Is it potassium peroxodicarbonate
C<sub>2</sub>O<sub>6</sub><sup>2−</sup>
"Rubidium peroxodicarbonate is a light blue crystalline solid that decomposes at 424 K. Its structure was published in 2003. In both salts, each of
the two carbonate units is planar. In the rubidium salt the whole molecule is planar, whereas in the potassium salt the two units lie on different and
nearly perpendicular planes, both of which contain the O-O bond."
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