Thèse Ordre de Charge et Couplage Exciton-Phonon dans les Perovskites à l'Or. H/F

Doctorat_Gouv

Établissement : Université Paris-Saclay GS Physique
École doctorale : Ondes et Matière
Laboratoire de recherche : Laboratoire de Physique des Solides
Direction de la thèse : Olivier PLANTEVIN ORCID 000000026131181X
Début de la thèse : 2026-10-01
Date limite de candidature : 2026-04-30T23:59:59

Les matériaux à structure pérovskite occupent une place centrale en physique du solide en raison de la forte interdépendance entre leur structure cristalline, leurs états de valence et leurs propriétés électroniques. Dans ces systèmes, de légères modifications structurales peuvent profondément transformer les propriétés physiques, donnant lieu à des phénomènes tels que les ondes de densité de charge, les transitions métal-isolant ou encore l'émergence d'états excitoniques fortement liés. Les doubles pérovskites inorganiques sans plomb de type CsBB'X constituent aujourd'hui une plateforme particulièrement prometteuse pour explorer ces couplages fondamentaux, tout en répondant aux enjeux de substitution du plomb dans les matériaux optoélectroniques.

Double perovskites are emerging as promising alternatives to lead halide perovskites, addressing critical challenges such as toxicity and chemical instability. This is a very competitive field which needs a combination of know-hows both in chemistry (crystal synthesis and doping) and in solid state physics. An appropriate collaboration between Lumin , IPVF and LPS will allow to address very efficiently the complexity of this material byt he use of advanced techniques both at laboratory and at synchrotron facilities.

This thesis project aims to study the effect of hydrostatic pressure on double perovskites containing noble metals such as gold and silver (AgBi, AgAu, Au(I)Au(III)). In the emblematic case of Cs [Au(I)Au(III)]X, derived from the CsAuX structure, the cations Au(I) and Au(III) are alternately organized in neighboring octahedrons. This alternation can be described as a local charge density wave, stabilized by a structural distortion of the Jahn-Teller type. Pressure is a particularly relevant external parameter to probe and control this delicate balance between load order, structural distortion and electronic hybridization.
By compressing the crystal lattice, the pressure changes the interatomic distances, bonding angles and the width of the electronic bands. It can thus influence the stability of mixed valence states, redistribute electron density and induce structural or electronic transitions. In some gold-based double perovskites, the application of pressures of a few gigapascals leads to a marked reduction of the electron gap, or even to a transition towards metallic behavior. Understanding the microscopic mechanisms at the origin of these evolutions is one of the central objectives of the thesis.

The doctoral work will be based on an experimental approach combining measurements under diamond anvils cells and complementary techniques allowing access to both the structure, the dynamics of the network and the local electronic structure. The X-ray diffraction under pressure will make it possible to precisely follow the evolution of mesh parameters, crystalline symmetries and octahedral distortions. Raman and infrared spectroscopies will provide access to phononic modes and optical conductivity, providing direct information on electron-phonon coupling and gap evolution. Photoluminescence measurements will characterize the nature of excitons, their binding energy and their sensitivity to structural distortions. Finally, X-ray absorption and emission spectroscopies (XAS/XES) will offer an element-specific probe of the oxidation states and electronic configurations of gold and silver, essential for understanding the evolution of valences under pressure.