As the UN summit in September 2015 addressed with the Sustainable Development Goals (SDG), our planet faces great challenges.[1] Not only since then has the role of synthetic materials chemistry been discussed in this context.[2–16] This not only concerns the development of new materials with outstanding properties such as catalysts, materials for energy conversion, and cost-efficient energy converting and storage materials, but also a reduction of the energy consumption of established functional material syntheses. Therefore, new approaches addressing the three main categories to promote the potential for energy and resource efficiency have been proposed: lowering the temperature of the synthetic processes, improving the yield and purity of the materials, and reducing the amount of waste materials. In this context a number of low-temperature processes have been established, in which mainly solvents, i.e. amines and alcohols, are used in combination with previously synthesised precursors as the solubility of the starting materials limits their usability and most elements are not soluble in these solvents. Novel solvents like ionic liquids (ILs) showcase growing interest as they are considered particularly resource-efficient.[17,18] ILs are defined as liquids that are comprised entirely of ions, with melting points below 100 °C. Continuing on from the reported work in this field, this thesis focuses on investigating the ability of ILs in the syntheses for known chalcogenides at lower temperatures and the synthesis of new materials. The main focus lies on conversions with high atom economy, especially by starting from the elements and completely recycling the IL afterwards, and mechanistic studies elucidating the intermediate dissolved species. Furthermore, as imidazolium based ILs, and their derived LEWIS-acidic ILs [BMIm]Cl/nAlCl3 mixtures, have proven to be good crystallisation media in inorganic syntheses, and the class of mostly room-temperature liquid ILs (RTILs) based on phosphonium cations [P66614]Cl showcased the ability to dissolve red phosphorous, Pred, and the heavier chalcogens S, Se, and Te, this work mainly focuses on these two IL systems. This was also chosen as an in-depth understanding of the activation and resource-efficient synthesis of these chalcogenides has still not been established. As they are RTILs, they also made the characterisation of the reactive and dissolved species by liquid state NMR, Raman, UV/Vis spectroscopy and electrochemical characterisation possible. This expands the knowledge of which main group elements and ore-like starting materials can be used in ionothermal synthesis. As a starting point the thermoelectrically interesting materials class of tellurides is addressed. The under normal conditions hard to dissolve element tellurium readily dissolves in phosphonium ILs with the cations [P66614]+ and [P4444]+. In ILs with carboxylate anions a deep purple hue of the IL already occurred by dissolving tellurium at temperatures of 60 °C. Investigations on the solutions in the acetate ILs revealed the formation of tellurium anions (Ten)2– with chain lengths up to at least n = 5, which are in a dynamic equilibrium with each other. Since external influences could be excluded and no evidence of an IL reaction was found, disproportionation of the tellurium is the only possible dissolution mechanism. However, the spectroscopic detection of tellurium cations in these solutions is difficult, but the coexistence of tellurium cations, such as (Te4)2+ and (Te6)4+, and tellurium anions could be proven by cyclic voltammetry and electrodeposition experiments. DFT calculations indicate that electrostatic interactions with the ions of the ILs are sufficient in stabilizing both types of tellurium ions in solution.[19] In contrast, the acetate ILs show insufficient conversion in reactions of coin metals (Cu, Ag, Au) with tellurium to the corresponding tellurides, especially at low temperatures, however the chloride ILs successfully synthesise Cu2–xTe, CuTe, AuTe2 and Ag2Te. As the synthesis of the tellurides in neat ILs at temperatures down to 60 °C was only sufficient for the system Ag-Te, with a full conversion of the elements to Ag2Te, this was chosen as a model system for further investigations. Even at room temperature, a quantitative yield was achieved by using either 2 mol% of [P66614]Cl in dichloromethane or a planetary ball mill. The unexpected finding that phosphane-free [P66614]Cl also allows the quantitative synthesis of Ag2Te at 60°C implies an additional activation mechanism independent from the phosphane, which is yet unknown.[20] Subsequently, the manifoldly-used lighter chalcogen sulphur is tested for the synthesis of sulphides. Direct synthesis of binary sulphides of B, Bi, Ge, Mo, Cu, Au, Sn, In, Ti, V, Fe, Co, Ga, Ni, Al, Zn, and Sb in [P66614]Cl was tested at 100 °C, i.e. below the melting point of sulphur. Under these conditions, substantial sulphide formation occurred only for nickel (Ni3S4, Ni3S2, NiS) and copper (Cu2S, CuS). Sb showed no formation of crystalline sulphide, but after addition of EtOH, an orange material precipitated which was identified as amorphous metastibnite.