The coordination (hydration) of rare earth ions in aqueous chloride solutions from x-ray diffraction. III. SmCl3, EuCl3, and series behavior

The inner sphere water coordination of Sm3+ and Eu3+ in concentrated (3.23 m) aqueous chloride solutions was obtained from x-ray diffraction measurements at 25 °C. The average water coordination is 8.8 for Sm3+ and 8.3 for Eu3+. The average RE3+–H2O distances are 2.474 Å for Sm3+ and 2.450 Å for Eu3+. The average RE3+⋅⋅⋅Cl− ion pair distances occur near 4.9 Å. A careful examination of the x-ray diffraction results for ten rare earth chloride solutions indicates that the inner sphere water coordination of the rare earth ions in aqueous solutions decreases from nine to eight due to the decreasing rare earth ionic radii. The ions La3+ through Nd3+ are nine coordinated, those between Nd3+ and Tb3+ are transitional between nine and eight, and those from Tb3+ to Lu3+ are eight coordinated.