atoms may be described by means of covalent interaction of sp 3 -type atomic orbitals.…”

“…However, the situation becomes less distinct for clathrate frameworks incorporating transition metal atoms (e.g. Rh, [5] Ir, [6] Ni, [7] Cu [8] ). When the content of a transition metal is too high for being integrated into E14-based clathrate frameworks, cage compounds with different structure motifs appear which include not only four-bonded atoms.…”

Superconductivity in gallium-substitutedBa8Si46clathrates

“…Therefore clathrates also have potential application in magnetic sensors and new magnetic semiconductors. [12][13][14] Inspired by the discovery of superconductivity in alkalimetal-doped C 60 fullerenes, efforts have been made to explore the superconductivity of group-IV clathrates with particular attention to the sp 3 -hybridized networks. In contrast to carbon, silicon and germanium do not form sp 2 -like networks.…”

Superconductivity and Magnetism in Silicon and Germanium Clathrates

“…These late transition metals exhibit filled d-bands when substituted on the silicon and germanium clathrates, shown for example by the formal valence of 1 for Cu atoms in the germanium type-I clathrate [69]. The earlier transition metals would be expected to induce magnetic moments, similar to observations in doped III-V and related semiconductors [86,87].…”

“…Germanium and tin clathrates commonly adhere closely to Zintl stability conditions, forming vacancies as needed. For example, the type-I compound initially reported as Ba 8 Cu 6 Ge 40 was found instead to form a composition close to Ba 8 Cu 5.3 Ge 40.7 , satisfying the Zintl criterion [69], with Cu treated as a simple s-p substituent having a valence of 1. On the other hand, siliconbased Ba 8 Cu x Si 46-x forms with varying composition not keeping to Zintl rules [59,70].…”