[21] As generating these elements from their ores is highly energy consuming, direct dissolution experiments of the crystalline stibnite in [P66614][OAc] and Cl– were investigated and resulted in yellow solutions, from which the amorphous form can be precipitated upon exposure to EtOH air without any sign of decomposition of the IL. In particular, follow-up investigations were conducted on the solubility of Sb2S3 for follow-up chemistry in the LEWIS-acidic IL [BMIm]Cl · 4.7 AlCl3 at 160 °C which resulted in the formation of the novel chloride-terminated [Sb13S16Cl2]5+ quadruple-heterocubane cation-containing compound [Sb13S16Cl2][AlCl4]5.[21] Addition of CuCl in a slightly modified reaction resulted in the formation of the layered semiconductor Cu(Sb2S3)[AlCl4]. From this the AlCl3 can be leached by treatment with 0.1 molar hydrochloric acid, yielding a compound with the presumed composition Cu(Sb2S3)Cl.[22] As ILs showed to be able to activate elements that are insoluble in common solvents, and the formation of Sb2S3 from reactions mixtures of the elements raised the question of whether only the sulphur forms a mobile species or if antimony could additionally activated, the synthesis of binary antimony compounds directly from elements was explored as they are highly discussed as replacements for silicon-based semiconductors. Therefore the 12 elements Ti–Cu, Al, Ga, In, and Te, which are known to form binary compounds with Sb, were reacted with Sb in [P66614]Cl under inert conditions in a simple closed glass flask with vigorous stirring for 16 h at 200 °C. This resulted in the formation of NiSb, InSb, Cu2Sb and Sb2Te3. The applied reaction temperature is several hundred degrees below the temperatures required for solvent-free conversions. Compared to reactions based on diffusions in the solid state, reaction times are much shorter. The IL is not consumed and can be recycled. Since the reaction with Cu showed almost complete conversion, the influences of reaction time, temperature and medium were further investigated. In a diffusion experiment, Cu2Sb formed on the copper, which indicates that antimony forms mobile species in these ILs. These systematic studies hence deliver a contribution to how ILs can help in the synthesis of new materials and how they can make a difference in the synthesis of inorganic materials as well in the context of “GREEN CHEMISTRY”. This can help in developing a more educated choice/toolbox of IL systems for reducing energy costs by reducing the temperature from high temperature inorganic syntheses to syntheses near room-temperature by using the elements as starting materials, with a high atom economy for the synthesis of known and new materials.

Ziel dieser Arbeit war es, Beziehungen zwischen den kristallchemischen Eigenschaften und dem beobachteten anomalen Verhalten im spezifischen elektrischen Widerstand (nicht-magnetischer Kondo-Effekt) aufzuzeigen und zusammenhängend zu interpretieren. Verbindungen, an denen dieser Effekt beobachtet wurde, werden aus einem Übergangs-, oder Actinidmetall mit je einem Vertreter der 15. (Pniktogene) und 16. Gruppe (Chalkogene) des Periodensystems gebildet und kristallisieren im PbFCl-Strukturtyp. Da zu ternären Actinidmetall-Pniktid-Chalkogeniden (z.B. ThAsSe, UPS) nur sehr wenige chemische und kristallographische Informationen existieren, wurden in dieser Arbeit umfassende Untersuchungen zur Kristallchemie ternärer Phasen aus den Systemen M-Pn-Q (M = Zr, Hf, La-Ce; Pn = As, Sb; Q = Se, Te durchgeführt. Der Schwerpunkt lag dabei auf der strukturellen Lokalisierung der beobachteten Widerstandsanomalie und der Erarbeitung chemisch-physikalischer Eigenschaftsbeziehungen. Die Darstellung der untersuchten ternären Phasen in Form von Einkristallen gelang über exothermen Chemischen Transport mit Jod. Da die erhaltenen Kristalle bis zu mehreren Millimetern groß sind, konnten an ein und demselben Kristallindividuum sowohl die stoffliche Charakterisierung (EDXS, WDXS, ICP-OES, LA-ICP-MS, CIC) und die strukturelle Charakterisierung, als auch die Messung der physikalischen Eigenschaften erfolgen. Es konnte u.a. gezeigt werden, dass ZrAs1,4Se0,5 und HfAs1,7Se0,2 ein ähnlich ungewöhnliches Verhalten im temperaturabhängigen elektrischen Widerstand zeigen, welches bereits an Thorium-Arsenid-Seleniden und Uran-Phosphid-Sulfiden beobachtet wurde. Desweiteren gelang es den beobachteten Verlauf im elektrischen Widerstand, mit seinem Minium bei etwa T = 15 K, auf intrinsische strukturelle Merkmale in der anionischen Arsen-Teilstruktur zurückzuführen. / The aim of this work was, to evaluate and interpret a relationship between the crystal-chemical properties and the observed unusual behavior