Structure and Thermal Expansion of Urea-Oxalic Acid (1:1)

Urea-oxalic acid, CH4N20.C2H204, mono- clinic, space group C2/c, a = 13.0625 (7), b = 6.6437 (2), c = 6.8478 (3) .A, fl = 92.474 (6) ° (at 295 (1) K), Z = 4, d c = 1.68 Mg m -a, R w = 5.9%. The structure consists of layers of urea and oxalic acid molecules held together by hydrogen bonds. The thermal-expansion tensor has been calculated from the changes of the lattice constants with temperature. The elements of the thermal-expansion tensor at 295.1 K are a~ = -1 (5), a 2 = 29 (6) and tt3 = 199 (10) × 10 -6 K-I.