Effect of Inclining Strain on the Crystal Lattice along an Extended Series of Lanthanide Hydroxysulfates Ln(OH)SO4(Ln = Pr−Yb, except Pm)

A series of trivalent lanthanide hydroxysulfates, Ln(OH)SO4, (Ln = Pr through Yb, except radioactive Pm) has been synthesized via hydrothermal methods from Ln2(SO4)3·8H2O by reaction with aqueous NaOH at 170 °C in Teflon lined Parr steel autoclaves, and were characterized by single crystal X-ray diffraction and FT-IR spectroscopy. Two types of arrangements were found in the solid state. The lighter (Ln = Pr−Nd, Sm−Gd) and heavier lanthanide(III) hydroxysulfates (Tb−Yb) are each isostructural. Both structure types exhibit the monoclinic space group P21/n, but the unit cell content is doubled with two crystallographically distinct LnO8polyhedra for the heavier lanthanide compounds. The lighter complexes maintain the coordination number 9, forming a three-dimensional extended lattice. The heavier counterparts exhibit the coordination number 8, and arrange as infinite columns of two crystallographically different LnO8polyhedra, while extending along the “c” axis. These columns of LnO8polyhedra are surrounded and separated by six columns of sulfate ions, also elongating in the “c” direction. The rigid sulfate entities seem to obstruct the closing in of the lighter LnO9polyhedra, and show an inclining degree of torsion into the “ac” layers. The crystal lattice of the lighter 4f complexes can sufficiently withstand the tension buildup, caused by the decreasing Ln3+radius, up to Gd(OH)SO4. The energy profile of this structural arrangement then seems to exceed levels at which this structure type is favorable. The lattice arrangement of the heavier Ln-analogues seems to offer a lower energy profile. This appears to be the preferred arrangement for the heavier lanthanide hydroxysulfates, whose crystal lattice exhibits more flexibility, as the coordination sphere of these analogues is less crowded. The IR absorbance frequencies of the hydroxide ligands correlate as a function of the Ln3+ionic radius. This corresponds well with the X-ray single crystal analysis data.