in the electrical resistivity (non-magnetic Kondo-effect). Compounds, which show such an effect, are formed by a transition- or actinide-metal with both a group 15 element and a group 16 element of the periodic table. All these compounds crystallizing in the PbFCl type of structure. Because of less crystallographic and chemical information about actinide-metal-pnictide-chalcogenides (i.e. ThAsSe, UPS), intensive investigation were made concerning the crystal-chemistry of ternary phases of the systems M-Pn-Q (M = Zr, Hf, La-Ce; Pn = As, Sb; Q = Se, Te. Our studies were focused on the structurally localization of the observed anomaly in the electrical resistivity and the evaluation of chemical-physical relations of properties. The synthesis of the investigated ternary phases was realized by exothermically Chemical Transport with iodine as transport agent. The dimension of the synthesized crystals allowed a chemical (EDXS, WDXS, ICP-OES, LA-ICP-MS, CIC) and structurally characterization, as well as a determination of the physical properties on one large single crystal. It could be shown, that ZrAs1,4Se0,5 and HfAs1,7Se0,2 reveal a similar unusual behavior in the temperature dependent electrical resistivity, as it was observed in thorium-arsenide-selenides and uranium-phosphide-sulphides. In conclusion, the non-magnetic Kondo-effect, which was found in the low-temperature range (about 15 K), arises from structurally features of the anionic sublattice with arsenic.

Ziel dieser Arbeit war es, Beziehungen zwischen den kristallchemischen Eigenschaften und dem beobachteten anomalen Verhalten im spezifischen elektrischen Widerstand (nicht-magnetischer Kondo-Effekt) aufzuzeigen und zusammenhängend zu interpretieren. Verbindungen, an denen dieser Effekt beobachtet wurde, werden aus einem Übergangs-, oder Actinidmetall mit je einem Vertreter der 15. (Pniktogene) und 16. Gruppe (Chalkogene) des Periodensystems gebildet und kristallisieren im PbFCl-Strukturtyp. Da zu ternären Actinidmetall-Pniktid-Chalkogeniden (z.B. ThAsSe, UPS) nur sehr wenige chemische und kristallographische Informationen existieren, wurden in dieser Arbeit umfassende Untersuchungen zur Kristallchemie ternärer Phasen aus den Systemen M-Pn-Q (M = Zr, Hf, La-Ce; Pn = As, Sb; Q = Se, Te durchgeführt. Der Schwerpunkt lag dabei auf der strukturellen Lokalisierung der beobachteten Widerstandsanomalie und der Erarbeitung chemisch-physikalischer Eigenschaftsbeziehungen. Die Darstellung der untersuchten ternären Phasen in Form von Einkristallen gelang über exothermen Chemischen Transport mit Jod. Da die erhaltenen Kristalle bis zu mehreren Millimetern groß sind, konnten an ein und demselben Kristallindividuum sowohl die stoffliche Charakterisierung (EDXS, WDXS, ICP-OES, LA-ICP-MS, CIC) und die strukturelle Charakterisierung, als auch die Messung der physikalischen Eigenschaften erfolgen. Es konnte u.a. gezeigt werden, dass ZrAs1,4Se0,5 und HfAs1,7Se0,2 ein ähnlich ungewöhnliches Verhalten im temperaturabhängigen elektrischen Widerstand zeigen, welches bereits an Thorium-Arsenid-Seleniden und Uran-Phosphid-Sulfiden beobachtet wurde. Desweiteren gelang es den beobachteten Verlauf im elektrischen Widerstand, mit seinem Minium bei etwa T = 15 K, auf intrinsische strukturelle Merkmale in der anionischen Arsen-Teilstruktur zurückzuführen. / The aim of this work was, to evaluate and interpret a relationship between the crystal-chemical properties and the observed unusual behavior in the electrical resistivity (non-magnetic Kondo-effect). Compounds, which show such an effect, are formed by a transition- or actinide-metal with both a group 15 element and a group 16 element of the periodic table. All these compounds crystallizing in the PbFCl type of structure. Because of less crystallographic and chemical information about actinide-metal-pnictide-chalcogenides (i.e. ThAsSe, UPS), intensive investigation were made concerning the crystal-chemistry of ternary phases of the systems M-Pn-Q (M = Zr, Hf, La-Ce; Pn = As, Sb; Q = Se, Te. Our studies were focused on the structurally localization of the observed anomaly in the electrical resistivity and the evaluation of chemical-physical relations of properties. The synthesis of the investigated ternary phases was realized by exothermically Chemical Transport with iodine as transport agent. The dimension of the synthesized crystals allowed a chemical (EDXS, WDXS, ICP-OES, LA-ICP-MS, CIC) and structurally characterization, as well as a determination of the physical properties on one large single crystal. It could be shown, that ZrAs1,4Se0,5 and HfAs1,7Se0,2 reveal a similar unusual behavior in the temperature dependent electrical resistivity, as it was observed in thorium-arsenide-selenides and uranium-phosphide-sulphides. In conclusion, the non-magnetic Kondo-effect, which was found in the low-temperature range (about 15 K), arises from structurally features of the anionic sublattice with arsenic.

