The low-temperature structure of Tl2SeO4 at 30 K

The structure of Tl2SeO4 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 P212121, 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-K2SO4 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 Tl2MoO4. A comparison to other Tl2TX4 compounds shows that the structural instabilities are closely connected to shortest Tl-O distances