Precipitation of ALn(CO3)2,xH2OALn(CO3)2,xH2O and Dy2(CO3)3,xH2ODy2(CO3)3,xH2O compounds from aqueous solutions for A+=Li+,Na+,K+,Cs+,NH4+ and Ln3+=La3+,Nd3+,Eu3+,Dy3+

Double carbonates of lanthanide (Ln) and alkaline or ammonium (A  ) ions, noted ALn(CO3)2,xH2OALn(CO3)2,xH2O, were precipitated from concentrated A2CO3A2CO3 aqueous solutions at room temperature and atmospheric pressure. Twelve hydrated compounds out of the twenty targeted ones have been obtained: Li(NdorEu)(CO3)2, NaLa(CO3)2NaLa(CO3)2, KNd(CO3)2,xH2OKNd(CO3)2,xH2O, Cs(LaorNd)(CO3)2, NH4(Nd,EuorDy)(CO3)2, Dy2(CO3)3Dy2(CO3)3 from concentrated A2CO3A2CO3 solutions and Na(Nd,EuorDy)(CO3)2 from concentrated AHCO3AHCO3 solutions. Although the trivalent lanthanide ions are often considered as analogs in solution, differences in their precipitation behaviour was observed, which is believed to have a kinetic origin in relation to the small differences in their ionic radii. The solid compounds were characterised by elemental analyses, thermogravimetry (TG), X-ray diffraction (XRD) and scanning electron microscope-energy dispersive spectroscopy (SEM-EDS). The powder diffraction patterns of nine solids were fitted using the tetragonal P4/mmmLau¨e class: LiNd(CO3)2,xH2OLiNd(CO3)2,xH2O: a=(12.16±0.02)A˚, c=(9.21±0.02)A˚, LiEu(CO3)2,3H2OLiEu(CO3)2,3H2O: a=(12.201±0.007)A˚, c=(9.23±0.01)A˚, KNd(CO3)2,xH2OKNd(CO3)2,xH2O: a=(13.28±0.04)A˚, c=(10.00±0.04)A˚, CsLa(CO3)2,xH2OCsLa(CO3)2,xH2O: a=(10.82±0.02)A˚, c=(8.18±0.02)A˚, CsNd(CO3)2,xH2OCsNd(CO3)2,xH2O: a=(10.81±0.07)A˚, c=(8.16±0.07)A˚ for NaLn(CO3)2,xH2ONaLn(CO3)2,xH2O: a=(11.10+1.75rLn3+)A˚ and c=(8.60+1.13rLn3+)A˚, where rLn3+rLn3+ is the ionic radius of Ln3+Ln3+ for a coordination number of 8 (rLa3+=1.16A˚, rNd3+=1.12A˚, rEu3+=1.07A˚ and rDy3+=1.03A˚). It is proposed that all the NaLn(CO3)2,xH2ONaLn(CO3)2,xH2O compounds are of very similar structure, as evidenced by their XRD patterns and by the linear variations of the lattice parameters with rLn3+rLn3+. The small differences in the lattice parameters can induce large modification of the precipitation pathways. Conversely, structural changes were evidenced within the A+A+ series for ANd(CO3)2,xH2OANd(CO3)2,xH2O. Dy2(CO3)3,xH2ODy2(CO3)3,xH2O was also obtained as a by-product. Its lattice parameters are in good agreement with Eu2(CO3)3,3H2OEu2(CO3)3,3H2O ones.

Linear relationships between the ionic radii of the lanthanides and the unit cell parameters of the corresponding NaLn(CO3)2,xH2ONaLn(CO3)2,xH2O compounds, extracted from three different studies (our experimental data (black) are compared with those of Mochizuki et al. (red) and Runde et al. (blue)).Figure optionsDownload as PowerPoint slide