Delicate, a study of the structural changes in ten-coordinated La(III), Ce(III), Pr(III), Nd(III), Sm(III) and Eu(III) sulfates.

Abstract

We recently presented a new method that allows for a direct structural comparison of coordination complexes. The main difference from other methods is that our AlignIt approach uses common scaling and orientation of the complexes when computing the symmetry deviation, σideal, values. Here, six apparently isostructural lanthanide(III) sulfates K6[(Ln)2(SO4)6]/K5Na[(Ln)2(SO4)6] with ten-coordinated lanthanide(III) sites (Ln(O)10) were prepared, and single-crystal structures were determined and compared using the symmetry deviation (σideal) values. The six structures were shown to fall into two groups: Pr(III) and Eu(III) are identical (σideal = 0.04) bar in size. With a maximum σideal of 0.07, the same was found to be true for La(III), Ce(III), Nd(III), Sm(III) structures. The two groups are shown to be significantly different (σideal > 2.7), yet the coordination geometries of all six are best described as bicapped square antiprisms (σideal = 1.15-1.68). The structures differ in more than symmetry as the smaller lanthanides are shown to crystallize with one sodium and five potassium ions, while the larger lanthanides crystallize with six potassium ions. This does not change the structure and we postulate that the structural variation is due to delicate size matching between alkali and lanthanide ions. For the first six lanthanides, this structure is very robust, which is confirmed as seven doped systems readily crystallized. These doped systems were prepared in order to use europium(III) luminescence as a structural probe. Unsurprisingly, only the doped Eu-La and Eu-Ce systems were found to be luminescent. Between these two and all the europium(III) systems, the intricate structural differences were shown to be enough to change both crystal field splitting and luminescence lifetime. We conclude that even in simple dilution experiments, where luminescent lanthanide(III) ions are introduced in 'innocent' hosts materials, the structure can act as a modulator for the observed properties.