ALBERT

Description: Experiments in laser-heated diamond anvil cells (LH DACs) are conducted to assess phase diagrams of planetary materials at high pressure-temperature (P-T) conditions; thus, reliable determination of temperature in LH DAC experiments is essential. Radiometric temperature determination in LH DACs relies on the assumption of sample's wavelength-independent optical properties (graybody assumption), which is not justified for major lower mantle materials. The result is that experimental phase diagrams contain systematic unconstrained errors. Here we estimate the systematic error in radiometric temperature of nongray polycrystalline bridgmanite (Bgm; Mg0.96Fe2+0.036Fe3+0.014Si0.99O3) in a LH DAC by modeling emission and absorption of thermal radiation in a sample with experimentally-constrained optical properties. A comparison to experimental data validates the models and reveals that thermal spectra measured in LH DAC experiments record the interaction of radiation with the hot nongray sample. The graybody assumption in the experiments on translucent Bgm (light extinction coefficient, k 〈 ∼250 cm-1 at 500–900 nm) yields temperatures ∼5% higher than the maximum temperature in the sample heated to ∼1900 K. In contrast, the graybody temperature of dark Bgm (k 〉 ∼1500 cm−1), such as that produced upon melt quenching in LH DACs, underestimates the maximum temperature by ∼10%. Our experimental results pose quantitative constraints on the effect of nongray optical properties on the uncertainty of radiometric temperature determination in Bgm in the LH DACs. Evaluating nongray temperature in the future would enable a revision of the Bgm to post-perovskite phase transition and the high-pressure melting curve of Bgm.

Description: Optical studies of materials at high pressure-temperature (P-T) conditions provide insights into their physical properties that may be inaccessible to direct determination at extreme conditions. Incandescent light sources, however, are insufficiently bright to optically probe samples with radiative temperatures above ~1000 K. Here we report on a system to perform optical absorption experiments in a laser-heated diamond anvil cell at T up to at least 4000 K. This setup is based on a pulsed supercontinuum (broadband) light probe and a gated CCD detector. Precise and tight synchronization of the detector gates (3 ns) to the bright probe pulses (1 ns) diminishes the recorded thermal background and preserves an excellent probe signal at high temperature. We demonstrate the efficiency of this spectroscopic setup by measuring the optical absorbance of solid and molten (Mg,Fe)SiO3, an important constituent of planetary mantles, at P~30 GPa and T~1200-4150 K. Optical absorbance of hot solid (Mg,Fe)SiO3 is moderately sensitive to temperature but increases abruptly upon melting and acquires a strong temperature-dependence. Our results enable quantitative estimates of the opacity of planetary mantles with implications to their thermal and electrical conductivity, all of which have never been constrained at representative P-T conditions, and call for an optical detection of melting in silicate-bearing systems to resolve the extant ambiguity in their high-pressure melting curves.

Description: The evolution of chemical bonding in ferropericlase, (Mg,Fe)O, with pressure may affect the physical and chemical properties of the Earth’s lower mantle. Here, we report highpressure optical absorption spectra of single-crystalline ferropericlase ((Mg0.87Fe0.13)O) up to 135 GPa. Combined with a re-evaluation of published partial fluorescence yield X-ray absorption spectroscopy data, we show that the covalency of the Fe−O bond increases with pressure, but the iron spin transition at 57−76.5 GPa reverses this trend. The qualitative crossover in chemical bonding suggests that the spin-pairing transition weakens the Fe−O bond in ferropericlase. We find, that the spin transition in ferropericlase is caused by both the increase of the ligand field-splitting energy and the decrease in the spin-pairing energy of high-spin Fe2+.

Description: We report the first direct measurements of the refractive index of silica glass up to 145 GPa that allowed quantifying its density, bulk modulus, Lorenz-Lorentz polarizability, and band gap. These properties show two major anomalies at ∼10 and ∼40 GPa. The anomaly at ∼10 GPa signals the onset of the increase in Si coordination, and the anomaly at ∼40 GPa corresponds to a nearly complete vanishing of fourfold Si. More generally, we show that the compressibility and density of noncrystalline solids can be accurately measured in simple optical experiments up to at least 110 GPa.

Description: Refractive index provides fundamental insights into the electronic structure of materials. At high pressure, however, the determination of refractive index and its wavelength dispersion is challenging, which limits our understanding of how physical properties of even simple materials, such as MgO, evolve with pressure. Here, we report on the measurement of room-temperature refractive index of MgO up to ∼140 GPa. The refractive index of MgO at 600 nm decreases by ∼2.4% from ∼1.737 at 1 atm to ∼1.696 (±0.017) at ∼140 GPa. Despite the index at 600 nm is essentially pressure independent, the absolute wavelength dispersion of the refractive index at 550–870 nm decreases by ∼28% from ∼0.015 at 1 atm to ∼0.011 (±8.04 × 10−4) at ∼103 GPa. Single-effective-oscillator analysis of our refractive index data suggests that the bandgap of MgO increases by ∼1.1 eV from 7.4 eV at 1 atm to ∼8.5 (±0.6) eV at ∼103 GPa.

