Bodo Böhme

Synthesis of the Clathrate-I Phase Ba8−xSi46 via Redox Reactions

The clathrate-I phase Ba(8-x)Si(46) (space group Pm3̅n) was synthesized by oxidation of Ba(4)Li(2)Si(6) with gaseous HCl. Microcrystalline powders of the clathrate phase were obtained within a few minutes. The reaction temperature and the pressure of HCl were optimized to achieve good-quality crystalline products with a composition range of 1.3 < x < 1.9. The new preparation route presented here provides an alternative to the high-pressure synthesis applied so far.

Synthesis of the intermetallic clathrate Na2Ba6Si46 by oxidation of Na2BaSi4 with HCl

Abstract A new preparation route to the intermetallic clathrate-I compound Na2Ba6Si46 is introduced, which allows one to make large amounts of product with standard laboratory equipment. The precursor Na2BaSi4 is oxidized with gaseous HCl at 673 K to Na2Ba6Si46, NaCl and BaCl2. Full-profile refinement of the crystal structure from the X-ray powder diffraction data revealed a composition close to Na2Ba6Si46 (Na1.94(1)Ba6.06(1)Si46, space group , a = 10.281(1) Å). Differential scanning calorimetry showed an exothermic effect at 874 K, indicating that Na2Ba6Si46 is metastable. The product was additionally characterized by scanning electron microscopy. The electronic structure of Na2Ba6Si46 was investigated by a first-principles, all-electron full-potential method, predicting metallic conductivity. Na2Ba6Si46 obtained by oxidation with HCl shows Pauli paramagnetism; no bulk superconductivity was found down to 1.8 K in a magnetic field of 20 Oe.

Synthesis and Electron Holography Studies of Single Crystalline Nanostructures of Clathrate-II Phases KxGe136 and NaxSi136

Crystalline nanosized particles of clathrate-II phases K(x)Ge(136) and Na(x)Si(136) were obtained from a dispersion of alkali metal tetrelides in ionic liquids based on DTAC/AlCl(3), which were slowly heated to 120-180 °C. The nanoparticles are bullet-shaped with typical dimensions of about 40 nm in width and 140-200 nm in length. Detailed structure investigations using high-resolution transmission electron microscopy (HRTEM) and electron holography reveal the crystallinity and dense morphology of the clathrate nanorods.

Distribution of Al atoms in the clathrate-I phase Ba8AlxSi46−x at x = 6.9

The clathrate-I phase Ba8AlxSi46-x has been structurally characterized at the composition x = 6.9 (space group Pm3[combining macron]n, no. 223, a = 10.4645(2) Å). A crystal structure model comprising the distribution of aluminium and silicon atoms in the clathrate framework was established: 5.7 Al atoms and 0.3 Si atoms occupy the crystallographic site 6c, while 1.2 Al atoms and 22.8 Si atoms occupy site 24k. The atomic distribution was established based on a combination of (27)Al and (29)Si NMR experiments, X-ray single-crystal diffraction and wavelength-dispersive X-ray spectroscopy.

Preparation of anionic clathrate-II K24-xGe136 by filling of Ge(cF136)

Abstract Metastable Ge(cF136) with empty clathrate-II crystal structure was successfully used for the preparation of otherwise hardly accessible germanium clathrate phases. On reaction of Ge(cF136) with potassium vapour at 280 °C for 6 days, the new clathrate-II phase K24Ge136 was formed, in which the cages of the germanium framework are completely filled by potassium atoms. The crystal structure of KxGe136 is discussed for different potassium contents (x=0, 8.6, 24).

