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Structure and thermolysis of NiC 2 O 4 ·2H 2 O ( I ) and Ni(HCO 2 ) 2 ·2H 2 O ( II ) were studied for the precipitated powders from the water solutions of precursors. The compound ( I ) has a disordered structure ( C2/c , a =11.7916(5), b = 5.31798(14), c = 9.7806(7) Å, β = 127.014(6)°, V= 489.73(4)Å 3 ) which is described via the displacement vector applied to the basis atoms and the part of the shifted atoms. Thermal dehydration of ( I ) leads to formation of the disordered β-NiC 2 O 4 ( III ), the structure of which ( P21/n , a =5.8294(9), b = 5.1685(13), c = 5.2712(4) Å, β = 117.793(12)°, V= 140.50(5)Å 3 ) was refined from the powder diffraction. Oxalates ( I ) and ( III ) are both characterized by the essential part of the shifted atoms causing the extinction of some lines in XRPD pattern. The final product of thermal decomposition of ( I ) in air is NiO, whereas thermolysis of ( II ) leads to formation of Ni/NiO mixture. The content of Ni in the latter depends on the thickness of the decaying layer in the sample because the gaseous products of the reaction prevent the oxidation of the nickel formed initially. [ABSTRACT FROM AUTHOR]