The low-temperature structure of Tl 2SeO 4 at [formula omitted

Abstract

The structure of Tl 2SeO 4 at 293, 100 and 30 K has been determined. Space group at 293 and 100 K is Pmcn, Z=4, lattice parameters at 293 K are a=6.0838(12)(100 K : 6.0336(19)), b=10.965(3) ( 100 K : 10.903(5)) and c=7.9385(15) A ̊ (100 K : 7.921(2)). At 30 K space group is P2 12 12 1, Z=4, with lattice parameters a=6.2333(17), b=10.533(3) and c=7.828(2). Measurement was carried out with a Stoe IPDS ( 293 K ) and a Nonius CCD using an Oxford He-cryostat for cooling (100 and 30 K ). Final agreement factors at 293 K after refinement with the program SHELXL97 are R( F)=0.061 ( 100 K : 0.097), wR(F ∗∗2)=0.051 ( 100 K : 0.156) for 566(100 K : 678) unique reflections and R( F)=0.039, wR(F ∗∗2)=0.097 for 1162 unique reflections at 30 K . The high-temperature phase is isotypical to the β-K 2SO 4 structure, characterized by isolated selenate tetrahedra and two different cation sites with coordination number 9 and 11. In the low-temperature phase, the tetrahedra are rotated and the coordination numbers of the Tl + ions are reduced to 8 and 10, respectively. Structural changes are similar to the ones observed in high- and low-temperature phases of Tl 2MoO 4. A comparison to other Tl 2 TX 4 compounds shows that the structural instabilities are closely connected to shortest Tl–O distances.