Die Hydrierung von Zintl-Phasen führt zur Bildung von Einlagerungshydriden, die ausschließlich von Kationen koordiniert sind, zu polyanionischen Hydriden, bei denen Wasserstoff kovalent an das stärker elektronegative Element bindet, oder zu einer Kombination von beiden Motiven. Es wurde eine Reihe neuer Verbindungen dargestellt und mittels Laborröntgen-, Synchrotron- und Neutronenpulverbeugung strukturell charakterisiert. Die meisten Beispiele werden durch die Hydrierung von Zintl-Phasen im CrB- oder FeB-Strukturtyp erhalten. Die beiden Typen sind strukturell eng verwandt. Sie sind durch das Auftreten von polyanionischen Zickzackketten gekennzeichnet. Die Einlagerungshydride LnTtH (Ln = La, Nd, Tt = Si, Ge, Sn) sind Oxidationsprodukte der formal metallischen Zintl-Phasen LnTt = Ln3+ Tt2- e- . Wasserstoff besetzt dabei Ln-Tetraederlücken. Die Produkte treten als gefüllter FeB- (P -Phase, LaGeH-Strukturtyp) oder als gefüllter CrB-Strukturtyp (C -Phase, NiZrH-Strukturtyp) auf. Die Hydrierung der elektronenpräzisen Zintl-Phasen AeTt (Ae = Sr, Ba, Tt = Ge, Sn, CrB-Strukturtyp) führt zu wasserstoffarmen (AeTtHy , y < 1) und wasserstoffreichen (AeTtHy , 1 < y ≤ 2) Phasen. Erstere weisen partiell gefüllte Ae4-Lücken auf. In Phasen mit kleinem y (< 0.4) wird der Wasserstoff statistisch über die Lücken verteilt (α-Phasen). Etwas höhere Gehalte führen zu partieller (β-SrGeHy , 0.47 < y < 0.75) oder vollständiger (β-BaGeH0.5 ) Ordnung. Die wasserstoffreichen Phasen AeTtHy , 1 < y ≤ 2 (γ-Phasen), zeigen sowohl die Strukturmotive von Einlagerungs- als auch von polyanionischen Hydriden. SrSiH1.6 und BaSiH1.9 als literaturbekannte Verbindungen wurden das erste mal strukturell charakterisiert. Die homologe Reihe konnte um SrGeH1.2 , BaGeH1.6 und BaSnH1.3 erweitert werden. Die Ae4 -Tetraeder sind in diesen Phasen vollständig mit Hydridionen besetzt. Zusätzlicher Wasserstoff bindet kovalent an die Polyanionen. Außerdem verknüpfen sich die Zickzackketten z. T. senkrecht zur Kettenrichtung. Es wurden insgesamt drei Strukturtypen differenziert, die alle strukturell eng verwandt sind. Das führt zu Problemen bei der Strukturbestimmung aus Pulverdaten. Der kovalente Charakter der Bindung wurde durch Festkörperkernresonanzspektroskopie und Dichtefunktionaltheorierechnungen charakterisiert. Typische Tetrel-Wasserstoff-Bindungslängen sind 155(2) pm (Si-H), 163(2) pm (Ge-H) und 186(1) pm (Sn-H). In situ -Neutron, Röntgen- und Synchrotronpulverbeugung wurden angewandt um Reaktionsabläufe aufzuklären. Beim Heizen unter Wasserstoffdruck treten im AeTt-H2 -System (Ae = Sr, Ba, Tt = Ge, Sn) verschiedene reversible Reaktionen zwischen den γ-, β- und α-Phasen auf, bevor ein irreversibler Zersetzungsschritt in die binären Hydride AeH2 und die Tt-reichen Zintl-Phasen AeTt2 beobachtet wird. Ein In situ Beugungsexperiment der Reaktion von NdGa mit Wasserstoff zeigt direkt die Bildung von NdGaH1+x (isostrukturell zu γ-AeTtHy ), das eine Zusammensetzung von mindestens x = 0.17 bis 0.80 aufweist. Die Ga-H Abstände sind lang (ca. 200 pm) und darum keine klassischen 2-Elektronen-2-Zentrenbindungen. In situ Beugung an den Reaktionen von KSi und CsSi mit Wasserstoff