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PhD thesis (H/F): Exploring oxygen diffusion mechanisms by neutron scattering

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General information

Reference : UMR5253-MONCER-001
Date of publication : Tuesday, November 26, 2019
Scientific Responsible name : Monica CERETTI
Type of Contract : PhD Student contract / Thesis offer
Contract Period : 36 months
Start date of the thesis : 1 February 2020
Proportion of work : Full time
Remuneration : 2 135,00 € gross monthly

Description of the thesis topic

Exploring oxygen diffusion mechanisms in Pr2NiO4+ under in situ conditions by neutron scattering: interplay between structure and lattice dynamics

The project aims to study the impact of lattice dynamics and oxygen ordering on the amplification of oxygen mobility at low temperature in the K2NiF4-type oxide Pr2NiO4+d. In particular, it will allow evaluating and extending a newly proposed phonon-assisted oxygen diffusion mechanism, enabling to rationalize oxygen mobility features in solid oxides, from room to moderate temperatures. This project has been funded by the French National Agency ANR in the frame of a bilateral contract with DFG/Germany. It is a collaboration between prof W. Paulus' group (Institut Charles Gerhard in Montpellier) and the reactor FRM2 in Munich (Germany).
In the last decades, Pr2NiO4+d has been extensively studied as a potential candidate for oxygen membranes or electrolytes in solid oxide fuel cells (SOFCs), as it shows a high oxygen diffusion rate at already moderate temperatures. It can reversibly take up a substantial amount of oxygen, yielding Pr2NiO4.25 as the maximum loading. The presence of intercalated oxygen atoms induces important local disorder, resulting in large anisotropic displacements of the apical oxygen atoms. These displacements are supposed to dynamically trigger shallow oxygen diffusion pathways between the apical and interstitial sites via a phonon assisted diffusion mechanism. Our motivation here is to explore the specific role of the interstitial oxygen atoms on the oxygen diffusion mechanism in situ at moderate temperatures. Structure and lattice dynamics of non-stoichiometric Pr2NiO4+ will be investigated on single crystals, combining elastic and inelastic neutron scattering experiments, carried out in-situ in a specially designed mirror furnace equipped with a quartz reaction cell, allowing to work under controlled gas atmosphere. These studies will be complemented by in situ experiments with synchrotron radiation allowing to follow the oxygen intercalation/de-intercalation on single crystals by diffraction and inelastic X-ray scattering at the ESRF. All experimental approaches will be accompanied by theoretical studies, based on density functional theory (DFT) especially for the investigation of lattice dynamics. The pioneering experimental approach here is that almost all experiments will be carried out in-situ on well defined, high quality single crystals. The availability of high-quality single crystals is thus mandatory to obtain valuable anisotropic information on the diffusion kinetics and related activation energies, but also for all structural and lattice dynamical investigations. For this reason, the growth of large high-quality single crystals of Pr2NiO4+δ is one of the most important milestones of the project.
In the frame of this project the PhD student will be focused principally on single crystal growth by travelling solvent floating zone method, as well as in structural studies (commensurate and incommensurate structure), in particular by in situ neutron/X-ray scattering experiments in house and to large scale facilities (neutron and synchrotron). A large part of the thesis work will take place at the Institute Charles Gerhard (Montpellier), but he/she will be likely to spend much time at the MLZ in Garching for neutron diffusion experiments to fulfill the tasks of the project. The candidate has to have a Master in one of the materials sciences, solid state chemistry/physics and will be enrolled at the doctorate school « Sciences Chimiques Balard » of the Montpellier University. The funding is already available and the contract can start immediately.

Work Context

The research project will take place within The Institute Charles Gerhardt Montpellier (ICGM). The Institute for Molecular Chemistry and Material Sciences is a Joint Research Unit (UMR 5253) created in January 2007 and supported by the Centre National de la Recherche Scientifique (CNRS), Université de Montpellier (UM) and Ecole Nationale Supérieure de Chimie de Montpellier (ENSCM). The ICGM is a multidisciplinary laboratory working in the fields of Energy, Environment and Health. Organized in three scientific departments, the research in the Institute is oriented around three main domains (i) Molecules to materials, (ii) advanced Materials and (iii) modelling. Our group is part of the Solid chemistry department and is actively working in exploring low temperature oxygen diffusion mechanisms in solid oxides. In this context, we are interested to better understand the underlying diffusion mechanisms on an atomic scale, having importance for technological applications in solid state ionics as the optimisation of battery materials, fuel cell membranes/electrolytes or sensors. The selected candidate will work within the solid state chemistry department, which offers access to a wide range of equipment suitable for the synthesis and the structural characterization of the materials considered in this project. In addition, the candidate will profit of stay at international large scale facilities, in particular at the FRM2 reactor in Munich (Germany) with our collaborators of the ANR project, to carry out neutron/synchrotron scattering experiments.

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