Scandium iodides without and with carbon

Abstract

There are two binary iodides of scandium, the rather trivial insulating ScI 3 and the scandium deficient Sc 0.9I 2, metallic above and insulating below about 100 K. At low temperatures, six out of eight excess electrons corresponding to Sc 0.89I 2 × 9 = Sc 8□ 1I 18 = (Sc 3+) 2(Sc 2+) 6(□ 1)(I −) 18 are localized in 3d states, and one regular metal site of a hypothetical CdI 2-like superstructure of ScI 2 is empty. At high temperatures, the electrons delocalize into a conduction band, in accord with the formulation (Sc 3+) 8(□ 1)(e −) 6(I −) 18. Excess electrons may also be added or consumed by a third partner. The first case is realized in the, again, scandium deficient ternary halides ASc x X 3 (A = Rb, Cs; X = Cl, Br, I). These have all, in principle, the CsNiCl 3-type of structure in which halide octahedra share common faces. Their centres are, in accord with, e.g., CsSc 0.75I 3 × 4 = Cs 4Sc 3□ 1I 12 = (Cs +) 4(Sc 3+) 3(□ 1)(e −) 1(I −) 12, occupied in three out of four cases. At sufficiently low temperatures, the one excess electron is localized at the middle scandium atom of a triad of scandium atoms. The scandium carbide iodides Sc 4C 2I 6 and Sc 6C 2I 11 are examples of reduced scandium iodides in which electrons are consumed by carbon dimers which occupy, as interstitials, the centres of scandium metal octahedra. In attempts to reproduce these in pure form, we have obtained Sc 24C 10I 30. It is built of nano-sized Sc 24C 10I 30 molecules which consist of a shell of 30 I − ions, a truncated and hollow T4 supertetrahedron. It envelopes a T3 supertetrahedron of 20 scandium atoms in which the tetrahedral interstices are occupied by carbon atoms. The T2 supertetrahedron C 10 itself incorporates a tetrahedron of scandium atoms. This inner scandium tetrahedron traps two electrons in a four-centre-two-electron molecular orbital. In total, Sc 24C 10I 30 may be viewed as (e −) 2(Sc 3+) 4(C 4−) 10(Sc 3+) 20(I −) 30.