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| Master Thesis | FZJ-2025-02540 |
2025
Jülich
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Please use a persistent id in citations: doi:10.34734/FZJ-2025-02540
Abstract: This thesis investigates the magnetic and structural properties of two batches of single crystals (freshly synthesized and aged) and polycrystalline material of the spin-crossover (SCO) compound [Fe(NQu3)(NCS)2]∙0.5 Et2O (Qu = quinoline and Et = ethyl). Magnetization measurements, carried out on in-house Superconducting Quantum Interference Device (SQUID) and on freshly synthesized single crystals, indicate that the compound exhibits abrupt SCO behaviour with a hysteresis width of 24.47 K. However, on freshly synthesized polycrystalline samples and on aged single crystals, only a paramagnetic signal was obtained across the entire temperature range (20-300 K) (measured on the Quantum Design Dynacool-PPMS (Physical Property Measurement System)), indicating the absence of a SCO. Based on these observations we believe that the particle size and the solvent molecules play a critical role in governing the SCO process.The compound crystallizes in the symmetry C2/c (measured on in-house SuperNova single-crystal diffractometer), with the lattice parameters and volume at room temperature as a = 9.9054(2) Å, b = 22.3828(7) Å, c = 25.8754(8) Å, β = 95.5840(20)°, and V = 5709.63(45) Å3. For both batches of single crystals, the symmetry was found to remain unchanged across the entire investigated temperature range (95-300 K). Based on the electron density map, the potential positions of the solvent molecules in freshly synthesized single crystals were identified. However, the electron density of solvent molecules at the same positions disappeared in aged single crystals.The fresh samples show a SCO transition temperature (T1/2) at 138 K in the cooling process and a temperature of 162.5 K in the warming process. The lattice parameters a and c, along with the angle β shrink significantly from HS to LS state, while the lattice parameter b expands. The FeN6 octahedron contracts non-uniformly from HS to LS state, in which the Fe-N bonds are shortened by approximately 0.12 Å to 0.32 Å. Additionally, we observed a gradual increase in mosaicity when cooling down the crystal.The results on the fresh crystals are compared and contrasted with the single crystal diffraction data of the “aged” single crystals, which do not exhibit a SCO transition, but instead show a smooth contraction of the unit cell volume with decreasing temperature. The compound [Fe(NQu3)(NCS)2]∙0.5 Et2O is an example of a spin crossover compound where the occurrence of the transition seems to be entirely governed by the concentration of solvent molecules. This serves as an excellent entry point for understanding and investigating the solvent-driven SCO phenomenon.
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