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

• Published in: CrystEngComm 2018-01, Vol.20 (26), p.3772-3783
• Main Authors: Pasechnik, L A, Tyutyunnik, A P, Enyashin, A N, Samigulina, R F, Ilyina, E A, Skachkov, V M, Medyankina, I S, Yatsenko, S P
• Format: Article
• Language: English
• Subjects: Ammonium sulfate; Crystal structure; Diffraction; Electronic structure; Infrared spectroscopy; Mathematical analysis; Phase transitions; Polymorphism; Scandium compounds; Scandium oxides; Scanning electron microscopy; Temperature dependence; Thermal dissociation; Thermal expansion; Unit cell; X ray powder diffraction
• ISSN: 1466-8033
• DOI: 10.1039/c8ce00593a

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) Å3) 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) Å3). 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.