Description: Physical properties of mantle minerals are essential for comprehensive geodynamic modelling. High-pressure experiments allow measurements of physical properties but fundamental insights into their evolution with pressure are often experimentally inaccessible. Here we report the first in situ experimental determination of the optical refractive index, its wavelength-dispersion, and optical absorption coefficient of ferropericlase up to ~140 GPa at room temperature. All these properties change gradually in dominantly high-spin (below ~50 GPa) and low-spin (above ~80 GPa) ferropericlase. However, in the mixed-spin state (i.e., significant presence of both high- and low-spin iron), the index dispersion and the absorption coefficient decrease by a factor of three and ~30 %, respectively. These anomalies suggest that charge transport by small polaron is reduced in mixed-spin ferropericlase, providing fundamental insights into the factor-of-three lower electrical conductivity of ferropericlase at ~50-70 GPa.

Description: Quaternary kesterite-type (KS) compounds have attracted worldwide attention from the scientific community as promising materials for solar cells. On the route to optimizing their performance, the effect of stress and strain constitutes a critical factor when it comes to thin film applications. Following a recent theoretical study, we report here joint experimental and computational high-pressure investigations on the KS Ag2ZnSnS4 and wurtz–kesterite (WZ–KS)-type Ag2CdSnS4 compounds. Our results reveal that both materials undergo successive transformations, first into a GeSb-type and then toward a CrN-type modification at ambient temperature. Our theoretical calculations predict a metallic character for all Ag2ZnSnS4 and Ag2CdSnS4 high-pressure phases. In addition, structural disorder is observed in KS Ag2ZnSnS4 upon moderate compression, prior to its KS → GeSb-type transition. Decompression leads to the recovery of a disordered zinc blende-type structure in the latter, whereas Ag2CdSnS4 retains the disordered GeSb-type modification. The similarities and deviations from the archetypical KS Cu2ZnSnS4 are discussed.

Description: Large parts of the Earth’s interior are inaccessible to direct observation, yet global geodynamic processes are governed by the physical material properties under extreme pressure and temperature conditions. It is therefore essential to investigate the deep Earth’s physical properties through in-situ laboratory experiments. With this goal in mind, the optical properties of mantle minerals at high pressure offer a unique way to determine a variety of physical properties, in a straight-forward, reproducible, and time-effective manner, thus providing valuable insights into the physical processes of the deep Earth. This thesis focusses on the system Mg-Fe-O, specifically on the optical properties of periclase (MgO) and its iron-bearing variant ferropericlase ((Mg,Fe)O), forming a major planetary building block. The primary objective is to establish links between physical material properties and optical properties. In particular the spin transition in ferropericlase, the second-most abundant phase of the lower mantle, is known to change the physical material properties. Although the spin transition region likely extends down to the core-mantle boundary, the ef-fects of the mixed-spin state, where both high- and low-spin state are present, remains poorly constrained. In the studies presented herein, we show how optical properties are linked to physical properties such as electrical conductivity, radiative thermal conductivity and viscosity. We also show how the optical properties reveal changes in the chemical bonding. Furthermore, we unveil how the chemical bonding, the optical and other physical properties are affected by the iron spin transition. We find opposing trends in the pres-sure dependence of the refractive index of MgO and (Mg,Fe)O. From 1 atm to ~140 GPa, the refractive index of MgO decreases by ~2.4% from 1.737 to 1.696 (±0.017). In contrast, the refractive index of (Mg0.87Fe0.13)O (Fp13) and (Mg0.76Fe0.24)O (Fp24) ferropericlase increases with pressure, likely because Fe Fe interactions between adjacent iron sites hinder a strong decrease of polarizability, as it is observed with increasing density in the case of pure MgO. An analysis of the index dispersion in MgO (decreasing by ~23% from 1 atm to ~103 GPa) reflects a widening of the band gap from ~7.4 eV at 1 atm to ~8.5 (±0.6) eV at ~103 GPa. The index dispersion (between 550 and 870 nm) of Fp13 reveals a decrease by a factor of ~3 over the spin transition range (~44–100 GPa). We show that the electrical band gap of ferropericlase significantly widens up to ~4.7 eV in the mixed spin region, equivalent to an increase by a factor of ~1.7. We propose that this is due to a lower electron mobility between adjacent Fe2+ sites of opposite spin, explaining the previously observed low electrical conductivity in the mixed spin region. From the study of absorbance spectra in Fp13, we show an increasing covalency of the Fe-O bond with pressure for high-spin ferropericlase, whereas in the low-spin state a trend to a more ionic nature of the Fe-O bond is observed, indicating a bond weakening effect of the spin transition. We found that the spin transition is ultimately caused by both an increase of the ligand field-splitting energy and a decreasing spin-pairing energy of high-spin Fe2+.

