The study of planetary interiors relies on understanding the composition and thermodynamic behavior of materials under extreme conditions. Seismological data suggest that the Earth’s core contains light elements in addition to iron, with silicon and oxygen being strong candidates. Fe-Si-O ternary alloys are of particular interest for their potential role in geodynamo generation, as SiO2 exsolution-driven buoyancy effects may influence core dynamics. However, direct experimental investigations at extreme pressures and temperatures remain challenging, necessitating the use of computational methods to complement experimental findings. In this study, ab initio simulations were performed to explore the properties of a Fe-Si-O alloy in a range of conditions relevant to rocky-planet interiors. The equation of state, structural, diffusion, and absorption properties were analyzed and compared with shock-compression and X-ray absorption spectroscopy (XAS) data from the ESRF ID24-ED beamline. Thermodynamic analysis confirmed that the alloy remains in the liquid phase across the investigated conditions, with no signs of phase transitions. The computed P-wave velocity exceeds seismological estimates for Earth's outer core, suggesting that the modeled Si and O concentrations are higher than those expected in reality. The simulated Hugoniot curve follows the experimental trend. Structural analysis through the radial distribution function (RDF) revealed a shoulder at 4000K, attributed to Si–O interactions. The Si–O coordination number remains below 2, ruling out SiO2 exsolution. This conclusion is supported by diffusion analysis, which suggests that observed clustering effects arise from simulation constraints rather than intrinsic alloy properties. XAS spectra seem to exhibit an isoabsorption range, suggesting that shock compression may not significantly alter the electronic energy bands of the alloy. The simulated near-edge region approximately aligns with experimental data, but large uncertainties in experimental conditions and low signal-to-noise ratio limit definitive conclusions regarding the reached structural state of the alloy under shock compression.
Lo studio dell’interno dei pianeti si basa sulla comprensione della composizione e del comportamento dei materiali in condizioni estreme. Dati sismologici indicano che il nucleo terrestre contiene elementi leggeri in aggiunta al ferro, con silicio e ossigeno tra i principali candidati. Le leghe ternarie Fe-Si-O sono di particolare interesse per il loro possibile ruolo nella generazione della geodinamo, poiché l’essoluzione di SiO2 potrebbe influenzare la dinamica del nucleo. Tuttavia, gli esperimenti ad alte pressioni e temperature presentano sfide tecnologiche e interpretative, rendendo essenziale l’uso di metodi computazionali per integrare i dati sperimentali. In questo lavoro, vengono utilizzate simulazioni ab initio per analizzare le proprietà termodinamiche e strutturali di una lega ternaria Fe-Si-O in condizioni rilevanti per l’interno dei pianeti rocciosi. Sono studiati l’equazione di stato, la struttura , la diffusione e l’assorbimento, confrontando i risultati con dati sperimentali di compressione da shock e spettroscopia di assorbimento, ottenuti presso la beamline ID24-ED dell’European Synchrotron Radiation Facility. L’analisi termodinamica conferma che la lega rimane liquida, non mostrando segni di transizioni di fase. La velocità delle onde P della lega supera le stime sismologiche, suggerendo maggiori concentrazioni di Si e O rispetto al nucleo terrestre. La curva di Hugoniot simulata segue la tendenza sperimentale. L’analisi strutturale a 4000K evidenzia l’influenza delle interazioni Si–O sulla Radial Distribution Function. Il numero di coordinazione Si–O inferiore a 2 esclude l’essoluzione di SiO2. Questo è supportato dall’analisi sulla diffusione, la quale mostra che l’effetto osservato è dovuto alla configurazione iniziale utilizzata nelle simulazioni più che a una proprietà intrinseca della lega. Gli spettri di assorbimento mostrano una zona di iso-assorbimento, suggerendo che la compressione da shock potrebbe non modificare significativamente le bande energetiche elettroniche della lega. Il confronto con gli spettri sperimentali mostra un buon accordo nella regione del picco, ma le incertezze sperimentali e il basso rapporto segnale-rumore alle alte energie limitano interpretazioni definitive sulla struttura della lega raggiunta sotto shock.
Investigation of a liquid Fe-Si-O alloy at planetary core conditions via atomistic simulations
FUSILLI, ANDREA
2024/2025
Abstract
The study of planetary interiors relies on understanding the composition and thermodynamic behavior of materials under extreme conditions. Seismological data suggest that the Earth’s core contains light elements in addition to iron, with silicon and oxygen being strong candidates. Fe-Si-O ternary alloys are of particular interest for their potential role in geodynamo generation, as SiO2 exsolution-driven buoyancy effects may influence core dynamics. However, direct experimental investigations at extreme pressures and temperatures remain challenging, necessitating the use of computational methods to complement experimental findings. In this study, ab initio simulations were performed to explore the properties of a Fe-Si-O alloy in a range of conditions relevant to rocky-planet interiors. The equation of state, structural, diffusion, and absorption properties were analyzed and compared with shock-compression and X-ray absorption spectroscopy (XAS) data from the ESRF ID24-ED beamline. Thermodynamic analysis confirmed that the alloy remains in the liquid phase across the investigated conditions, with no signs of phase transitions. The computed P-wave velocity exceeds seismological estimates for Earth's outer core, suggesting that the modeled Si and O concentrations are higher than those expected in reality. The simulated Hugoniot curve follows the experimental trend. Structural analysis through the radial distribution function (RDF) revealed a shoulder at 4000K, attributed to Si–O interactions. The Si–O coordination number remains below 2, ruling out SiO2 exsolution. This conclusion is supported by diffusion analysis, which suggests that observed clustering effects arise from simulation constraints rather than intrinsic alloy properties. XAS spectra seem to exhibit an isoabsorption range, suggesting that shock compression may not significantly alter the electronic energy bands of the alloy. The simulated near-edge region approximately aligns with experimental data, but large uncertainties in experimental conditions and low signal-to-noise ratio limit definitive conclusions regarding the reached structural state of the alloy under shock compression.File | Dimensione | Formato | |
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https://hdl.handle.net/10589/236130