arXiv subjects

Anibal J. Ramirez-Cuesta

Publications and source records attributed to Anibal J. Ramirez-Cuesta.

3 recordsLinked to original sources

arXiv2023

Dynamic crystallography reveals spontaneous anisotropy in cubic GeTe

Cubic energy materials such as thermoelectrics or hybrid perovskite materials are often understood to be highly disordered. In GeTe and related IV-VI compounds, this is thought to provide the low thermal conductivities needed for thermoelectric applications. Since conventional crystallography cannot distinguish between static disorder and atomic motions, we develop the energy-resolved variable-shutter pair distribution function technique. This collects structural snapshots with varying exposure times, on timescales relevant for atomic motions. In disagreement with previous interpretations, we find the time-averaged structure of GeTe to be crystalline at all temperatures, but with anisotropic anharmonic dynamics at higher temperatures that resemble static disorder at fast shutter speeds, with correlated ferroelectric fluctuations along the $<$100$>$c direction. We show that this anisotropy naturally emerges from a Ginzburg-Landau model that couples polarization fluctuations through long-range elastic interactions. By accessing time-dependent atomic correlations in energy materials, we resolve the long-standing disagreement between local and average structure probes, and show that spontaneous anisotropy is ubiquitous in cubic IV-VI materials.

cond-mat.mtrl-sci↗

arXiv2021

Proton polarons in HxWO3 by synchrotron photoemission and DFT modelling

The measurement of hydrogen induced changes on the electronic structure of transition metal oxides by X-ray photoelectron spectroscopy is a challenging endeavor, since the origin of the photoelectron cannot be unambiguously assigned to hydrogen. The H-induced electronic structure changes in tungsten trioxide have been known for more than 100 years, but are still being controversially debated. The controversy stems from the difficulty in disentangling effects due to hydrogenation from the effects of oxygen deficiencies. Using a membrane approach to X-ray photoelectron spectroscopy, in combination with tuneable synchrotron radiation we measure simultaneously core levels and valence band up to a hydrogen pressure of 1000 mbar. Upon hydrogenation, the intensities of the W$^{5+}$ core level and a state close to the Fermi level increase following the pressure-composition isotherm curve of bulk H$_x$WO$_3$. Combining experimental data and density-functional theory the description of the hydrogen induced coloration by a proton polaron model is corroborated. Although hydrogen is the origin of the electronic structure changes near the Fermi edge, the valence band edge is now dominated by tungsten orbitals instead of oxygen as is the case for the pristine oxide having wider implication for its use as (photo-electrochemical) catalyst.

cond-mat.mtrl-sci↗

arXiv2020

Study of anharmonicity in Zirconium Hydrides using inelastic neutron scattering and ab-initio computer modeling

The anharmonic phenomena in Zirconium Hydrides and Deuterides, including ε-ZrH2, γ-ZrH, and γ-ZrD, have been investigated from aspects of inelastic neutron scattering (INS) and lattice dynamics calculations within the framework of density functional theory (DFT). The observed multiple sharp peaks below harmonic multi-phonon bands in the experimental spectra of all three materials did not show up in the simulated INS spectra based on the harmonic approximation, indicating the existence of strong anharmonicity in those materials and the necessity of further explanations. We present a detailed study on the anharmonicity of zirconium hydrides/deuterides by exploring the 2D potential energy surface of hydrogen/deuterium atoms, and solving the corresponding 2D single-particle Schrodinger equation to get the eigenfrequencies. The obtained results well describe the experimental INS spectra and show harmonic behavior in the fundamental modes and strong anharmonicity at higher energies.

cond-mat.mtrl-sci↗