arXiv subjects

Carmen Sanchez-Valle

Publications and source records attributed to Carmen Sanchez-Valle.

4 recordsLinked to original sources

arXiv2026

Iron spin crossover in ferropericlase and its effect on lower-mantle thermal conductivity

Thermal conductivity of Earths lower mantle controls heat transfer across the core-mantle boundary (CMB) and strongly influences mantle convection. We report direct measurements of the thermal conductivity of single-crystal ferropericlase (Mg$_{1-x}$Fe$_x$O, $x = 0.09$-0.13), the second most abundant lower-mantle mineral, using optical laser flash and X-ray free-electron laser heating in diamond-anvil cells up to $\sim2200$~K and 130~GPa. These experiments provide the first conductivity data for ferropericlase at simultaneous lower-mantle pressures and temperatures. A marked reduction in conductivity between 60 and 100~GPa at $\sim1700$~K is consistent with the iron spin crossover. Combined with our previous results for Fe- and Fe,Al-bearing bridgmanite, the data define a lower-mantle conductivity profile that increases with pressure to $\sim10$~W\,m$^{-1}$\,K$^{-1}$ near the CMB, constraining mantle heat flux, plume buoyancy, and long-term geodynamic evolution.

physics.geo-ph↗

arXiv2025

Compressional rate-dependent stability of ammonia hydrates crystallized from water-rich ammonia-water solutions

Understanding the crystallization pathways of water-rich ammonia-water (NH3-H2O) solutions and the stability of ammonia hydrates is key to unraveling the behavior of complex hydrogen-bonding networks as well as for planetary interior modelling. Yet, there are still inconsistencies in the crystallization sequence reported upon pressure-induced crystallization of H2O-rich NH3-H2O solutions at room temperature. Here, we investigate the effect of compression rates on the crystallization pathways of 25 wt% NH3 aqueous solutions at room temperature using dynamically compressed diamond anvil cells (dDAC) coupled with time-resolved X-ray diffraction. We show that compression rates exceeding 0.5 GPa/sec promote direct crystallization of a body-centered cubic (bcc) phase (DMA') with possible AMH stoichiometry coexisting with H2O ice VII, while rates below 0.2 GPa/sec stabilize monoclinic NH3-rich AHH-II and ice VII phases. Intermediate rates between 0.2-0.5 GPa/sec produce a mixture of both hydrates alongside ice VII, hence demonstrating the role of compression rate on the crystallization sequence of ammonia solutions. The compression behavior and phase stability of the distinct phase assemblies (AHH-II/DMA' + ice VII) are investigated further to place constraints on the composition of the DMA' phase, the effect of ice VII on the compressibility of ammonia hydrates, and the plausible incorporation of NH3 impurities within the lattice of high-pressure ice phases.

cond-mat.mtrl-sci↗

arXiv2025

Evidence for ultra-water-rich ammonia hydrates stabilized in icy exoplanetary mantles

Understanding the behavior of the water-ammonia system at high pressure-high temperature conditions is important for modeling the internal dynamics of exoplanet icy mantles. Conventionally, mixtures of ammonia hemihydrate AHH (2:1 ammonia-water molar ratio) and H2O ice VII have been regarded as the ultimate solid phase assembly in the system. Here we report evidence for chemical reactions between AHH and ice VII above 750 K and 16 GPa that stabilize water-rich ammonia hydrates, including a novel ultra-water rich hydrate NH3.6H2O (1:6 ratio) coexisting with ammonia dihydrate ADH (1:2 ratio) and excess ice VII. This assembly is stable up to at least 30 GPa and 1600 K and can be quenched to room temperature. Our results demonstrate that water-rich ammonia hydrates are favored in the icy mantle of 1-2 MEarth exoplanets regardless of the ammonia content of the hydrate crystallized during accretion and/or evolution as long as excess H2O ice is available. The buoyancy contrast between water-rich hydrates and ice VII may lead to chemical stratification in exoplanet icy mantles, hence affecting their thermal evolution.

astro-ph.EP↗

arXiv2023

Pressure dependence of intra- and interlayer excitons in 2H-MoS$_2$ bilayers

The optical and electronic properties of multilayer transition metal dichalcogenides differ significantly from their monolayer counterparts due to interlayer interactions. The separation of individual layers can be tuned in a controlled way by applying pressure. Here, we use a diamond anvil cell to compress bilayers of 2H-MoS$_2$ in the gigapascal range. By measuring optical transmission spectra, we find that increasing pressure leads to a decrease in the energy splitting between the A and interlayer exciton. Comparing our experimental findings with ab initio calculations, we conclude that the observed changes are not due to the commonly assumed hydrostatic compression. This effect is attributed to the MoS$_2$ bilayer adhering to the diamond, which reduces in-plane compression. Moreover, we demonstrate that the distinct real-space distributions and resulting contributions from the valence band account for the different pressure dependencies of the inter- and intralayer excitons in compressed MoS$_2$ bilayers.

cond-mat.mes-hall↗