arXiv subjects

Renaud Podor

Publications and source records attributed to Renaud Podor.

3 recordsLinked to original sources

arXiv2026

A Topotactic Phase Transition in the Uranium Oxide System

A topotactic phase transition involves the transformation of one crystalline solid to another, which may include the loss or gain of material, where the orientation of the parent crystal determines the orientation of the daughter. We set out an experimental approach, based on polyepitaxial thin film deposition, where the precise transformation mechanism in important physico-chemical processes can be revealed in brilliant detail. Here, we find a reversible topotactic transition from (001) cubic UO2 to a (130) orthorhombic U3O8 structure; a >35% expansion/contraction. This remarkable result solves a puzzle that has eluded researchers for decades, and presents a method for determining the mechanism of crystallographic transformation in many other compounds.

cond-mat.mtrl-sci↗

arXiv2024

Polyepitaxial grain matching to study the oxidation of uranium dioxide

Although the principal physical behaviour of a material is inherently connected to its fundamental crystal structure, the behaviours observed in the real-world are often driven by the microstructure, which for many polycrystalline materials, equates to the size and shape of the constituent crystal grains. Here we highlight a cutting edge synthesis route to the controlled engineering of grain structures in thin films and the simplification of associated 3-dimensional problems to less complex 2D ones. This has been applied to the actinide ceramic, uranium dioxide, to replicate structures typical in nuclear fission fuel pellets, in order to investigate the oxidation and subsequent transformation of cubic UO$_{2}$ to orthorhombic U$_{3}$O$_{8}$. This article shows how this synthesis approach could be utilised to investigate a range of phenomena, affected by grain morphology, and highlights some unusual results in the oxidation behaviour of UO$_{2}$, regarding the phase transition to U$_{3}$O$_{8}$.

cond-mat.mtrl-sci↗

arXiv2021

Dynamic instability of individual carbon nanotube growth revealed by in situ homodyne polarization microscopy

Understanding the kinetic selectivity of carbon nanotube growth at the scale of individual nanotubes is essential for the development of high chiral selectivity growth methods. Here we demonstrate that homodyne polarization microscopy can be used for high-throughput imaging of long individual carbon nanotubes under real growth conditions (at ambient pressure, on a substrate), and with sub-second time resolution. Our in situ observations on hundreds of individual nanotubes reveal that about half of them grow at a constant rate all along their lifetime while the other half exhibits stochastic changes in growth rates, and switches between growth, pause and shrinkage. Statistical analysis shows that the growth rate of a given nanotube essentially varies between two values, with similar average ratio (~1.7) regardless of whether the rate change is accompanied by a change in chirality. These switches indicate that the nanotube edge or the catalyst nanoparticle fluctuates between different configurations during growth.

cond-mat.mtrl-sci↗