Crystal Structure of K2[C2O6]—First Proof of Existence and Constitution of a Peroxodicarbonate Ion This work was supported by the Deutsche Forschungsgemeinschaft (DFG) and the Fonds der Chemischen Industrie. Research was carried out in part at the National Synchrotron Light Source at Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Division of Materials Sciences and Division of Chemical Sciences. The SUNY X3 beamline at NSLS is supported by the Division of Basic Energy Sciences of the US Department of Energy under Grant No. DE-FG02-86ER45231. Work at Stony Brook was partially supported by the National Science Foundation. We thank Michael Knapp from beamline B2 (Hasylab) for the measurements at Hasylab.