STRUCTURE DETERMINATION OF K2ZNBR4 AT 291-K AND 144-K

The room-temperature phase of K2ZnBr4 is isomorphous with Sr2GeS4 (P2(1)/m) while the low-temperature structure (P2(1)) is slightly distorted [the phase transition occurs at 155 K]. Both structures contain highly deformed tetrahedral [ZnBr4]2- molecules with Br(3)-Zn-Br(3') angles of 103.06(5) and 102.49 (9)degrees at 291 and 144 K, respectively. This distortion is caused by the repulsion of Br atoms whose distance 3.712 (1) and 3.661 (2) angstrom at 291 and 144 K, respectively, is below the Br-Br van der Waals distance (3.9 angstrom). The phase transition is accompanied by minor shifts of cations and [ZnBr4]2- tetrahedra which are simultaneously rotated about a small angle. Below the phase transition point an inversion twin develops whose twin-fraction parameter was refined to 0.459 (65).