Au<sub>2</sub>(SeO<sub>3</sub>)<sub>2</sub>(SeO<sub>4</sub>):  Synthesis and Characterization of a New Noncentrosymmetric Selenite−Selenate

The reaction of elemental gold and selenic acid in Teflon-lined steel autoclaves leads to orange-yellow single crystals of Au<sub>2</sub>(SeO<sub>3</sub>)<sub>2</sub>(SeO<sub>4</sub>) (orthorhombic, <i>Z</i> = 4, <i>Cmc</i>2<sub>1</sub> (No. 36), <i>a</i> = 1689.1(3) pm, <i>b</i> = 630.13(8) pm, <i>c</i> = 832.7(1) pm, <i>V</i> = 886.2(2) Å<sup>3</sup>, <i>R</i><sub>all</sub> = 0.0452). In the crystal structure, Au<sup>3+</sup> is surrounded by four oxygen atoms of just as many monodentate SeO<sub>3</sub><sup>2-</sup> ions in a square planar manner. The linkage of the polyhedra leads to double chains in the [001] direction which are connected to puckered layers by SeO<sub>4</sub><sup>2-</sup> groups. The noncentrosymmetric space group could be proved by the observation of an SHG effect upon irridation at 1064 nm that shows an efficiency of about 43% compared to a KDP reference. Upon heating, Au<sub>2</sub>(SeO<sub>3</sub>)<sub>2</sub>(SeO<sub>4</sub>) decomposes at about 370 °C in one step yielding elemental gold. The presence of selenite and selenate groups in the compounds is also obvious from the IR and Raman spectra which show the characteristic bands of both species. Furthermore, solid-state NMR spectra reveal the different surroundings of the selenium atoms in the compound.