Forschung

Solide-State Chemistry

The synthesis of solids has many facets. Methods range from brute force, e. g. high temperature and high pressure, to soft chemistry, chimie douce, at ambient conditions, and utilize multifarious techniques. They comprise solid-state and solid-gas reactions, chemical vapor deposition and transport, solvothermal and precursor methods, topotactic reactions, melt methods, self-propagating reactions and precipitation from solution, just to name a few. Most of them are used in an explorative way to synthesize new compounds, which may or may not have useful properties to be exploited in applied research. This genetic-like approach to functional materials is in fact often more effective than planned syntheses to materials with desired properties. Synthesis planning is hampered by the fact that in general little is known about the basic steps of the formation of solids. Therefore, in situ studies revealing reaction pathways are highly desirable and yield a wealth of information useful for planning and optimization of preparation procedures. They provide a feedback loop between synthesis and characterization (see scheme), which enhances the effectivity of solid-state syntheses.

Our research group uses explorative approaches to aim at new solid compounds and functional materials. Where appropriate, we try to get a detailed understanding of reaction pathways at the level of the most basic steps of the formation of solids by in situ methods (X-ray and neutron diffraction, thermal analysis, Raman spectroscopy). In situ methods are also applied for the improvement of common synthesis reactions and technical processes. In the following, a few examples highlight the current progress.

In situ neutron powder diffraction on solid-gas reactions

Solid-gas reactions play a crucial role in many technologically important processes such as ore smelting, heterogeneous catalysis, synthesis of solids, hydrogen storage, and corrosion of metals and alloys. They are thus key for the production, use and wear of many functional materials. The study of such reactions is technically demanding due to the oftentimes harsh conditions. Neutron diffraction is ideally suited to serve as a probe for such reactions because common gases, e. g. H2, N2, O2, H2O, CO, CO2, contain light elements, which can often be better located in crystal structures by neutron as compared to X-ray diffraction. Moreover, the bulky sample environment for maintaining in situ conditions is easier to penetrate by neutrons than by X-rays due to the smaller absorption coefficients for most elements. Further, neutrons can probe many properties of materials, e. g. crystal structure, diffusion, magnetism, vibrational properties, and thus yield extensive information. For time-resolved investigation of solid-gas reactions, we have constructed a gas pressure cell for elastic neutron diffraction (Fig. 1). By proper orientation of a single-crystal sapphire tube as sample holder, Bragg peaks from the container material can be completely avoided, thus yielding high-quality powder diffraction data with very clean diffraction background. This enables the extraction of high precision crystal structure data as a function of gas pressure and temperature (Fig. 1) in time-resolved studies, the identification of intermediates, and optimization of synthesis protocols.

A. Götze, H. Auer, R. Finger, T. C. Hansen, H. Kohlmann, A sapphire single-crystal cell for in situ neutron powder diffraction of solid-gas reactions, Phys. B (Amsterdam, Neth.) 2018, 551, 395-400, DOI: 10.1016/j.physb.2017.11.024

Magnetic materials

SmCo5 is one of the strongest permanent magnet materials with one of the highest coercitivities known do date. Its high Curie temperature around 1020 K and effective magnetic moment between seven and eight Bohr magnetons per formula unit qualify the use in so-called supermagnets with application in turbines, compressors, electric motors, guitars, headphones or NMR spectrometers. Surprising for a well-known textbook example, the magnetic structure above room temperature was not known as yet and a predicted switch to a ferrimagnetic state above 350 K never experimentally tackled. Neutron diffraction is a powerful tool to determine magnetic structures and has in this case unambiguously shown that samarium’s magnetic moment goes through a minimum of 0.2 Bohr magnetons around 650 K (tiny green arrows in Fig. 2), but stays positive and aligned parallel to the magnetic moments of cobalt. More than 50 years after its discovery, this work presents crystal and magnetic structure data of SmCo5 from the Curie temperature down to 5 K and proves that it is a ferromagnet in the whole temperature range.

H. Kohlmann, T. C. Hansen, V. Nassif, Magnetic Structure of SmCo5 from 5 K to the Curie Temperature, Inorg. Chem. 2018, 57, 1702-1704, DOI: 10.1021/acs.inorgchem.7b02981

Luminescent materials

Among the numerous rare-earth metal ion substituted materials, those containing Eu(II) as activator are of special interest for photoluminescence properties. In contrast to most metal ions, the electronic structure of Eu2+ allows for the adjustment of the emission wavelength by choosing a suitable host structure. The excited state (4f65d1) are influenced by ligand-field splitting and moreover by shifting the barycenter of d-states by covalent interaction with ligands (nephelauxetic effect). For halogenides the latter is negligible and the ligand field is weak, resulting in energetically high excited state and a short wavelength emission (violet-blue). Ionic metal hydrides have been considered as host materials just recently. Due to the strong nephelauxetic effect of the hydride ion, the barycenter of the excited state is significantly lowered. Therefore, Eu(II)-substituted alkaline earth hydrides and perovskite related alkaline alkaline-earth hydrides possess strong red shifts of the emission wavelength to 530 nm (LiBaH3), 680 nm (NaMgH3) and 764 nm (CaH2)with respect to corresponding fluorides. Based on the structural analogy of fluorides and ionic hydrides, solid solution series AMH3-xFx (A = alkaline, M = alkaline earth) allow for emission wavelength tuning (Fig. 3).

N. Kunkel, A. Meijerink, H. Kohlmann, Bright yellow and green Eu(II) luminescence and vibronic fine structures in LiSrH3, LiBaH3 and their corresponding deuterides, Phys. Chem. Chem. Phys. 2014, 16, 4807-4813, DOI: 10.1039/C3CP55102D

C. Pflug, A. Franz, H. Kohlmann, Crystal structure and europium luminescence of NaMgH3-xFx, J. Solid State Chem. 2018, 258, 391-396, DOI: 10.1016/j.jssc.2017.10.034

Zintl phase hydrides

Zintl phase are polar intermetallic compounds on the border between ionic salts and metals consisting of a group one or two metal M and a group 13 to 16 element X. They contain polyanions, which can be rationalized by the general 8-N rule assuming an electron transfer from M to X atoms. By reaction with hydrogen a subtle game of redox chemistry can be played with incorporation of hydrogen atoms into interstitial voids, oxidation of the polyanion, formation of new X-X and X­-H bonds or insertion of hydrogen atoms into X-X bonds (Fig. 4). The hydrogenation of Zintl phases yields hydrides with diverse structure and bonding motifs. Hydrogenation is a tool to cut and glue polyanions in solids and thus offers a vast playground for experimental and theoretical solid-state chemistry.

H. Auer, R. Schlegel, O. Oeckler, H. Kohlmann, Structural and Electronic Flexibility in Hydrides of Zintl Phases with Tetrel-Hydrogen and Tetrel-Tetrel Bonds, Angew. Chem. 2017, 129, 12515–12518, DOI: 10.1002/ange.20170652; Angew. Chem. Int. Ed. 2017, 56, 12344-12347, DOI: 10.1002/anie.20170652

A. Werwein, H. Auer, L. Kuske, H. Kohlmann, From Metallic LnTt (Ln = La, Nd; Tt = Si, Ge, Sn) to Electron-precise Zintl Phase Hydrides LnTtH, Z. Anorg. Allg. Chem. 2018, 644, 1532–1539, DOI: 10.1002/zaac.201800062

letzte Änderung: 02.12.2019