In dieser Arbeit wird das Wachstum von dünnen quasi-kristallinen Ge-Sb-Te (GST) Schichten mittels Molekularstrahlepitaxie demonstriert, die zu einer geordneten Konfiguration von intrinsischen Kristallgitterfehlstellen führen. Es wird gezeigt, wie es eine Strukturanalyse basierend auf Röntgenstrahlbeugungssimulationen, Dichtefunktionaltheorie und Transmissionselektronenmikroskopie ermöglicht, eine eindeutige Beurteilung der Kristallgitterlückenanordnung in den GST-Proben vorzunehmen. Das Verständnis für die Ordnungsprozesse der Gitterfehlstellen erlaubt eine gezielte Einstellung des Ordnungsgrades selbst, der mit der Zusammensetzung und der Kristallphase des Materials in Zusammenhang steht. Auf dieser Basis wurde ein Phasendiagramm mit verschiedenen Wachstumsfenstern für GST erstellt. Des Weiteren wird gezeigt, dass man eine hohe Ordnung der Gitterfehlstellen in GST auch durch Ausheizprozesse und anhand von Femtosekunden-gepulster Laserkristallisation von amorphem Material erhält, das zuvor auf einem als Kristallisationsgrundlage dienenden Substrat abgeschiedenen wurde. Diese Erkenntnis ist bemerkenswert, da sie zeigt, dass sich kristalline GST Schichten mit geordneten Kristallgitterlücken durch verschiedene Herstellungsprozesse realisieren lassen. Darüber hinaus wurde das Wachstum von GeTe/Sb2Te3 Übergittern durchgeführt, deren Struktur die von GST mit geordneten Gitterfehlstellen widerspiegelt. Die Möglichkeit den Grad der Gitterfehlstellenordung in GST gezielt zu manipulieren wurde mit einer Studie der Transporteigenschaften kombiniert. Die Anwendung von großflächigen Charakterisierungsmethoden wie XRD, Raman und IR-Spektroskopie, erlaubte die Bestimmung der Phase und des Fehlstellenordnungsgrades von GST und zeigte eindeutig, dass die Fehlstellenordnung für den Metall-Isolator-Übergang (MIT) verantwortlich ist. Insbesondere wird durch das Vergleichen von XRD-Messungen mit elektrischen Messungen gezeigt, dass der Übergang von isolierend zu leitend erfolgt, sobald eine Ordnung der Kristallgitterlücken einsetzt. Dieses Phänomen tritt in der kubischen Kristallphase auf, wenn Gitterfehlstellen in GST von einem ungeordneten in einen geordneten Zustand übergehen. Im zweiten Teil des Kapitels wird eine Kombination aus FIR- und Raman-Spektroskopie zur Untersuchung der Vibrationsmoden und des Ladungsträgerverhaltens in der amorphen und der kristallinen Phase angewendet, um Aktivierungsenergien für die Elektronenleitung, sowohl für die kubische, als auch für die trigonale Kristallphase von GST zu bestimmen. Hier ist es wichtig zu erwähnen, dass, in Übereinstimmung mit Ergebnissen aus anderen Untersuchungen, das Auftauchen eines MIT beim Übergang zwischen der ungeordneten und der geordneten kubischen Phase beobachtet wurde. Schlussendlich wurden verschiedene sogenannte Pump/Probe Technik, bei der man das Material mit dem Laser anregt und die Röntgenstrahlung oder Terahertz (THz)-spektroskopie als Sonde nutzt, angewandt. Dies dient um ultra-schnelle Dynamiken zu erfassen, die zum Verständnis der Umschaltmechanismen nötig sind. Die Empfindlichkeit der THz-Messungen hinsichtlich der Leitfähigkeit, sowohl in GST, als auch in GeTe/Sb2Te3 Übergittern zeigte, dass die nicht-thermische Natur der Übergitterumschaltprozesse mit Grenzflächeneffekten zusammenhängt und . Der Ablauf wird mit beeindruckender geringer Laser-Fluenz erreicht. Dieses Ergebnis stimmt mit Berichten aus der Literatur überein, in denen ein Kristall-zu Kristallwechsel von auf Übergittern basierenden Speicherzellen für effizienter gehalten wird als GST Schmelzen, was zu einen ultra-schwachen Energieverbrauch führt. / The growth by molecular beam epitaxy of Ge-Sb-Te (GST) alloys resulting in quasi-single-crystalline films with ordered configuration of intrinsic vacancies is demonstrated. It is shown how a structural characterization based on transmission electron microscopy, X-ray diffraction and density functional theory, allowed to unequivocally assess the vacancy