Substitution chemistry in the rhombohedral R(7)X(12)Z-type structure.

摘要

A series of quaternary lanthanide halide cluster compounds AR₆I ₁₂Z (R = La, Pr) with transition metal interstitials (Z) and alkali or alkaline earth metal cations (A) have been synthesized by high temperature solid state techniques. The compounds were structurally characterized by single crystal and powder x-ray diffraction methods, as well as energy dispersive spectroscopy on selected examples. The LiLa₆I₁₂Os compound was further studied by neutron powder diffraction and 7Li nuclear magnetic resonance in order to locate the lithium cations more confidently.; Like many other lanthanide and early transition metal halides, the new phases, which are isostructural with rhombohedral R₇X₁₂Z (R = Sc, Y, La-Lu; X = Cl, Br, I; Z = transition metal or main group element), are constituted of nominally octahedral R₆X₁₂ units. Electronic requirements of the relatively electron-poor lanthanide clusters demand the incorporation of an interstitial atom (Z) at the center of each octahedron. In addition to the six R atoms in the octahedral cluster, a seventh R cation occupies an isolated position between clusters, surrounded only by halides. In the present work, alkali and alkaline earth metals (Li, Na, Mg, Ca, Sr) preferentially substitute for this isolated R atom such that the position is fractionally occupied as (A_xR _1–x) with x ≤ 1; the balance of the structure remains essentially unchanged.; The electrical and magnetic properties of several ALa₆I ₁₂Z compounds were investigated through bulk resistivity and susceptibility measurements. All were semiconductors and exhibited Curie-Weiss paramagnetism at low temperatures but had more complicated mixed-state magnetic characters at room temperature.; This research was supported by the National Science Foundation - Solid State Chemistry - Grants DMR -9809850, -9510278, -0129785, and was carried out in the facilities of the Ames Laboratory, U.S. Department of energy.