konnte gezeigt werden, dass die Hydride KSiH3 und CsSiH3 in einem Schritt gebildet werden. Diese Phasen weisen SiH3--Anionien auf, die isoelektronisch zu PH3 sind. Weiteres Heizen unter Wasserstoffdruck führt zur Zersetzung in KH und K8Si46 oder zur Rückbildung von CsSi. Außerdem wurde eine Reihe weiterer Verbindungen auf die Reaktivität gegenüber Wasserstoff untersucht. Die Phasen AeTt2 , AGe und ASixGe1-x (A = K, Rb, Cs) bilden keine Hydride unter den untersuchten Bedingungen (mindestens 5 MPa H2, 700 K). Die Gallide CaGa, Sr8Ga7 und Ba8Ga7 weisen Reaktivität gegenüber Wasserstoff auf. Diese Beispiele zersetzen sich in binäres Hydrid und die galliumreichen Phasen Ca3Ga8, SrGa4 und BaGa4 . In situ Laborröntgenbeugung der Reaktion von CaGa mit Wasserstoff führt zur Bildung einer neuen, kristallinen Phase. Bildung und Zersetzung laufen in einem sehr schmalen Temperaturfenster ab. Die neue Phase konnte noch nicht charakterisiert werden. / The hydrogenation of Zintl phases leads to interstitial hydrides that are coordinated exclusively by cations, polyanionic hydrides that exhibit a covalent bond to the more electronegative element, or a combination of both motifs. A series of new compounds is prepared and structurally characterised by laboratory X-ray, synchrotron and neutron powder diffraction. Most examples can be derived via hydrogenation of CrB- or FeB-type Zintl phases. These structure types are closely related and characterised by polyanionic zigzag chains. The interstitial hydrides LnTtH (Ln = La, Nd, Tt = Si, Ge, Sn) are oxidation products of the formally metallic Zintl phases LnTt = Ln3+ Tt2- e- . Hydrogen occupies tetrahedral Ln4-voids. The products occur as a filled FeB-type phase (P-phase, LaGeH-structure type) or a filled CrB-type phase (C-phase, ZrNiH-structure type). The hydrogenation of electron-precise Zintl phases AeTt (Ae = Sr, Ba, Tt = Ge, Sn, CrB-structure type) leads to hydrogen-poor (AeTtHy , y < 1) and hydrogen-rich phases (AeTtHy , 1 < y ≤ 2). The first show partially hydrogen-filled Ae4-voids. For low contents y < 0.4, hydrogen is statistically distributed over the voids (α-phases). Slightly increased hydrogen contents lead to partial (β-SrGeHy , 0.47 ≤ y ≤ 0.75) or full ordering (β-BaGeH0.5 ). The hydrogen-rich phases AeTtHy, 1 < y ≤ 2 (γ-phases), combine interstitial and polyanionic hydride motifs. The literature-known phases SrSiH1.6 and BaSiH1.9 could be structurally characterised for the first time. The homologue series was extended to SrGeH1.2, BaGeH1.6 and BaSnH1.3 . Tetrahedral Ae4-voids are totally filled with hydride anions. The additional hydrogen binds to the polyanions. Furthermore, some of the zigzag chains are interconnected perpendicular to the chain direction. Three different structure types exhibiting a close structural relationship were identified. This leads to difficulties in structure determination from powder diffraction. The covalent character of the bond is characterised by solid-state nuclear magnetic resonance and density functional theory calculations. Typical tetrel-hydrogen bond lengths are 155(2) pm (Si-H), 163(2) pm (Ge-H) and 186(1) pm (Sn-H). In situ neutron, X-ray