Temperature-Induced Polymorphism of LiSc(SO ₄ ) ₂ ·2H ₂ O Allows the Control of the Yield of Scandium during Precipitation from Diluted Aqueous Solutions

Two polymorphs of lithium-scandium sulfate hydrate LiSc(SO₄)₂·2H₂O are isolated by chimie douce crystallization. Neutron- and X-ray powder diffraction data unveil their monoclinic shape (sp.gr. C2/c, m109-LSS) and orthorhombic lattices (sp.gr. P2₁2₁2₁, o-LSS). An elevated temperature during prolonged crystallization yields solely o-LSS. Thermal dehydration of both polymorphs to an anhydrous state proceeds as a lineage of intermediates LiSc(SO₄)₂·(2 - x)H₂O identified for the first time by a combination of high-temperature neutron- and X-ray powder diffractions, thermal analysis, and vibrational and NMR spectroscopies. DFT calculations establish the phase priority during dehydration and the inevitable coexistence of different intermediates. Remarkably, heating up to 50 °C or X-ray radiation stimulates a reversible phase transition from m109-LSS to another monoclinic phase m107-LSS (sp.gr. C2/c). NMR data confirm that the lattices of both monoclinic as well as orthorhombic polymorphs possess a different coordination environment of water molecules and lithium ions in the space between the scandium sulfate layers. A greater strength of hydrogen bonds of "rigidly bound" water molecules is obtained within orthorhombic structure than that within a monoclinic one.