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On the crystal structures of lithium thiocyanate monohydrate LiSCN ⋅ 1 H2O and the phase diagram LiSCN – H2O
Journal of Physics and Chemistry of Solids  (IF3.995),  Pub Date : 2021-08-04, DOI: 10.1016/j.jpcs.2021.110299
Markus Joos, Maurice Conrad, Sebastian Bette, Rotraut Merkle, Robert E. Dinnebier, Thomas Schleid, Joachim Maier

Hydration of anhydrous LiSCN by stoichiometric amounts of water vapor leads to the formation of LiSCN $\cdot$ 1 H2O. At room temperature, LiSCN $\cdot$ 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, $\beta$ = 96.147(6) ° and V = 761.32(2) Å³ (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) Å³. 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 ${p}_{{\mathrm{H}}_{2}\mathrm{O}}$, differential scanning calorimetry (DSC) and direct measurements of equilibrium ${p}_{{\mathrm{H}}_{2}\mathrm{O}}$. The results indicate that LiSCN $\cdot$ 1 H2O is a special case when compared to LiSCN $\cdot$ 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 $\cdot$ n H2O hydrates.