and synchrotron powder diffraction were used to elucidate reaction schemes. The AeTt-H2 systems (Ae = Sr, Ba, Tt = Ge, Sn) show several reversible reaction steps between γ-, β- and α-phases upon heating under hydrogen pressure. Finally, an irreversible decomposition into the binary hydrides AeH2 and Tt-rich Zintl phases AeTt2 occurs. In situ diffraction of the reaction of NdGa with hydrogen leads directly to NdGaH1+x (isostructural to γ-AeTtHy ) which shows a large compositional range from at least x = 0.17 to 0.80. Ga-H distances are long (about 200 pm) and, thus, not classical 2-electron-2-center bonds. In situ diffraction of the reactions of KSi and CsSi with hydrogen show a one step formation of the corresponding hydrides KSiH3 and CsSiH3 . They exhibit SiH3--anions which are isoelectronic to PH3 . Further heating under hydrogen pressure leads to decomposition into KH and K8Si46 or reformation of CsSi, respectively. Finally, further compounds were tested for reactivity towards hydrogen. The phases AeTt2 (Ae = Ca, Sr, Ba, Tt = Si, Ge), AGe and ASixGe1-x (A = K, Rb, Cs) do not form corresponding hydrides under the investigated conditions (at least 5 MPa H2, 700 K). The gallides CaGa, Sr8Ga7 and Ba8Ga7 show reactivity towards hydrogen. They decompose into binary hydride and the gallium-rich phases Ca3 Ga8 , SrGa4 or BaGa4. Furthermore, laboratory in situ diffraction of the reaction of CaGa with hydrogen indicates the formation of a new, crystalline phase. Formation and decomposition occur in a relative small temperature window. The new phase could not be characterised, yet.

Von der Verbindungsklasse der heteroanionischen Hydride sind nur relativ wenige Vertreter bekannt, dabei zeigen sie großes Potential für Funktionsmaterialien. In dieser Arbeit werden verschiedene heteroanionische Hydride der seltenenen Erden in Kombination mit O2– -, F– - und N3– -Anionen untersucht. Die ternären Verbindungen REHO dienen dabei als Ausgangspunkt und der Fokus liegt auf der Kristallstrukturanalyse per Pulverneutronenbeugung. Verschiedene Synthesestrategien werden mit Unterstützung durch thermodynamische Rechnung auf Grundlage quantenchemischer Studien angewandt. Zwei neue REHO-Modifikationen werden identifiziert: der orthorhombische Anti-LiMgN-Typ mit Raumgruppe Pnma f¨ur RE =Y und Lu, sowie der kubische Halb-Heusler LiAlSi-Typ mit Raumgruppe F¯43m f¨ur RE =Dy–Er und vermutlich Lu. Der Anti-LiMgN-Typ zeigt ein für Seltenerdverbindungen untypisches Verhalten, da seine Ausbildung scheinbar nicht mit den Kationenradien korreliert ist. Die Berechnung von Zustandsdichten und Bader-Analysen unterstützen das Bild eines vornehmlich ionischen Bindungstyps. Eine Neutronenbeugungsuntersuchung an SmHO zeigt Überstrukturreflexe, die vermutlich aus einer langreichweitigen Ordnung von Anionen auf der Oktaederl¨ucke stammen. Ihre Stabilität an Luft wird durch in situ Pulverröntgenbeugung und Differentialthermoanalyse sowie ex situ Pulverröntgenbeugung untersucht. YHO ist bis 500 K, HoHO bis 540K stabil an Luft. Bei höheren Temperaturen zersetzt sich HoHO unter Bildung von Ho2O3 und HoH3 über ein Intermediat, das vermutlich Ho2O3–xH2x ist; YHO verhält