Electronic Fine Tuning of the Structures of Reduced Rare-Earth Metal Halides

Electronic band-structure calculations on Pr3I3Ru and Y3I3Ru have been performed in order to analyze the large
structural differences found in these isoelectronic compounds. These constitute two structural extremes within
the family of monoclinic RE3I3Ru phases (RE = rare-earth metal) that exhibit distortions ranging from one-dimensional double chains of trans-edge-sharing octahedra (bioctahedral chains, BOH) to one-dimensional chains
of trans-edge-sharing square pyramidal units bonded base to base (bisquare pyramidal chains, BSP). The structure
of La3I3Ru was established by single-crystal X-ray diffraction (monoclinic, P21/m, Z = 4, a = 9.343(1) Å, b =
4.3469(8) Å, c = 12.496(3) Å, β = 93.42(2)°) and found to be isomorphous with the BOH Pr3I3Ru. It is determined
that the structural variation in this RE3I3Z family of materials depends largely on the differences in orbital energies
between the corresponding rare-earth metal and the interstitial. These bonding considerations can be generalized
to account for structural variations in a variety of other rare-earth halides as well as several group 4 or 5 reduced
metal halide cluster phases.