On the crystal structures of lithium thiocyanate monohydrate LiSCN [formula omitted] 1 H2O and the phase diagram LiSCN – H2O

Abstract

Hydration of anhydrous LiSCN by stoichiometric amounts of water vapor leads to the formation of LiSCN ⋅ 1 H2O. At room temperature, LiSCN ⋅ 1 H2O crystallizes in the monoclinic space group C2/m with a = 15.0271(3) Å, b = 7.5974(1) Å, c = 6.7070(1) Å, β = 96.147(6) ° and V = 761.32(2) ų (α-phase). At 49 °C, the material transforms into a high temperature β-phase with the orthorhombic space group Pnam and a = 13.2258(2) Å, b = 7.0619(9) Å, c = 8.1663(1) Å and V = 762.72(2) ų. Structural and thermodynamic investigations were conducted using infrared (IR) spectroscopy, ex situ and in situ X-ray powder diffraction (XRPD) while varying temperature or water partial pressure pH2O, differential scanning calorimetry (DSC) and direct measurements of equilibrium pH2O. The results indicate that LiSCN ⋅ 1 H2O is a special case when compared to LiSCN ⋅n H2O (n = 0 or 2), both regarding crystal structure and hydration thermodynamics. These results are combined with literature date into an updated phase diagram of the LiSCN – H2O system, showing a detailed picture of the phase formation behavior of the LiSCN ⋅n H2O hydrates.