Die Kristallstrukturen von Verbindungen des Typs A2TeX6 (A = K, NH4, Rb, Cs; X = Cl, Br, I)

Abstract

With the determination of the crystal structure of (NH4)2TeI6 the list of the descriptions of A2TeX6 structures is further completed. At 293 K three structure types are observed with an antifluorite packing of cations and anions: The cubic K2PtCl6 type structure (space group Fm 3̄ m, Z = 4), the tetragonal Rb2TeI6 type structure (space group P4/mnc, Z = 2), and the monoclinic K2TeBr6 type structure (space group P21/n, Z = 2). The latter type was found for (NH4)2TeI6 with a = 8.0694(7), b = 8.0926(9), c = 11.7498(8) Å and β = 89.605(8)° and refined to a final Rw of 0.065. From ionic radii ratios the symmetry of the A2MX6 type structures can be predicted including a prediction of low temperature instability (soft mode condensation). Group-subgroup relationships connect the corresponding space groups and the space groups of the high/low temperature polymorphs.