Solid State Chemistry of Clathrate Phases: Crystal Structure, Chemical Bonding and Preparation Routes

Clathrates represent a family of inorganic materials called cage compounds. The key feature of their crystal structures is a three-dimensional (host) framework bearing large cavities (cages) with 20–28 vertices. These polyhedral cages bear—as a rule—guest species. Depending on the formal charge of the framework, clathrates are grouped in anionic, cationic and neutral. While the bonding in the framework is of (polar) covalent nature, the guest-host interaction can be ionic, covalent or even van-der Waals, depending on the chemical composition of the clathrates. The chemical composition and structural features of the cationic clathrates can be described by the enhanced Zintl concept, whereas the composition of the anionic clathrates deviates often from the Zintl counts, indicating additional atomic interactions in comparison with the ionic-covalent Zintl model. These interactions can be visualized and studied by applying modern quantum chemical approaches such as electron localizability.

Application of n-Dodecyltrimethylammonium Chloride for the Oxidation of Intermetallic Phases

The thermal decomposition products of ionic liquids based on n-dodecyltrimethylammonium chloride (DTAC) were used for the preparation of the metastable allotrope Ge(cF136) by oxidation of Na12Ge17 in gas-solid reactions. This method of preparation provides a promising low-temperature route for the synthesis of intermetallic phases and elemental modifications. In order to explore the reaction mechanism, we investigated the thermal decomposition of DTAC as well as of the ionic liquids DTAC/MgCl2 and DTAC/AlCl3 by in-situ mass spectrometry and by powder X-ray diffraction. The results have revealed HCl, CH3Cl and 1-chlorododecane to act as oxidizing agents in the gas-solid redox reactions. Graphical Abstract Application of n-Dodecyltrimethylammonium Chloride for the Oxidation of Intermetallic Phases

Formation of Vacancies in Si- and Ge-based Clathrates: Role of Electron Localization and Symmetry Breaking

The formation of framework vacancies in Si- and Ge-based type-I clathrates is studied using density-functional theory as a function of filling the cages with K and Ba atoms. Our analysis reveals the relevance of structural disorder, geometric relaxation, and electronic saturation as well as vibrational and configurational entropy. In the Si clathrates, we find that vacancies are unstable, but very differently, in Ge clathrates, up to three vacancies per unit cell can be stabilized. This contrasting behavior is largely driven by the different energy gain on populating the electronic vacancy states, which originates from the different degree of localization of the valence orbitals of Si and Ge. This also actuates a qualitatively different atomic relaxation of the framework.

[Cs6Cl][Fe24Se26]: A Host-Guest Compound with Unique Fe-Se Topology

The novel host-guest compound [Cs6Cl][Fe24Se26] (I4/mmm; a=11.0991(9), c=22.143(2) Å) was obtained by reacting Cs2Se,CsCl, Fe, and Se in closed ampoules. This is the first member of a family of compounds with unique Fe-Se topology, which consists of edge-sharing, extended fused cubane [Fe8Se6Se8/3] blocks that host a guest complex ion, [Cs6Cl](5+). Thus Fe is tetrahedrally coordinated and divalent with strong exchange couplings, which results in an ordered antiferromagnetic state below TN =221 K. At low temperatures, a distribution of hyperfine fields in the Mössbauer spectra suggests a structural distortion or a complex spin structure. With its strong Fe-Se covalency, the compound is close to electronic itinerancy and is, therefore, prone to exhibit tunable properties.

A type-II clathrate with a Li-Ge framework

Abstract Na16Cs8LixGe136−x (x≈2.8, space group Fd3̅m) is the first intermetallic type-II clathrate with a lithium-substituted framework. The phase was obtained from the elements in sealed Ta ampoules by annealing at 650°C for 5 days. Samples were investigated by synchrotron X-ray powder diffraction, solid-state NMR, microstructure and chemical analysis. The substitution of Ge by Li atoms causes a marked shrinking of the lattice parameter. Studies by 7Li NMR confirmed the presence of Li in the clathrate phase and the 23Na and 133Cs NMR spectra consistently showed distinct changes as compared to the ternary Na16Cs8Ge136. The SEDOR technique revealed a distance between Li and Cs atoms in agreement with the result of crystal structure refinement, indicating Li substitution at site 96g. The distinct Knight shift of all NMR signals implies metallic behaviour of the clathrate phase, measurements of the magnetic susceptibility indicate diamagnetic behaviour.