Description: Geodynamische Prozesse werden von den physikalischen Materialeigenschaften unter den extremen Druck- und Temperaturbedingungen des Erdinneren gesteuert, gerade diese Areale sind aber faktisch nicht für direkte Beobachtungen zugänglich. Umso wichtiger ist es, die physikalischen Eigenschaften unter Bedingungen des Erdinneren zu untersuchen. Mit diesem Ziel vor Augen erlaubt das Studium der optischen Eigenschaften von Mineralen des Erdmantels, eine große Bandbreite an physikalischen Materialeigenschaften, in einer einfachen, reproduzierbaren und effizienten Art und Weise zu bestimmen. Dadurch bieten sich wichtige Einblicke in die physikalischen Prozessen des Erdinneren. Die vorliegende Arbeit konzentriert sich auf das System Mg-Fe-O, im Speziellen auf Periklas (MgO) und seine Eisen-haltige Variante Ferroperiklas ((Mg,Fe)O), ein wichtiger Baustein planetarer Körper. Das Hauptziel der Arbeit besteht darin Verbindungen zwischen optischen Eigenschaften und physikalischen Materialeigenschaften zu finden. Gerade der Spin-Übergang in Ferroperiklas, der zweithäufigsten Phase des unteren Erdmantels, ist dabei von Bedeutung, da damit Veränderungen in den physikalischen Materialeigenschaften einhergehen. Obwohl sich der Spinübergangsbereich vermutlich bis zur Kern-Mantel-Grenze erstreckt, sind die Auswirkungen des gemischten Spin-Zustandes, bei dem sowohl Hoch- als auch Tief-Spin präsent sind, nur unzureichend untersucht. Die hier vorgestellten Studien zeigen, wie optische Eigenschaften mit anderen wichtigen physikalischen Eigenschaften wie elektrischer und thermischer Leitfähigkeit, Viskosität oder auch mit der chemischen Bindung verbunden sind. Daraus lässt sich auch ableiten wie der Spin-Übergang in Ferroperiklas diese Eigenschaften beeinflusst. Von Raumbedingungen bis zu ~140 GPa sinkt der Brechungsindex von MgO um ~2.4 % von 1.737 auf 1.696 (±0.017). Im Gegensatz dazu steigt der Brechungsindex von (Mg0.87Fe0.13)O (Fp13) und (Mg0.76Fe0.24)O (Fp24) Ferroperiklas mit dem Druck an. Dies ist auf Fe-Fe Wechselwirkungen zwischen benachbarten Eisenpositionen zurückzuführen, die eine starke Verringerung der Polarisierbarkeit, wie im Falle von reinem MgO mit zunehmender Dichte, behindern. Eine Analyse der Dispersion des Brechungsindexes von MgO (Abnahme um ~23 % von 1 Atm zu ~103 GPa) offenbart eine Verbreiterung der Bandlücke von ~7.4 eV bei 1 Atm zu ~8.5 (±0.6) eV bei ~103 GPa. Die Messung der Dispersion (zwischen 550 und 870 nm) in Fp13 zeigt eine starke Abnahme über den Bereich des Spin-Überganges (~44–100 GPa) bis zu einem Faktor von ~3. Die Bandlücke nimmt in der Region des gemischten Spin-Zustandes signifikant auf bis zu ~4.7 eV zu (entspricht einer Zunahme um den Faktor ~1.7). Dies deutet auf eine Verringerung der Elektronen-Mobilität zwischen benachbarten Fe2+-Positionen mit unterschiedlichem Spin-Zustand hin, was die bereits in früheren Arbeiten beobachtete Abnahme der elektrischen Leitfähigkeit im Bereich des gemischten Spin-Zustandes erklärt. Absorptionsspektren an Fp13 zeigen eine Druck-bedingte Zunahme der Kovalenz der Fe-O Bindung für Ferroperiklas im Hoch-Spin Zustand, wohingegen Tief-Spin Ferroperiklas einen Trend zu einer mehr ionischen Fe-O Bindung auf-weist, was auf einen Bindungs-schwächenden Effekt des Spin-Wechsels hinweist. Der Übergang von Hoch- zu Tiefspin ist letztlich auf eine Zunahme der Ligandenfeldaufspaltungsenergie sowie eine abnehmende Spinpaarungsenergie von Hoch-Spin Fe2+ zurückzuführen.