Synthese und Charakterisierung von sp2-, sp3- und Pyrocarbonaten

Die nachfolgende Dissertation wurde an der Goethe-Universität Frankfurt am Institut für Geowissenschaften (FB 11) in der Arbeitsgruppe Kristallographie und Mineralogie (AG Winkler) verfasst. Die Betreuung der hier durchgeführten Arbeiten erfolgte hauptsächlich durch Prof. B. Winkler in Zusammenarbeit mit Dr. L. Bayarjargal, PD Dr. E. Haussühl und PD Dr. V. Vinograd. Bei dem vorliegenden Manuskript handelt es sich um eine kumulative bzw. publikationsbasierte Dissertation, welche die Forschungsergebnisse verschiedener bereits veröffentlichter wissenschaftlicher Fachartikel zusammenfasst. Die Arbeit beschreibt verschiedene Synthesen und Untersuchungen an Carbonaten und teilt sich im Wesentlichen in zwei Abschnitte. Zum einen wurden Experimente mit Carbonaten bei Extrembedingungen bzw. unter hohen Drücken und hohen Temperaturen durchgeführt, wie sie auch im Inneren der Erde zu finden sind. Im zweiten Teil wurden Carbonate bei Raumbedingungen synthetisiert und der Einbau von Seltenerdelementen untersucht. Grundsätzlich werden jedoch in beiden Teilen dieser Arbeit die Strukturen und Eigenschaften verschiedener Carbonate und eine mögliche Kationensubstitution bzw. die Synthese isostruktureller Verbindungen erforscht

Incorporation of Europium into (Ba,Ca)2_2(CO3_3)2_2

Synchrotron-based powder diffraction measurements in combination with inductively coupled plasma optical emission spectrometry, Raman and fluorescence spectroscopy show that (Ba,Ca)$_2$(CO$_3$)$_2$ can incorporate significant amounts (up to 6 ​mol%) of europium. This solid solution is therefore of potential interest for the solidification of nuclear waste streams involving aqueous nitrate solutions of lanthanides. Europium replaces Ba/Ca on lattice sites and is not incorporated as an interstitial defect. Charge compensation is likely due to the presence of OH$^−$-groups as we could exclude a coupled substitution involving Na$^+$. The Eu-containing compound is stable to at least 723 ​K. We show that the one-phase-field of (Bax,Ca($_{1−x}$))CO$_3$ solid solutions at ambient conditions is larger (0.36 $< x <$ 0.51) than previously thought. The synthesis routes employed here lead to compounds which have similar molar volumes than those of the naturally occurring (Ba,Ca)-double carbonates, in noted contrast to another synthetic phase, “balcite”

Structural, Physical, and Thermodynamic Properties of Aragonitic Cax_xSr1–x_{1–x} CO3_3 Solid Solutions

ingle-phase Ca$_x$Sr$_{1–x}$ CO$_3$ solid solutions with an aragonite structure and x = 0–1 were obtained by precipitation synthesis and characterized with respect to their structural, physical, and thermodynamic properties. While there is no measurable excess volume of mixing, relaxation microcalorimetry showed a noticeable excess enthalpy of mixing, $ΔH_{298}$$^{ex}$. While the values determined here are much smaller than those obtained in an earlier study by solution calorimetry, they are consistent with the results of density functional theory (DFT)-based calculations. A combination of Raman spectroscopy and atomistic modeling confirms the finding based on the analysis of the diffraction data that there is no ordering of the Ca and Sr atoms. For the atomistic models, both quasi-random special structures and the virtual crystal approximation were employed. The dependence of the bulk modulus on composition was obtained by fitting an equation of state (EOS) to high-pressure synchrotron powder diffraction data and by DFT-based calculations, where stress–strain and compression data were employed. Both, the experimental and the DFT pressure-dependent data show an anomalous pressure-induced elongation of the b-axis around 20–30 GPa, depending on composition, but no structural pressure-induced phase transition until 40 GPa, the highest pressure investigated here

