Zirconium Oxalates: Zr(OH)₂(C₂O₄), (H₁₁O₅)₂[Zr₂(C₂O₄)₅(H₂O)₄], and MM′[Zr(C₂O₄)₃]·<i>x</i>H₂O with M and M′ = Ammonium, Alkali Metal, and Hydroxonium Ion H_2<i>n</i>+1O_<i>n</i>⁺ (<i>n</i> = 2, 3, 4)

Crystallized powder of dihydroxide zirconium oxalate Zr­(OH)2(C2O4) (ZrOx) was obtained by precipitation and the structure determined from powder X-ray data. The three-dimensional (3D) framework observed in (ZrOx) results from the interconnection of zirconium hydroxide chains 1∞[Zr­(OH)2]2+ and zirconium oxalate chains 1∞[{Zr­(C2O4)}2+]. Single crystals of (H11O5)2[Zr2(C2O4)5(H2O)4] (H2Zr2O5) were obtained by evaporation. The structure contains dimeric anions [Zr2(C2O4)5(H2O)4]2– connected through hydrogen bonds to hydroxonium ions (H11O5)+ to create a 3D supramolecular framework. The addition of ammonium or alkali nitrate led to the formation of single crystals of Na2[Zr­(C2O4)3]·2H2O (Na2ZrOx3), M­(H7O3)­[Zr­(C2O4)3]·H2O, M = K (KHZrOx3), M = NH4 (NH4HZrOx3), M­(H5O2)0.5(H9O4)0.5[Zr­(C2O4)3], M = Rb (RbHZrOx3), and M = Cs (CsHZrOx3). For the five compounds, the structure contains ribbons 1∞[{ZrOx3}2–] formed by entities Zr­(C2O4)4 sharing two oxalates. In (Na2ZrOx3), the shared oxalates are in cis positions and the chain 1∞[Zr–Ox] is stepped with a Zr–Zr–Zr angle of 98.27(1)°. In the other compounds, the shared oxalates are in trans positions and the chains 1∞[Zr–Ox] are corrugated with Zr–Zr–Zr angles in the range 140.34(1)–141.07(1)°. In the compounds (MHZrOx3), the cohesion between the ribbons is ensured by the alkaline or ammonium cations and the hydroxonium ions (H7O3)+ for M = K, NH4, (H5O2)+, and (H9O4)+ for M = Rb and Cs. During the thermal decomposition of the alkaline-free zirconium oxalates (ZrOx), (H2Zr2Ox5), and (NH4HZrOx3), the formed amorphous zirconia is accompanied by carbon; the oxidation of carbon at about 540 °C to carbon dioxide is concomitant with the crystallization of the stabilized tetragonal zirconia.