ordering in GST samples, which was so far only predicted. The understanding of the ordering process enabled the realization of a fine tuning of the ordering degree itself, which is linked to composition and crystalline phase. A phase diagram with the different growth windows for GST is obtained. High degree of vacancy ordering in GST is also obtained through annealing and via femtosecond-pulsed laser crystallization of amorphous material deposited on a crystalline substrate, which acts as a template for the crystallization. This finding is remarkable as it demonstrates that it is possible to create a crystalline GST with ordered vacancies by using different fabrication procedures. Growth and structural characterization of GeTe/Sb2Te3 superlattices is also obtained. Their structure resembles that of ordered GST, with exception of the Sb and Ge layers stacking sequence. The possibility to tune the degree of vacancy ordering in GST has been combined with a study of its transport properties. Employing global characterization methods such as XRD, Raman and Far-Infrared spectroscopy, the phase and ordering degree of the GST was assessed, and unequivocally demonstrated that vacancy ordering in GST drives the metal-insulator transition (MIT). In particular, first it is shown that by comparing electrical measurements to XRD, the transition from insulating to metallic behavior is obtained as soon as vacancies start to order. This phenomenon occurs within the cubic phase, when GST evolves from disordered to ordered. In the second part of the chapter, a combination of Far-Infrared and Raman spectroscopy is employed to investigate vibrational modes and the carrier behavior in amorphous and crystalline phases, enabling to extract activation energies for the electron conduction for both cubic and trigonal GST phases. Most important, a MIT is clearly identified to occur at the onset of the transition between the disordered and the ordered cubic phase, consistently with the electrical study. Finally, pump/probe schemes based on optical-pump/X-ray absorption and Terahertz (THz) spectroscopy-probes have been employed to access ultrafast dynamics necessary for the understanding of switching mechanisms. The sensitivity of THz-probe to conductivity in both GST and GeTe/Sb2Te3 superlattices showed that the non-thermal nature of switching in superlattices is related to interface effects, and can be triggered by employing up to one order less laser fluences if compared to GST. Such result agrees with literature, in which a crystal to crystal switching of superlattice based memory cells is expected to be more efficient than GST melting, therefore enabling ultra-low energy consumption.

Die epitaktische Wachstum von GeTe Dünnschichten und Sb2Te3/GeTe Übergittern durch Molekularstrahlepitaxie wird auf drei verschiedenen Silizium Oberflächen gezeigt: Si(111)−(7×7), Si(111)−(√3×√3)R30°−Sb, und Si(111)−(1×1)−H. Mit Röntgenstrukturanalyse wird bewiesen, dass die epitaktische Beziehung der GeTe Schicht von der Oberflächepassievierung abhängig ist; auf einer passivierten Fläche können verdrehte Domänen unterdrückt sein. Dieses Verhalten ähnelt dem, welches bei 2D Materialien zu erwarten wäre, und wird auf die Schwäche der Resonanten ungebundenen Zustände zurückgeführt, die durch Peierls Verzerrung noch schwächer werden. / The growth by molecular beam epitaxy of GeTe and Sb2Te3/GeTe superlattices on three differently reconstructed Si(111) surfaces is demonstrated. Namely, these are the Si(111)−(7×7), Si(111)−(√3×√3)R30°−Sb, and Si(111)−(1×1)−H reconstructions. Through X-ray diffraction, the epitaxial relationship of GeTe is shown to depend on the passivation of the surface; in-plane twisted and twinned domains could be suppressed on a passivated surface. This behavior which resembles what would be expected from lamellar materials, is attributed to the relative weakness of resonant dangling bonds, that are further weakened by Peierls distortion.