Abstract
The compounds Er 4I 5 and Er 7I 10 are formed as single crystals by reacting ErI 3 and erbium in sealed tantalum capsules at temperatures between 1000 and 1200 °C. Both compounds crystallize in the monoclinic system (space group C2 m ; a 0 = 1852.1 pm, b 0 = 401.5 pm, c 0 = 847.8 pm, β = 103.1° for Er 4I 5 and a 0 = 2096.6 pm, b 0 = 418.7 pm, c 0 = 1458.5 pm, β = 96.6° for Er 7I 10). The structures contain one-dimensional infinite sections linked by Er-Er bonds which are discussed in terms of single strings (Er 4I 5) and double strings (Er 7I 10) of trans-edge-sharing Er 6-octahedra. The iodine atoms are attached to the free edges corresponding to the atomic arrangement in the isolated M 6X 12-cluster (M metal; X halogen). Iodine atoms form closest sphere packings together with Er 6-octahedra. Isolated Er 3+ ions occupy octahedral sites formed by iodine atoms. The reduced halides LnX n (Ln lanthanide; n < 2) derived from the Ln 6X 12 cluster reveal close structural relationships. The compounds Ln 5X 8 and Ln 7X 10 are first in a series of general composition Ln2 a + 3 X 2 a + 6 , whereas Ln 4X 5 and Ln 6X 7 set up a series of composition Ln 2 a + 2 X 2 a + 3 . The index a represents the number of strings of single octahedra which are condensed in the structures.
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