PhD Defense of Ruiqi CHEN

Mme Astrid HOLZHEID 

Rapportrice

University of Kiel 

Mme Céline DARIE 

Rapporteur

Grenoble INP –UGA University

Mme Elena Solana MADRUGA 

Examinateur

Universidad Complutense de Madrid

Mme Marie COLMONT

Directeur de thèse

M. Angel AREVALO-LOPEZ

Co-directrice de thèse

Université de Lille 

M. Oleg SIIDRA

Co-encadrant de thèse

Saint-Petersburg State University

Oxides minerals, metamict minerals, radioactive waste, thermal expansion, magnetic materials, solid phase synthesis, crystal structure, magnetic properties

This study investigates six metamict minerals and a series of synthetic compounds. Radioactive minerals can mimic high-level waste and are a valuable subject of research. Samples of four mineral types – brannerite, davidite, fergusonite, thorite, and zirconolite – were identified as metamict. Their mineralogical characteristics and chemical composition were thoroughly described and measured using electron microprobe analysis, and Raman spectroscopy... All mineral samples, when heated to 1200-1300 °C, recrystallized into expected structures, accompanied by the formation of new phases in different percentages. To discuss the mechanism of recrystallization, the processes of recrystallization and phase evolution were investigated using X-ray diffraction and thermal analysis. The thermal expansion of the crystallized minerals was studied, and the obtained thermal expansion coefficients contributed to the database of matrices for high-level waste disposal. Among the studied mineral structures, davidite, belonging to the crichtonite structural group, attracted interest in materials science due to its high chemical capacity. As part of this work, a series of compounds with the crichtonite structure were synthesized, and their magnetic properties were investigated. This study demonstrates the ferrimagnetic arrangement of Mn²⁺ and Ti³⁺ spins in CaMTO, SrMTO, BaMTO, CaFTO, and SrFTO. In contrast, LaMTO, CeMTO, NdMTO, LaMTFO, and SrMTFO exhibit spin glass behavior. These observations suggest that the incorporation of rare earth elements in the dodecahedral positions and iron in the octahedral positions disrupts the ferrimagnetic order within the AMTO crichtonite.