Crystal structures and phase transition of Rb 2TeBr 6 (300-12.5 K)

Abstract

Using 298 and 160(3) K diffractometer intensity data the structure of Rb 2TeBr 6 has been determined by single crystal X-ray technique and refined to a final R w of 0.041 and 0.037, respectively (K 2PtCl 6 type structure, space group Fm3 m, Z = 4 with a 298 = 10.773(4) Å and a 160 = 10.713(6) Å). The powder diffraction pattern from 300 to 12.5 K was recorded by a low-temperature Guinier diffractometer and camera. Below 45(5) K a second-order phase transition leads to a tetragonally distorted structure corresponding to a softening of the T 1 g ( Γ) rotary phonon. The powder pattern measured with the diffractometer at 12.5 K was used for structural refinement ( R = 0.092). This low-temperature phase shows a ferrorotative displacement of the TeBr 6-octahedra with a tilt angle of 4.7(1) deg. (space group I4 m , Z = 2, a = 7.4726(3) Å and c = 10.7008(5) Å). The structural results indicate that there is no stereochemical activity of the lone pair electrons at Te(IV) even at very low temperatures. Phase transitions of Rb 2TeBr 6 and Rb 2TeI 6 (A 2g( X)-condensation) are compared. The results of a lattice dynamic calculation shows that only the T 1 g ( Γ) condensation can be explained with a rigid ion model.