sich scheinbar analog. An Luft werden sie langsam durch Luftfeuchtigkeit hydrolysiert, wobei der Grad mit dem Kationenradius im Vergleich verschiedener REHO abnimmt. Zwei Gestaltungskonzepte dienen der Entdeckung neuer heteroanionischer Hydride. Durch H/F-Metathese an SmOF wird die neue Verbindung SmHxOF1–x erhalten. Sie kristallisiert in einer Auff¨ullungsvariante des Bixbyit- bzw. im Anti-Li3ScN2-Typen mit Raumgruppe Ia¯3 und weist zwei Anionenlagen auf, von der auf einer H– , O2– und F– mischen. Durch eine formale aliovalente Anionensubstitution werden das erste Seltenerd-Nitridhydrid LiLa2N1.5H2.5 und das erste Seltenerd-Nitridhydridoxid LiLa2NH2O entdeckt. Gemeinsam mit dem Hydridionenleiter LiLa2HO3 stellen sie eine aliovalente Substitutionsreihe dar. Beide kristallisieren im tetragonalen K2NiF4-Typ mit gemischten N3–/H– - bzw. N3–/O2– -Lagen. / Heteroanionic hydrides are a compound class with relatively few members but simultaneous great potential for use in functional materials. In this work, several rare earth heteroanionic hydrides containing O2– -, F– -, or N3– -anions are studied starting from the ternary REHO compounds with focus on the crystal structure analysis by powder neutron diffraction. Different synthesis routes are examined and accompanied by thermodynamic calculations based on quantum chemical studies. Two new REHO polymorphs are identified: the orthorhombic anti-LiMgN-structure type with space group Pnma for RE =Y and Lu, and the cubic half-Heusler LiAlSi-type with space group F¯43m, that is adopted by RE =Dy–Er and probably also Lu. Density of states calculations and Bader charge analyses support the picture of a predominant ionic bonding type. The Anti-LiMgN-Type shows a rare-earth unusual behavior, as its adaption is apparently not correlated to the cationic radii. SmHO shows super structure reflections in the powder neutron diffraction experiments, which probably result from a long-range ordering of anions on the octahedral interstice. The air-resistivity is investigated by in situ powder X-ray diffraction and differential scanning calorimetry as well as ex situ powder X-ray diffraction. YHO is stable up to 500K and HoHO up to 540K in air. HoHO decomposes at higher temperatures to Ho2O3 and HoH3 via an intermediate, which is probably Ho2O3–xH2x ; YHO apparently behaves analogously. A degradation upon storage in air could be attributed to hydrolysis by air humidity and its degree declines along with the cationic radii among different REHO compounds. Two design concepts are employed to discover new heteroanionic hydrides: The H/Fmetathesis on SmOF lead to the discovery of SmHxOF1–x , samarium hydride oxide fluoride, which crystallizes in a filled bixbyite- or anti-Li3ScN2-type with space group Ia¯3 and exhibits two anion sites with one shared by H– , O2– , and F– . By a formal aliovalent anion substitution, the first rare earth nitride hydride LiLa2N1.5H2.5 and the first nitride hydride oxide LiLa2NH2O were discovered, which represent an aliovalent substitution series with the hydride ion conductor LiLa2HO3. Both crystallize in the tetragonal K2NiF4-type with mixed N3–/H– resp. N3–/O2– sites.

