The theme Formulation-Synthesis-Structure is mainly concerned by the synthesis and the characterization of new glasses and glass-ceramics. The targeted applications from these non-conventional materials are very varied. Historically, the research in our team was mainly focused on the optical properties of the chalcogenide glasses. At present time, new objectives are also to develop electrolytes for all-solid batteries, materials for the conversion of solar energy by photovoltaic or photocatalytic effect, and materials for the conversion of thermal energy by thermoelectric effect. In other words, this research theme is upstream of other themes developed in our team.
The great interest of the vitreous state resides in their plasticity allowing a functional system to be thermoformed. To keep the benefit of functional efficiency, the development of glass-ceramics is an interesting strategy, which has to be amplified.
To continuously optimize the composition of all glasses or glass-ceramics following the covered applications and consequently the required properties, it is necessary to investigate the structure of these materials. The numerous tools dedicated to the structural analyses available in our institute are not sufficient to solve the structure of glasses, which only possess a short- and medium-range order. So, complementary to the usual tools such as Raman spectroscopy, solid NMR, and X-ray diffraction we carry out experiment on high-scale facilities, which allow collecting valuable information on the structure of glasses.
Another objectives, more fundamental, are devoted to aim a better understanding of the origin of the vitreous state. Some collaboration works are already under way with colleagues from the team Chimie Théorique Inorganique to combine synthesis, spectroscopy, experimental functions of correlation (originated from Neutron diffusion and High-energy X-ray diffraction), molecular dynamics, calculation of spectroscopic parameters (NMR, Raman) or calculation of physical quantities (energy diagram, link vibration, electronic and ionic conductivities ...). The main objectives are to understand the structural organization of these atypical materials and to draw the quintessence in terms of applications. A strong ambition is also to implement these high temperature structural techniques to understand the origin of the formation of a glass in the liquidus. Chalcogenide glasses are a formidable playground from this point of view since their characteristic temperatures are lower than those of the oxides and there is a multitude of glassy systems with varied cation / anion stoichiometry, which is more important than in the silicate or phosphate glasses.
Experimental and theoretical insights into the structure of tellurium chloride glasses
The structure of the binary chalcohalide glasses Te1–xClx (0.35 ≤ x ≤ 0.65) is considered by combining experimental and theoretical results. The structural network properties are influenced by a competition between ionic and covalent bonding in such glasses. At first, a focus is placed on the detailed information available by using the complementary high-energy X-ray and the neutron diffractions in both the reciprocal and real spaces. The main characteristic suggested by the structure factors S(Q) concerns the presence of three length scales in the intermediate range order. The total correlation function T(r) lets us also suppose that the structure of these glasses is more complicated than Te-chain fragments with terminal Cl as demonstrated in crystalline Te3Cl2. Molecular dynamics simulations were subsequently performed on Te3Cl2 and Te2Cl3, and coupled with the experimental data, a highly reticulated network of chalcogen atoms, with a fair amount of chlorine atoms bonded in a bridging mode, is proposed. The simulations clearly lead to a glass description that differs markedly from the simple structural model based on only Te atom chains and terminal Cl atoms. Solid-state NMR experiments and NMR parameters calculations allowed validation of the presence of Te highly coordinated with chlorine in these glasses.
Short and medium range structures of 80GeSe2-20Ga2Se3 chalcogenide glasses
The short and medium range structures of the 80GeSe2-20Ga2Se3 (or Ge23.5Ga11.8Se64.7) chalcogenide glasses have been studied by combining ab initio molecular dynamics (AIMD) simulations and experimental neutron diffraction studies. Structure factor and total correlation function were calculated from glass structures generated from AIMD simulations and compared with neutron diffraction experiments showing reasonable agreement. The atom structures of the ternary chalcogenide glasses were analyzed in detailed and it was found that gallium atoms are four-fold coordinated by selenium and form [GaSe4] tetrahedra. Germanium atoms on average also have around four-fold coordination, among which 3.5 is selenium with the remaining being Ge-Ge homo-nuclear bonds. Ga and Ge tetrahedra link together through mainly corner-sharing and some edge-sharing. No homo-nuclear bonds were observed among Ga atoms or between Ge and Ga. In addition, selenium-selenium homo-nuclear bonds were observed and form Se chains with various lengths. A small fraction of selenium atom triclusters that bond to three cations of Ge and Ga were also observed, confirming earlier proposals from Se solid state NMR studies. Furthermore, electronic structure of the ternary chalcogenide glasses were studied in terms of atomic charge, chemical bonding and electronic density to gain insights on the chemical bonding in the glass and electronic properties and to provide explanation of observed atomic structures.