Zintl Defects in Intermetallic Clathrates

Abstract

In many cases, idealized crystal structure models cannot rationalize the actual properties of intermetallic compounds. For a realistic approach in materials research, microstructures and defects need to be taken into account. In case of clathrate compounds, particularly the intrinsic framework vacancies (denoted as Zintl defects) demand consideration. Consequently, clathrate research produces evidence that modern‐day structure chemistry involves the utilization of advanced X‐ray diffraction methods combined with elaborated bulk phase analyses, the investigation of phase relations, and the study of mutual interrelations in the triangle chemical bonding–structure–properties. Herein, we review some fundamental contributions to the specific defect chemistry of intermetallic clathrates.

The impact of boron atoms on clathrate-I silicides: composition range of the borosilicide K8−xBySi46−y

“…Sample 3, prepared in an iron crucible, had a lower purity than the other samples. The result confirms the absence of framework vacancies in the structure as the diamagnetic contributions of (3b)Si − anions 23 would exceed the elemental increments of (4b)Si 0 significantly. 34…”

“…Amorphous boron powder (Alfa Aesar), which typically contains oxidic impurities in the range of 1-3 mass%, was purified in a streaming hydrogen plasma. 23 For this purpose, the boron powder was placed in a corundum crucible situated in a silica glass Schlenk-tube. Plasma treatment under controlled H 2 flow was realized in a microwave furnace (MLS) for 20 minutes (P = 1200 W, p(H 2 ) ≈ 29 Si NMR signals (static echo measurements) show a small Knight shift typical for widely electronically balanced silicon clathrate-I compounds.…”

“…Typically, the replacement of group 14 elements in intermetallic clathrates by heteroatoms starts at site 6c (Si1). As in all binary clathrate silicides M 8−x Si 46 the Si2 atoms show the shortest framework distances, 22,23 this site should be the most suitable one for boron atoms. However, the partial substitution at site 16i inevitably causes a structural disorder, because 16i atoms appear as bonded pairs.…”

Mastering extreme size constraints in the clathrate‐I borosilicide Cs8B8Si38

“…The crystal structures of the hitherto characterized clathrate‐I borosilicides K 8− x B y Si 46− y [5,6] and Rb 8 B y Si 46− y [7] are exceptional compared to other intermetallic clathrates. Usually, vacancy formation or substitution by transition metal preferably occurs at site 6 c (Si1), which connects plane six‐membered rings to chains along the {100} directions [20] (light blue in Figure 3). By introducing heteroatoms or vacancies at 6 c , the tension in the crystal structure is reduced, as planar six‐rings do not allow for the adaption to an ideal tetrahedral environment characteristic for four‐bonded tetrel atoms.…”

Temperature-dependent crystal structure investigation of 4f hybridized thermoelectric clathrate Ba8–x Ce x Au y Si46–y

“…Transition metals have generally been observed to order on the 6c site in the host structure (Johnsen et al, 2006(Johnsen et al, , 2007Baitinger et al, 2020;Herrmann et al, 1999) and Au was confined to this position in the initial model used here, with Si being present on all sites. From refining the relative Au/Si occupancy on 6c, the site was found to be preferentially occupied by Au: occ Au (6c)/occ Si (6c) = 5.9 (1).…”