<p>  </p> <p>Thermal energy storage (TES) is used to reduce the operating costs of heating, ventilation, and air conditioning (HVAC) systems by shifting loads away from on-peak periods, to reduce the maximum heating or cooling capacity needed from the HVAC system, and to store excess energy generated by on-site solar power. The most commonly-used form of TES is ice storage with air conditioning (A/C) systems in commercial buildings. There has been extensive research into many other forms of TES for use with HVAC systems, both in commercial and residential buildings. However, this research is often limited to use with either heating or cooling systems.</p> <p>Year-round, high-density storage for both heating and cooling would yield significantly larger cost savings than existing TES systems, particularly for residential buildings, where heating loads are often larger than cooling loads. This dissertation examines the feasibility of using metal hydrides for year-round storage, as well as analyzing the potential of variable-temperature energy storage for optimizing system performance beyond allowing for year-round use.</p> <p>Metal hydrides are metals that exothermically absorb and endothermically desorb hydrogen. Since the temperature this reaction occurs at depends on the hydrogen pressure, hydrides can be used for energy storage at varying temperatures. System architecture for using metal hydrides with an HVAC system is developed. A thermodynamic model which combines a dynamic model of the hydride reactors with a static model of the HVAC system is used to calculate operating costs, compared to a conventional HVAC system, for different utility rates and locations. The payback period of the system is unacceptably high, due to the high initial cost of metal hydrides and the operating costs of compressing hydrogen to move it between hydride reactors.</p> <p>In addition to the metal hydride system model, a generalized model of a variable-temperature TES system is used to determine the potential cost savings from dynamically altering the storage temperature to achieve optimal cost savings. Dynamic tuning does result in cost savings but is most effective for storage tank sizes significantly smaller than the optimal tank size. An alternate system design where the storage tank is charged with the outlet flow from the house achieves larger cost savings even for the optimally-sized tanks. Payback periods calculated for optimal sizing show that year-round storage has a lower payback period than separate cold and heat storage if the year-round storage system is not more expensive than two separate storage tanks. </p>

Many Zintl phases exhibiting a CrB type structure form hydrides. Systematic studies of AeTtHx (Ae = Ca, Sr, Ba; Tt = Si, Ge, Sn), LnTtHx (Ln = La, Nd; Tt = Si, Ge, Sn), and LnGaHx (Ln = Nd, Gd) showed the vast structural diversity of these systems. Hydrogenation reactions on REGa (RE = Y, La, Tm) and RESi (RE = Y, Er, Tm) were performed in steel autoclaves under hydrogen pressure up to 5 MPa and temperatures up to 773 K. The products were analyzed by X-ray and neutron powder diffraction. RESi (RE = Y, Er, Tm) form hydrides in the C-LaGeD type. LaGaD1.66 is isostructural to NdGaD1.66 and shows similar electronic features. Ga-D distances (1.987(13) Å and 2.396(9) Å) are considerably longer than in polyanionic hydrides and not indicative of covalent bonding. In TmGaD0.93(2) with a distorted CrB type structure deuterium atoms exclusively occupy tetrahedral voids. Theoretical calculations on density functional theory (DFT) level confirm experimental results and suggest metallic properties for the hydrides.

<p>This thesis is divided into two parts. In the first part we study thermodynamics of the two-dimensional Heisenberg ferromagnet with dipolar interaction. This interaction breaks the conditions of the Mermin-Wagner theorem, resulting in a finite transition temperature. Our calculations are done within the framework of the self-consistent spin-wave theory (SSWT), which is modified in order to include the dipolar interaction. Both quantum and classical versions of the Heisenberg model are considered.</p><p>The second part of the thesis investigates the chemical bonding in solids from the first principles calculations. A new chemical bonding indicator called balanced crystal orbital overlap population (BCOOP) is developed. BCOOP is less basis set dependent than the earlier indicators and it can be used with full-potential density-functional theory (DFT) codes. We apply BCOOP formalism to the chemical bonding in the high-T_c superconductor MgB2 and the theoretically predicted MAX phase Nb3SiC2. We also study how the chemical bonding results in a repulsive hydrogen–hydrogen interaction in metal hydrides. The role of this interaction in the structural phase transition in Ti3SnHx is investigated.</p>

