Ion transport mechanism in anhydrous lithium thiocyanate LiSCN part II: frequency dependence and slow jump relaxation.

Specific aspects of the Li+ cation conductivity of anhydrous Li(SCN) are investigated, in particular the high migration enthalpy of lithium vacancies. Close inspection of impedance spectra and conductivity data reveals two bulk relaxation processes, with comparatively fast ion transport at high frequencies and slow ion migration at low frequencies. The impedance results are supported by solid state nuclear magnetic resonance (ssNMR), and pair distribution function (PDF) analysis. This behavior reflects a frequency dependent conductivity, which is related to the extremely slow thiocyanate (SCN)- anion lattice relaxation that occurs when a Li+ cation jumps to the next available site. Two possible migration models are proposed: the first model considers an asymmetric energy landscape for Li+ cation hopping, while the second model is connected to the jump relaxation model and allows for 180° rotational disorder of the (SCN)- anion. A complete kinetic analysis for the hopping of Li+ cations is presented, which reveals new fundamental insights into the ion transport mechanism of materials with complex anions.

Ion transport mechanism in anhydrous lithium thiocyanate LiSCN part III: charge carrier interactions in the premelting regime.

In lithium thiocyanate Li(SCN), the temperature regime below the melting point (274 °C) is characterized by excess conductivities over the usual Arrhenius behavior (premelting regime). Here, the Schottky defect pair concentration is high, and the point defect chemistry can no longer be considered as dilute. Coulomb interactions of Schottky pairs are expected to occur lowering the formation energy of new carriers and hence leading avalanche-like to a transition into a fully defective superionic state. The respective non-linear behavior is investigated using the cube-root law approach characterized by a defect interaction parameter J, which is a measure of the effective defect-lattice energy. In the case of Li(SCN), the rather pronounced volume expansion is to be included in the model. A literature comparison with other materials emphasizes to what degree defect formation as well as defect interactions depend not only on the dominant mobile defect, but also on the respective sublattice. Overall, a quantitative description of the defect chemistry of Li(SCN) in the premelting regime is derived.

Ion transport mechanism in anhydrous lithium thiocyanate LiSCN Part I: ionic conductivity and defect chemistry.

This work reports on the ion transport properties and defect chemistry in anhydrous lithium thiocyanate Li(SCN), which is a pseudo-halide Li+ cation conductor. An extensive doping study was conducted, employing magnesium, zinc and cobalt thiocyanate as donor dopants to systematically vary the conductivity and derive a defect model. The investigations are based on impedance measurements and supported by other analytical techniques such as X-ray powder diffraction (XRPD), infrared (IR) spectroscopy, and density functional theory (DFT) calculations. The material was identified as Schottky disordered with lithium vacancies being the majority mobile charge carriers. In the case of Mg2+ as dopant, defect association with lithium vacancies was observed at low temperatures. Despite a comparably low Schottky defect formation enthalpy of (0.6 ± 0.3) eV, the unexpectedly high lithium vacancy migration enthalpy of (0.89 ± 0.08) eV distinguishes Li(SCN) from the chemically related lithium halides. A detailed defect model of Li(SCN) is presented and respective thermodynamic and kinetic data are given. The thiocyanate anion (SCN)- has a significant impact on ion mobility due to its anisotropic structure and bifunctionality in forming both Li-N and Li-S bonds. More details about the impact on ion dynamics at local and global scale, and on the defect chemical analysis of the premelting regime at high temperatures are given in separate publications (Part II and Part III).

Synthesis and characterisation of two lithium-thiocyanate solvates with tetrahydrofuran: Li[SCN]·THF and Li[SCN]·2THF.

Li[SCN]·THF and Li[SCN]·2THF can be obtained from solutions of anhydrous Li[SCN] in tetrahydrofuran (C4H8O, THF). Both compounds are very hygroscopic and slowly decompose even at room temperature. At ambient conditions Li[SCN]·THF crystallizes in the monoclinic space group P21/c with the lattice parameters a = 574.41(2), b = 1643.11(6), c = 830.15(3) pm and ß = 99.009(1)° for Z = 4 as determined by laboratory X-ray powder diffraction. Its crystal structure contains Li+ cations surrounded by one THF molecule and three thiocyanate anions [SCN]- forming {Li[NCS]2[SCN](OC4H8)}2- tetrahedra, which join together as pairs via shared N⋯N edges. CHNS combustion analysis and vibrational spectroscopy confirmed its composition, whereas differential scanning calorimetry and thermogravimetric analysis coupled with a mass spectrometer were applied to record its thermal behaviour. Li[SCN]·2THF crystallises in a primitive monoclinic lattice as well, but in the space group P21/n with the lattice parameters a = 1132.73(3), b = 1637.98(3), c = 1264.88(2) pm and ß = 94.393(2)° for Z = 8 as determined from single-crystal X-ray diffraction data at 100 K. Its structure contains two crystallographically independent Li+-centred tetrahedra {Li[NCS]2(OC4H8)2}-, which form dimers {(C4H8O)2Li[µ2-NCS]2Li(OC4H8)2} via shared N⋯N edges. They are merely stabilised by weak agostic H⋯S interactions between some CH2-groups of the C4H8O molecules and the [NCS]- ligands.

Synthesis, characterization and thermal behaviour of solid phases in the quasi-ternary system Mg(SCN)2-H2O-THF.

Mg(SCN)2·4H2O can be converted into previously unknown compounds Mg(SCN)2·(4 - x) H2O·xTHF with x = 0, 2 and 4 by multiple recrystallization in tetrahydrofuran (THF). The phases were characterized by infrared spectroscopy (IR), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), and their crystal structures were solved from X-ray powder diffraction (XRPD) data. In the crystal structures isolated Mg(NCS)2(H2O)4-x(THF)x units form layered motifs. The thermal behavior of Mg(SCN)2·4H2O and Mg(SCN)2·4THF was investigated by temperature dependent in situ XRPD, where Mg(SCN)2·4THF was found to acquire a room temperature (α-form) and high temperature modification (ß-form). The phase transformation is associated with an order-disorder transition of the THF molecules and with a reversion of the stacking order of the layered motifs. Further heating eventually leads to the formation of Mg(SCN)2·2THF. There thiocyanate related sulfur atoms fill the voids in the coordination sphere of magnesium, which leads to the formation of one dimensional electroneutral ∞[Mg(NCS)2/2(SCN)2/2(THF)2] chains. All investigated Mg(SCN)2·(4 - x) H2O·xTHF phases exhibit a remarkable anisotropic thermal expansion, and Mg(SCN)2·4H2O and Mg(SCN)2·2THF were found to show both positive and negative thermal expansion coefficients.