Pressure-induced Pb-Pb bonding and phase transition in Pb2SnO4

High-pressure single-crystal to 20 GPa and powder diffraction measurements to 50 GPa, show that the structure of Pb2SnO4 strongly distorts on compression with an elongation of one axis. A structural phase transition occurs between 10 GPa and 12 GPa, with a change of space group from Pbam to Pnam. The resistivity decreases by more than six orders of magnitude when pressure is increased from ambient conditions to 50 GPa. This insulator-to-semiconductor transition is accompanied by a reversible appearance change from transparent to opaque. Density functional theory-based calculations show that at ambient conditions the channels in the structure host the stereochemically-active Pb 6s2 lone electron pairs. On compression the lone electron pairs form bonds between Pb2+ ions. Also provided is an assignment of irreducible representations to the experimentally observed Raman bands

Sr[C2O5]Sr[C_2O_5] is an Inorganic Pyrocarbonate Salt with [C2O5]2–[C_2O_5]^{2–} Complex Anions

The synthesis of a novel type of carbonate, namely of the inorganic pyrocarbonate salt Sr[C$_2$O$_5$], which containsisolated [C$_2$O$_5$]$^{2−}$-groups, significantly extends the crystal chemistry of inorganic carbonates beyond the established sp$^{2}$ and sp$^{3}$-carbonates. We synthesized Sr[C$_2$O$_5$] in a laser-heated diamond anvil cell by reacting Sr[CO$_3$] with CO$_2$. By single crystalsynchrotron diffraction, Raman spectroscopy, and density functional theory (DFT) calculations, we show that it is a pyrocarbonatesalt. Sr[C$_2$O$_5$] is the first member of a novel family of inorganic carbonates. We predict, based on DFT calculations, that furtherinorganic pyrocarbonates can be obtained and that these will be relevant to geoscience and may provide a better understanding ofreactions converting CO$_2$ into useful inorganic compounds

Sr3_3[CO4_4]O Antiperovskite with Tetrahedrally Coordinated sp3^3-Hybridized Carbon and OSr6_6 Octahedra

We have synthesized the orthocarbonate Sr$_3$[CO$_4$]O in a laser-heated diamond anvil cell at 20 and 30 GPa by heating to ≈3000 (300) K. Afterward, we recovered the orthocarbonate with [CO$_4$]$^{4–}$ groups at ambient conditions. Single-crystal diffraction shows the presence of [CO$_4$]$^{4–}$ groups, i.e., sp$^3$-hybridized carbon tetrahedrally coordinated by covalently bound oxygen atoms. The [CO$_4$]$^{4–}$ tetrahedra are located in a cage formed by corner-sharing OSr$_6$ octahedra, i.e., octahedra with oxygen as a central ion, forming an antiperovskite-type structure. At high pressures, the octahedra are nearly ideal and slightly rotated. The high-pressure phase is tetragonal (I$_4$/mcm). Upon pressure release, there is a phase transition with a symmetry lowering to an orthorhombic phase (Pnma), where the octahedra tilt and deform slightly

Synthesis and Structure of Pb[C2O5]Pb[C_2O_5]: An Inorganic Pyrocarbonate Salt

We have synthesized $Pb[C_2O_5]$, an inorganic pyrocarbonate salt, in a laser-heated diamond anvil cell (LH-DAC) at 30 GPa by heating a $Pb[CO_3]$ + $CO_2$ mixture to ≈2000(200) K. Inorganic pyrocarbonates contain isolated $[C_2O_5]^{2–}$ groups without functional groups attached. The $[C_2O_5]^{2–}$ groups consist of two oxygen-sharing [CO3]3– groups. $Pb[C_2O_5]$ was characterized by synchrotron-based single-crystal structure refinement, Raman spectroscopy, and density functional theory calculations. $Pb[C_2O_5]$ is isostructural to $Sr[C_2O_5]$ and crystallizes in the monoclinic space group $P2_1/c$ with Z = 4. The synthesis of $Pb[C_2O_5]$ demonstrates that, just like in other carbonates, cation substitution is possible and that therefore inorganic pyrocarbonates are a novel family of carbonates, in addition to the established $sp^2$ and $sp^3$ carbonates