<p>Energy is one of the basic needs of human beings and is extremely crucial for continued development of human life. Our work, leisure and our economic, social and physical welfare all depend on the sufficient, uninterrupted supply of energy. Therefore, it is essential to provide adequate and affordable energy for improving human welfare and raising living standards. Global concern over environmental climate change linked to fossil fuel consumption has increased pressure to generate power from renewable sources [1]. Although substantial advances in renewable energy technologies have been made, significant challenges remain in developing integrated renewable energy systems due primarily to mismatch between load demand and source capabilities [2]. The output from renewable energy sources such as photo-voltaic, wind, tidal, and micro-hydro fluctuate on an hourly, daily, and seasonal basis. As a result, these devices are not well suited for directly powering loads that require a uniform and uninterrupted supply of input energy.</p>

This thesis is divided into two parts. In the first part we study thermodynamics of the two-dimensional Heisenberg ferromagnet with dipolar interaction. This interaction breaks the conditions of the Mermin-Wagner theorem, resulting in a finite transition temperature. Our calculations are done within the framework of the self-consistent spin-wave theory (SSWT), which is modified in order to include the dipolar interaction. Both quantum and classical versions of the Heisenberg model are considered. The second part of the thesis investigates the chemical bonding in solids from the first principles calculations. A new chemical bonding indicator called balanced crystal orbital overlap population (BCOOP) is developed. BCOOP is less basis set dependent than the earlier indicators and it can be used with full-potential density-functional theory (DFT) codes. We apply BCOOP formalism to the chemical bonding in the high-T_c superconductor MgB2 and the theoretically predicted MAX phase Nb3SiC2. We also study how the chemical bonding results in a repulsive hydrogen–hydrogen interaction in metal hydrides. The role of this interaction in the structural phase transition in Ti3SnHx is investigated.

<p>Energy is one of the basic needs of human beings and is extremely crucial for continued development of human life. Our work, leisure and our economic, social and physical welfare all depend on the sufficient, uninterrupted supply of energy. Therefore, it is essential to provide adequate and affordable energy for improving human welfare and raising living standards. Global concern over environmental climate change linked to fossil fuel consumption has increased pressure to generate power from renewable sources [1]. Although substantial advances in renewable energy technologies have been made, significant challenges remain in developing integrated renewable energy systems due primarily to mismatch between load demand and source capabilities [2]. The output from renewable energy sources such as photo-voltaic, wind, tidal, and micro-hydro fluctuate on an hourly, daily, and seasonal basis. As a result, these devices are not well suited for directly powering loads that require a uniform and uninterrupted supply of input energy.</p>

Magister Scientiae - MSc / Energy is one of the basic needs of human beings and is extremely crucial for continued development of human life. Our work, leisure and our economic, social and physical welfare all depend on the sufficient, uninterrupted supply of energy. Therefore, it is essential to provide adequate and affordable energy for improving human welfare and raising living standards. Global concern over environmental climate change linked to fossil fuel consumption has increased pressure to generate power from renewable sources [1]. Although substantial advances in renewable energy technologies have been made, significant challenges remain in developing integrated renewable energy systems due primarily to mismatch between load demand and source capabilities [2]. The output from renewable energy sources such as photo-voltaic, wind, tidal, and micro-hydro fluctuate on an hourly, daily, and seasonal basis. As a result, these devices are not well suited for directly powering loads that require a uniform and uninterrupted supply of input energy. / South Africa

Confining light metal hydrides in micro- or mesoporous scaffolds is considered to be a promising way to overcome the existing challenges for these materials, e.g. their application in hydrogen storage. Different techniques exist which allow us to homogeneously fill pores of a host matrix with the respective hydride, thus yielding well defined composite materials. For this report, the ordered mesoporous carbon CMK-3 was taken as a support for LiAlH₄ realized by a solution impregnation method to improve the hydrogen desorption behavior of LiAlH₄ by nanoconfinement effects. It is shown that upon heating, LiAlH₄ is unusually oxidized by coordinated tetrahydrofuran solvent molecules. The important result of the herein described work is the finding of a final composite containing nanoscale aluminum oxide inside the pores of the CMK-3 carbon host instead of a metal or alloy. This newly observed unusual oxidation behavior has major implications when applying these compounds for the targeted synthesis of homogeneous metal–carbon composite materials.