Dynamic Disorder of Bi 8 2+ Clusters in the Plastic Phase (Bi 8 ) 3 Bi[InI 4 ] 9

Black, air‐insensitive crystals of the bismuth‐rich tetraiodido‐indate(III) (Bi 8 ) 3 Bi[InI 4 ] 9 have been prepared from melts of the contributing elements. X‐ray diffraction on a single crystal at 293 K and 220 K revealed a cubic structure with [InI 4 ] – tetrahedra, isolated Bi 3+ ions and rotationally disordered Bi 8 2+ square antiprisms ace group Fm $\bar{3}$ c ; a = 20.162(2) Å at T = 293(1)]. The principal arrangement of the complex ions corresponds to the AuCu 3 type. The small bulk modulus B 0 = 12(1) GPa and its large pressure derivative B 0 ′ = 11(1) as well as the low Debye temperature θ D = 52 K are in accordance with a plastic behavior. Cooling below 148(1) K induces the first order transition into an orderedphase with 0.43 % higher density but much larger unit cell and lower crystallographic point group. The transition enthalpy Δ H ≈ 3.8 kJ·mol –1 and transition entropy Δ S ≈ 3 R were determined from specific heat data. A similar reversible phase transition into a lower symmetric structure is observed at a pressure of approximately 5 GPa. (Bi 8 ) 3 Bi[InI 4 ] 9 is a diamagnetic semiconductor.