Precipitation of [formula omitted] and [formula omitted] compounds from aqueous solutions for [formula omitted] and [formula omitted

Abstract

Double carbonates of lanthanide ( Ln) and alkaline or ammonium ( A) ions, noted ALn ( CO 3 ) 2 , x H 2 O , were precipitated from concentrated A 2 CO 3 aqueous solutions at room temperature and atmospheric pressure. Twelve hydrated compounds out of the twenty targeted ones have been obtained: Li ( Nd or Eu ) ( CO 3 ) 2 , NaLa ( CO 3 ) 2 , KNd ( CO 3 ) 2 , x H 2 O , Cs ( La or Nd ) ( CO 3 ) 2 , NH 4 ( Nd , Eu or Dy ) ( CO 3 ) 2 , Dy 2 ( CO 3 ) 3 from concentrated A 2 CO 3 solutions and Na ( Nd , Eu or Dy ) ( CO 3 ) 2 from concentrated A HCO 3 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 P 4 / mmm La u ¨ e class: LiNd ( CO 3 ) 2 , x H 2 O : a = ( 12.16 ± 0.02 ) A ˚ , c = ( 9.21 ± 0.02 ) A ˚ , LiEu ( CO 3 ) 2 , 3 H 2 O : a = ( 12.201 ± 0.007 ) A ˚ , c = ( 9.23 ± 0.01 ) A ˚ , KNd ( CO 3 ) 2 , x H 2 O : a = ( 13.28 ± 0.04 ) A ˚ , c = ( 10.00 ± 0.04 ) A ˚ , CsLa ( CO 3 ) 2 , x H 2 O : a = ( 10.82 ± 0.02 ) A ˚ , c = ( 8.18 ± 0.02 ) A ˚ , CsNd ( CO 3 ) 2 , x H 2 O : a = ( 10.81 ± 0.07 ) A ˚ , c = ( 8.16 ± 0.07 ) A ˚ for Na Ln ( CO 3 ) 2 , x H 2 O : a = ( 11.10 + 1.75 r Ln 3 + ) A ˚ and c = ( 8.60 + 1.13 r Ln 3 + ) A ˚ , where r Ln 3 + is the ionic radius of Ln 3 + for a coordination number of 8 ( r La 3 + = 1.16 A ˚ , r Nd 3 + = 1.12 A ˚ , r Eu 3 + = 1.07 A ˚ and r Dy 3 + = 1.03 A ˚ ). It is proposed that all the Na Ln ( CO 3 ) 2 , x H 2 O compounds are of very similar structure, as evidenced by their XRD patterns and by the linear variations of the lattice parameters with r Ln 3 + . The small differences in the lattice parameters can induce large modification of the precipitation pathways. Conversely, structural changes were evidenced within the A + series for A Nd ( CO 3 ) 2 , x H 2 O . Dy 2 ( CO 3 ) 3 , x H 2 O was also obtained as a by-product. Its lattice parameters are in good agreement with Eu 2 ( CO 3 ) 3 , 3 H 2 O ones.