Atomic surrounding of Co implanted in AlN at high energy

AlN bulk ceramic has been implanted with energetic Co ions. In order to accurately characterise the atomic surrounding of the implanted ions, X-ray absorption measurements were carried out at 80 K in the fluorescence mode at the Co K edge in the as-implanted and annealed states. Simulation of the EXAFS oscillations allowed us to identify a first stage where Co is inserted in the AlN matrix followed by a second stage where Co precipitates form.Fil: Traverse, Agnès. Lure, Universidad Paris-sud; FranciaFil: Delobbe, Anne. Lure, Universidad Paris-sud; FranciaFil: Zanghi, Didier. Lure, Universidad Paris-sud; FranciaFil: Rentería, Mario. Lure, Universidad Paris-sud; Francia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; ArgentinaFil: Gailhanou, Marc. Lure, Universidad Paris-sud; Franci

Atomic surrounding of Co implanted in AlN at high energy

AlN bulk ceramic has been implanted with energetic Co ions. In order to accurately characterise the atomic surrounding of the implanted ions, X-ray absorption measurements were carried out at 80 K in the fluorescence mode at the Co K edge in the as-implanted and annealed states. Simulation of the EXAFS oscillations allowed us to identify a first stage where Co is inserted in the AlN matrix followed by a second stage where Co precipitates form.Facultad de Ciencias ExactasInstituto de Física La Plat

Local structural analyses on molten terbium fluoride in lithium fluoride and lithium–calcium fluoride mixtures

X-ray absorption fine structure (XAFS) measurements on terbium fluoride in molten lithium fluoride and in molten lithium–calcium fluoride mixtures, (e.g. 0.20TbF3–0.80LiF, 0.20TbF3–0.62LiF–0.18CaF2, 0.20TbF3–0.48LiF–0.32CaF2, 0.50TbF3–0.50LiF, and 0.50TbF3–0.38LiF–0.12CaF2), have been carried out. In the solid state, coordination number of terbium (Ni) and inter ionic distances between terbium and fluorine in the first neighbor (ri) are nearly constant in all mixtures. In 0.20TbF3–0.80LiF, 0.20TbF3–0.62LiF–0.18CaF2 and 0.50TbF3–0.50LiF mixtures, Ni's decrease from ca. 8 to 6 and ri's also decrease from ca. 2.29 to 2.26 Å on melting. On the other hands, in molten 0.20TbF3–0.48LiF–0.32CaF2 and 0.50TbF3–0.38LiF–0.12CaF2 mixtures, Ni's are slightly larger than 6 and ri's do not change. These facts correspond to the amount of F− supplied by solvent melts, i.e. the effect of CaF2 becomes predominant at bCaF2 > 0.32 in ternary 0.20TbF3–aLiF–bCaF2 mixtures and at bCaF2 > 0.12 in ternary 0.50TbF3–aLiF–bCaF2 mixtures

Neutron diffraction of calcium aluminosilicate glasses and melts

International audienceThe combination of neutron diffraction with aerodynamic levitation and laser heating, pioneered by Neville Greaves and co-workers about 15 years ago, is an important tool for studying the structure of liquid melts. Since the first work on liquid Al2O3 published in 2001, the technique has been largely improved and experiments are now routinely performed at neutron sources, providing interesting structural information on various materials.In this paper, the structure of glass-forming compounds in the system CaO-Al2O3-SiO2 was measured by applying neutron diffraction with aerodynamic levitation. Results obtained in the liquid state above the melting point and from the glass at room temperatures are presented. Various compositions were studied by increasing the silica content and by changing the ratio CaO/Al2O3. As observed using other methods, the main structural changes relate to modification of the Al-O short range order

Highly Nonstoichiometric YAG Ceramics with Modified Luminescence Properties

Y3Al5O12 (YAG) is a widely used phosphor host. Its optical properties are controlled by chemical substitution at its YO8 or AlO6/AlO4 sublattices, with emission wavelengths defined by rare-earth and transition-metal dopants that have been explored extensively. Nonstoichiometric compositions Y3+xAl5-xO12 (x ≠ 0) may offer a route to new emission wavelengths by distributing dopants over two or more sublattices simultaneously, producing new local coordination environments for the activator ions. However, YAG typically behaves as a line phase, and such compositions are therefore challenging to synthesize. Here, a series of highly nonstoichiometric Y3+xAl5-xO12 with 0 ≤ x ≤ 0.40 is reported, corresponding to ≤20% of the AlO6 sublattice substituted by Y3+, synthesized by advanced melt-quenching techniques. This impacts the up-conversion luminescence of Yb3+/Er3+-doped systems, whose yellow-green emission differs from the red-orange emission of their stoichiometric counterparts. In contrast, the YAG:Ce3+ system has a different structural response to nonstoichiometry and its down-conversion emission is only weakly affected. Analogous highly nonstoichiometric systems should be obtainable for a range of garnet materials, demonstrated here by the synthesis of Gd3.2Al4.8O12 and Gd3.2Ga4.8O12. This opens pathways to property tuning by control of host stoichiometry, and the prospect of improved performance or new applications for garnet-type materials.Financial support was provided by the ANR-18-CE08-0012 PERSIST and ANR-20-CE08-0007 CAPRE projects of the French National Research Agency (ANR) and the CNRS, the I+D+I Grants PID2021-122328OB-100 funded by MCIN/AEI/10.13039/501100011033 and by “ERDF A way of making Europe”. PhD studentships for WC and XF were financed by the Chinese Scholarship Council (project numbers 201808450100 and 202008450026). Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. EXAFS beamtime was provided by the SOLEIL synchrotron (Gif-sur-Yvette, France) under project 99210047. The project benefitted from the microscopy facilities of the Platform MACLE-CVL which was co-funded by the European Union and Centre-Val de Loire Region (FEDER).Peer reviewe

Structural, Dynamic, and Thermodynamic Study of KF–AlF 3 Melts by Combining High-Temperature NMR and Molecular Dynamics Simulations

International audienc

Study of the NaF-ScF3 system as a molten bath for production of Sc alloys: A combination of NMR and molecular dynamics simulations

International audienc

Study of NaF–AlF 3 melts by coupling molecular dynamics, density functional theory, and NMR measurements

International audienceImprovement of the industrial electrolytic process for aluminum production necessitates a thorough understanding of the underlying ionic structure of the electrolyte, which mainly comprises NaF and AlF3 at around 965 °C. The chemical and physical properties of this melt strongly depend on the aluminum speciation, which requires a multipronged approach in order to clarify its properties. Here we parametrize a new polarizable ion model (PIM) interatomic potential for the molten NaF−AlF3 system, which is used to study the liquid properties up to 50 mol % of AlF$_3$ at high temperatures. The potential parameters are obtained by force fitting to density functional theory (DFT) reference data. Molecular dynamics (MD) simulations are combined with further DFT calculations to determine NMR chemical shifts for $^{27}$Al, $^{23}$Na, and $^{19}$F. An excellent agreement is obtained with experimental values. This enables the study of the dynamic properties of the melts such as viscosity, electrical conductivity, and self-diffusion coefficient