The conversion of La2O3 with HCl, subsequent neutralization and precipitation using (NH4)2CO3 in an industrial process leads to the previously unknown La2(CO3)3 ⋅ 5H2O. The phase was characterized by X-ray powder diffraction leading to an averaged crystal structure model, thermal analysis, vibrational and optical spectroscopy. La2(CO3)3 ⋅ 5H2O crystallizes in space group Pbca(61) with a=9.0167(4) Å, b=12,842(3) Å, c=9.6331(4) Å and V=1115.4(3) Å3. The structure consists of (Formula presented.) zig zag layers and a highly disordered interlayer space occupied by water molecules and carbonate anions. Partially occupied water-related oxygen positions theoretically allow for a flexible hydrate water content La2(CO3)3 ⋅ xH2O, with x ranging from 1.5 to 5.0. Although a strong structural relationship between La2(CO3)3 ⋅ 5H2O and La2(CO3)3 ⋅ 8H2O and similar substance properties were found, the title compound cannot be obtained by partial dehydration of the octahydrate. Instead, both lanthanum carbonate hydrates transform into strongly disordered La2(CO3)3 in a one-step dehydration process.
