Scandium iodides without and with carbon

There are two binary iodides of scandium, the rather trivial insulating ScI3 and the scandium deficient Sc0.9I2, metallic above and insulating below about 100 K. At low temperatures, six out of eight excess electrons corresponding to Sc0.89I2 × 9 = Sc8□1I18 = (Sc3+)2(Sc2+)6(□1)(I−)18 are localized in 3d states, and one regular metal site of a hypothetical CdI2-like superstructure of ScI2 is empty. At high temperatures, the electrons delocalize into a conduction band, in accord with the formulation (Sc3+)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 AScxX3 (A = Rb, Cs; X = Cl, Br, I). These have all, in principle, the CsNiCl3-type of structure in which halide octahedra share common faces. Their centres are, in accord with, e.g., CsSc0.75I3 × 4 = Cs4Sc3□1I12 = (Cs+)4(Sc3+)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 Sc4C2I6 and Sc6C2I11 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 Sc24C10I30. It is built of nano-sized Sc24C10I30 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 C10 itself incorporates a tetrahedron of scandium atoms. This inner scandium tetrahedron traps two electrons in a four-centre-two-electron molecular orbital. In total, Sc24C10I30 may be viewed as (e−)2(Sc3+)4(C4−)10(Sc3+)20(I−)30.