Formation of chondrule analogs aboard the International Space Station

Chondrules are thought to play a crucial role in planet formation, but the mechanisms leading to their formation are still a matter of unresolved discussion. So far, experiments designed to understand chondrule formation conditions have been carried out only under the influence of terrestrial gravity. In order to introduce more realistic conditions, we developed a chondrule formation experiment, which was carried out at long‐term microgravity aboard the International Space Station. In this experiment, freely levitating forsterite (Mg2SiO4) dust particles were exposed to electric arc discharges, thus simulating chondrule formation via nebular lightning. The arc discharges were able to melt single dust particles completely, which then crystallized with very high cooling rates of >105 K h−1. The crystals in the spherules show a crystallographic preferred orientation of the [010] axes perpendicular to the spherule surface, similar to the preferred orientation observed in some natural chondrules. This microstructure is probably the result of crystallization under microgravity conditions. Furthermore, the spherules interacted with the surrounding gas during crystallization. We show that this type of experiment is able to form spherules, which show some similarities with the morphology of chondrules despite very short heating pulses and high cooling rates.Carl Zeiss Meditec AG http://dx.doi.org/10.13039/501100002806BIOVIA Science Ambassador programBundesministerium für Wirtschaft und Energie http://dx.doi.org/10.13039/501100006360Deutsches Zentrum für Luft‐ und Raumfahrt http://dx.doi.org/10.13039/501100002946NanoRacks LLCDreamUpDeutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659Dr. Rolf M. Schwiete Stiftun

Novel Calcium sp 3 Carbonate CaC2_2O5_5-I4ˉI\bar{4}2d May Be a Carbon Host in Earth’s Lower Mantle

CaC$_2$O$_5$-$I\bar{4}$2d was obtained by reacting CO$_2$ and CaCO$_3$ at lower Earth mantle pressures and temperatures ranging between 34 and 45 GPa and between 2000 and 3000 K, respectively. The crystal structure was solved by single-crystal X-ray diffraction and contains carbon atoms tetrahedrally coordinated by oxygen. The tetrahedral CO$_4$$^{4−}$groups form pyramidal [C$_4$O10]$^{4−}$complex anions by corner sharing. Raman spectroscopy allows an unambiguous identification of this compound, and the experimentally determined spectra are in excellent agreement with Raman spectra obtained from density functional theory calculations. CaC$_2$O$_5$-$I\bar{4}$2d persists on pressure release down to ∼18 GPa at ambient temperature, where it decomposes into calcite and, presumably, CO$_2$ under ambient conditions. As polymorphs of CaCO$_3$ and CO$_2$ are believed to be present in the vicinity of subducting slabs within Earth’s lower mantle, they would react to give CaC$_2$O$_5$-$I\bar{4}$2d, which therefore needs to be considered instead of end-member CaCO$_3$ in models of the mantle mineralogy

Formation of fused aggregates under long‐term microgravity conditions aboard the ISS with implications for early solar system particle aggregation

In order to gain further insights into early solar system aggregation processes, we carried out an experiment on board the International Space Station, which allowed us to study the behavior of dust particles exposed to electric arc discharges under long‐term microgravity. The experiment led to the formation of robust, elongated, fluffy aggregates, which were studied by scanning electron microscopy, electron backscatter diffraction, and synchrotron micro‐computed tomography. The morphologies of these aggregates strongly resemble the typical shapes of fractal fluffy‐type calcium‐aluminum‐rich inclusions (CAIs). We conclude that a small amount of melting could have supplied the required stability for such fractal structures to have survived transportation and aggregation to and compaction within planetesimals. Other aggregates produced in our experiment have a massy morphology and contain relict grains, likely resulting from the collision of grains with different degrees of melting, also observed in some natural CAIs. Some particles are surrounded by igneous rims, which remind in thickness and crystal orientation of Wark–Lovering rims; another aggregate shows similarities to disk‐shaped CAIs. These results imply that a (flash‐)heating event with subsequent aggregation could have been involved in the formation of different morphological CAI characteristics.BIOVIANordlicht GmbHDeutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659Bundesministerium für Wirtschaft und Energie http://dx.doi.org/10.13039/501100006360NanoRacks LLCDr. Rolf M. Schwiete Stiftung http://dx.doi.org/10.13039/501100020027Deutsches Zentrum für Luft‐ und Raumfahrt http://dx.doi.org/10.13039/501100002946DreamUpCarl Zeiss Meditec AG http://dx.doi.org/10.13039/50110000280