Das Lanthaniodidethanid o‐La5I9(C2) – Die orthorhombische Hochtemperaturmodifikation

The Lanthanumiodideethanide o‐La₅I₉(C₂) – The Orthorhombic High Temperature Modification o‐La₅I₉(C₂) is synthesized by reaction of LaI₃, La metal and graphite powder in sealed Ta containers at 850 °C < T < 900 °C. It crystallizes in the orthorhombic space group Pbca with a = 8.0247(16) Å, b = 16.887(3) Å, c = 35.886(7) Å. o‐Ce₅I₉(C₂) is isotypic with the lattice parameters a = 7.9284(4) Å, b = 16.714(1) Å, c = 35.530(3) Å. o‐La₅I₉(C₂) transforms at 800 °C to the triclinic low temperature modification t‐La₅I₉(C₂). The transformation is reversible. The La atoms form trigonal bipyramids centered by C₂ groups. These units are connected by iodine atoms above the faces (f), edges (e) and corners according to La₅(C₂)I(f)ⁱI(e)^i?i_2/2I(e)^i?a_7/2I(e)^a?i_7/2. The C‐C distance in the C₂ unit is 1.45(2) Å. The crystals with greenish luster are moisture sensitive.