Thèse Propriétés Optiques et Spectroscopiques d'Oxydes Pyrochlores et Grenats à Haute Entropie H/F

Doctorat.Gouv.Fr

Établissement : Université Paris-Saclay GS Chimie École doctorale : Sciences Chimiques : Molécules, Matériaux, Instrumentation et Biosystèmes Laboratoire de recherche : Institut de Chimie Moléculaire et des Matériaux d'Orsay Direction de la thèse : David BERARDAN ORCID 000000022682998X Début de la thèse : 2026-10-01 Date limite de candidature : 2026-06-30T23:59:59 Ce projet, qui s'inscrit dans le cadre d'une collaboration entre l'Université Paris Saclay et l'Université Ewha de Séoul, en Corée, vise à concevoir de nouveaux matériaux oxydes à haute entropie destinés à des applications optiques. Réunissant une équipe interdisciplinaire composée de chimistes et de physiciens spécialisés dans les matériaux, il s'étendra du design de nouvelles compositions spécialement conçues pour améliorer la compréhension des liens entre les compositions complexes des oxydes à haute entropie, leur désordre local et leurs propriétés spectroscopiques et optiques, à l'optimisation des performances des matériaux. High entropy oxides (HEOx) constitute a new class of materials discovered in 2015. When a sufficient number of binary oxides are mixed and heated at high temperature, the contribution of configurational entropy to free enthalpy can become dominant. It leads to the formation of a single-phase compound crystallizing in a simple structure with a random distribution of cations on the cationic sublattice of the structure, even for binary oxides that do not form a solid solution between them, and do not all crystallize in the same structure.
Following pioneering work carried out by our team on the study of the functional properties of these compounds, numerous research avenues have been opened up, highlighting their potential applications in many fields. They include for examples the field of energy, with colossal dielectric constants observed for several compositions and structures, compositions with high potential for lithium battery anodes or solid electrolytes, and encouraging catalytic or photocatalytic properties. Although the origin of these properties is not yet understood, their observation highlights the potential of high-entropy oxides for the development of new functional materials, one of their main advantages being the possibility of finely adjusting their properties by varying their stoichiometry or composition.
Among the many intriguing physical properties of high-entropy oxides, their optical and spectroscopic properties have been scarcely studied to date. Questions may arise, for example, about the influence of very strong local structural disorder (caused by the presence of several cations of different sizes at the same crystal site) on infrared and Raman vibration modes, or about possible local symmetry breaking causing changes in the crystal field and thus in transitions between energy levels, or on the possibility of finely controlling the energy or intensity of emission lines by manipulating the composition and local structure. This project proposes to fill this gap, following encouraging preliminary results obtained through a collaboration between ICMMO and Ewha University (Korea). Broadly speaking, it aims to study the optical and spectroscopic properties of entropy-stabilized oxides build from pyrochlore and garnet parent compounds that are already of significant interest for their optical properties, which has never been reported to date.
The main goal of the project is to study the optical and spectroscopic properties (including typically photoluminescence, Raman, infrared and UV-visible spectroscopy) of entropy-stabilised oxides compounds belonging to the pyrochlore RE2M2O7 (RE = rare earth, M = equimolar mixture of cations) or garnet RE3A2B3O12 (RE = rare earth, A and B = equimolar mixture of cations) families, whose parent single-cation compounds are of significant interest for many optical or photonic applications. The objective of this study is to understand the link(s) between complex chemical composition (and its associated local structural disorder) and the optical and spectroscopic properties, in order to develop new compounds with improved and finely tuneable properties. Our methodology will span from the design of new compositions to the characterization of the materials performances. We plan to focus on entropy-stabilized oxides belonging to the pyrochlore and garnet families. The first compositions that we will study will be based on the first preliminary results that have been obtained in the collaboration between ICMMO and Ewha university. We will first design series of compounds in order to finely control the local chemical, structural and charge disorders so as to understand their influence on the optical properties. In a second step, based on this understanding we will design new compositions targeting optimized performances.

The experimental research work will include:
- The synthesis of high-entropy pyrochlore and garnet oxides in polycrystalline form, mainly by solid-phase synthesis, and the post-processing of these materials by natural sintering or more advanced techniques, for example Spark Plasma Sintering, to obtain the dense compounds necessary to optimise surface quality for spectroscopic studies,
- The synthesis of certain compositions in single-crystalline form using the vertical melting zone method (the advantage of single crystals is that they avoid diffusion through grain boundaries, but their production is much more complex and time-consuming, which limits their use to a few carefully selected compositions),
- The chemical, structural and microstructural characterisation of the synthesised compounds (X-ray diffraction, X-ray Photoemission Spectroscopy, electron microscopy coupled with chemical analysis by Energy Dispersive X-ray Spectroscopy),
- The study of their optical and spectroscopic properties (IR, Raman and UV-visible spectroscopy, photoluminescence measurements, etc.).
The project will also require the use of large-scale facilities, mainly synchrotrons for high-resolution X-ray diffraction, EXAFS studies or spectroscopic studies in energy ranges that are more difficult to probe using laboratory spectrometers. Inelastic neutron scattering measurements may also be considered to characterise the crystal electric field around rare earths that should significantly influence the absorption and emission properties.

Proposed timeline, including 2x6 months secondment in Korea:
- month 1: training of the candidate (synthesis and characterizations of reference compounds whose synthesis route and properties are well known in the team),
- month 2-7: synthesis, as well as laboratory chemical, structural and microstructural characterizations of the series of high-entropy pyrochlore and garnet oxides in polycrystalline form,
- month 8-13: 6 months secondment in Ewha University to study their optical and spectroscopic properties,
- month 14-24: synthesis of new high-entropy compositions designed from the results obtained during the 6 previous months in Korea, synthesis of selected single-crystalline compositions, thorough characterizations using large scale facilities,
- month 25-30: 6 months secondment in Ewha University to study the optical and spectroscopic properties of the new compositions as well as the single-crystalline compounds,
- month 31-36: finalization of the last characterizations and last data processing + PhD dissertation writing and thesis defense.