Polymorphism and properties of ammonium scandium sulfate (NH4)3Sc(SO4)3: new intermediate compound in scandium production

Abstract

Anhydrous complex scandium–ammonium sulfate (NH₄)₃Sc(SO₄)₃ – one of the intermediate compounds in technologically feasible production of scandium oxide – has been prepared using a hydrothermal route. The sample was characterized by X-ray powder diffraction (XRPD), IR spectroscopy, scanning electron microscopy, and thermal and elemental analysis. The high-temperature XRPD and thermal studies revealed an enantiotropic first-order phase transition at 81.6 °C and standard pressure. In the temperature region of 80–100 °C, the low-temperature stable monoclinic (P21/c, LT, CSD 434420) polymorph (unit cell parameters: a = 9.0524(1) Å, b = 15.4622(2) Å, c = 9.21650(6) Å, β = 108.4263(6)°, V = 1223.89(2) ų) coexists with the high-temperature rhombohedral (R3c, HT, CSD 434422) polymorphic modification (a = 15.4435 (3) Å, c = 9.2305(2) Å, V = 1906.55(6) ų). According to the XRPD and IR measurements, the crystal structure of both LT and HT modifications consists of infinite columns of the formed ScO₆ and SO₄ polyhedra, although with different variations of intercolumnar clearance occupied by ammonium cations. The temperature-dependent transformation of the unit cell is accompanied by variations in the thermal expansion coefficients. The decomposition of HT modification to Sc₂O₃ proceeds in both air and Ar atmosphere in 3 stages according to thermal dissociation. The DFT calculations confirm the relative stability of (NH₄)₃Sc(SO₄)₃ polymorphs and predict the wide band gaps in their electronic structure.