Synthesis, crystal structure, and solid state phase transition of Te4[AsF6]2•SO2

The oxidation of tellurium with AsF5 in liquid SO2 yields Te-4(2+)[AsF6-](2) which can be crystallized from the solution in form of dark red crystals as the SO2 solvate. The crystals are very sensitive against air and easily lose SO2, so handling under SO2 atmosphere or cooling is required. The crystal structure was determined at ambient temperature, at 153 K, and at 98 K. Above 127 K Te-4[AsF6](2)(SO2)-S-. crystallizes orthorhombic (Pnma, a = 899.2(1), b = 978.79(6), c = 1871.61(1) pm, V = 1647.13(2)(.)10(6) pm(3) at 297 K, Z = 4). The structure consists of square-planar Te-4(2+) ions (Te-Te 266 pm), octahedral [AsF6](-) ions and of SO2 molecules which coordinate the Te-4 rings with their O atoms in bridging positions over the edges of the square. At room temperature one of the two crystallographically independent [AsF6](-) ions shows rotational disorder which on cooling to 153 K is not completely resolved. At 127 K Te-4[AsF6](2)(SO2)-S-. undergoes a solid state phase transition into a monoclinic structure (P112(1)/a, a = 866.17(8), b = 983.93(5), c = 1869.10(6) pm, gamma = 96.36(2)degrees, V = 1554,2(2)(.)10(6) pm(3) at 98 K, Z = 4). All [AsF6](-) ions are ordered in the low temperature form. Despite a direct supergroup-subgroup relationship exists between the space groups, the phase transition is of first order with discontinuous changes in the lattice parameters. The phase transition is accompanied by crystal twinning. The main difference between the two structures lies in the different coordination of the Te-4(2+) ion by O and F atoms of neighbored SO2 and [AsF6](-) molecules.