Structural study of polymorphism and thermal behavior of CaZr(PO4)2

International audienceThe crystal structure of CaZr(PO4)2 has been revised by ab initio Rietveld analysis of X-ray powder diffraction data. At room temperature, CaZr(PO4)2 crystallizes in the orthorhombic space group Pna21 (Z = 4). Differential thermal analysis suggests a reversible second order transition at 1000°C confirmed by high temperature XRD analysis that brings out the existence of a high temperature form, very similar to the room temperature one, but more symmetrical (Pnma, Z = 4). Analysis of the crystal parameters evolution during heating reveals that CaZr(PO4)2 exhibits a quite low thermal expansion coefficient of 6.11 10-6K-1. This value stems from a combination of several mechanisms, including Coulombic repulsion and bridging oxygen rocking motion

Transmission Kikuchi Diffraction, a powerful imaging technique for nanoscale structural characterisation of cultural heritage materials

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Structure et propriétés des verres de silicates de plomb

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High-temperature behavior of dicesium molybdate Cs2MoO4: Implications for fast neutron reactors

International audienceDicesium molybdate (Cs 2 MoO 4)'s thermal expansion and crystal structure have been investigated herein by high temperature X ray diffraction in conjunction with Raman spectroscopy. This first crystal-chemical insight at high temperature is aimed at predicting the thermostructural and thermomechanical behavior of this oxide formed by the accumulation of Cs and Mo fission products at the periphery of nuclear fuel rods in sodium-cooled fast reactors. Within the temperature range of the fuel's rim, Cs 2 MoO 4 becomes hexagonal P6 3 /mmc, with disordered MoO 4 tetrahedra and 2D distribution of Cs-O bonds that makes thermal axial expansion both large (50 rα l r 70 10 À 6 1C À 1 , 500-800 1C) and highly anisotropic (α c À α a ¼ 67 Â 10 À 6 1C À 1 , hexagonal form). The difference with the fuel's expansion coefficient is of potential concern with respect to the cohesion of the Cs 2 MoO 4 surface film and the possible release of cesium radionuclides in accidental situations

Structure and properties of lead silicate glasses

International audienceGiven their specific properties, glasses containing lead oxide have a very broad field of application. This extends from crystal glasses, enamels and glazes, to optical glasses with high refractive index, passing through glasses absorbing ionizing radiation and glasses for low-temperature welding of metals, ceramics or glasses. This chapter focuses first of all on the chemistry of lead, and more specifically on the properties and stereochemistry of the Pb$^{2+}$ ion (species under which lead is present in oxide glasses), the nature of the Pb-O chemical bond and the crystalline structure of pure PbO oxide (stereochemistry and mode of connection of the polyhedrons). Then, since the lead-based oxide glasses of interest are overwhelmingly silicate glasses, the focus is on how the Pb$^{2+}$ ions get inserted in the glassy network of silica and modify its structure, first of all in the case of the simple binary SiO2-PbO system (for which the evolution of the structure of the binary crystalline phases will also be approached depending on their PbO content compared to glasses), then in the case of more complex systems such as the ternary SiO$_2$-PbO-R$_2$O (R: alkaline) and SiO$_2$-PbO-Al$_2$O$_3$ systems

Structural study and luminescence of TlSrLa(AsO4)2

This work presents the crystal structure and luminescent properties of TlSrLa(AsO4)2. In this phase Tl+ ions are located in large tunnels delimited by chains of alternating (AsO4) and (Sr,La)O8 polyhedra. Thallium atoms are eightfold coordinated with C1 symmetry. Large Tl---O distances are observed revealing a low stereochemical activity of the 6s2 lone pair..

Microstructure imaging of Florentine stuccoes through X-ray tomography: A new insight on ancient plaster-making techniques

International audienceGypsum-based plasters or stuccoes, in spite of their importance and diffusion, received little attention in cultural heritage materials studies. This work introduces a new, non-destructive methodology, using micro-tomography to measure the water/plaster ratio and the morphology of the hemihydrate powder used to make plasters on < 1 mm3 samples. This methodology give insight in both the raw material (and ultimately provenance) and the technique used to make plaster. The methodology was tested first on mock-up samples of known composition, then in a case study on 13 low-relief cast plaster sculptures from 15th century Florentine artists. Preliminary conclusions on this limited corpus show relative uniformity across most reliefs in terms of raw materials and techniques. The casts of one model (Nativity, attributed to Donatello and B. Bellano) were made with a different raw material, in line with prior geochemical analyses; these results support the previous attribution to a North Italian rather than Florentine origin. The casts of a second model (Virgin and Child, type of Saint Petersburg, attributed to Antonio Rossellino) were prepared with a different technique. This surprising result was not expected from Art History or previous studies

Site occupancy and mechanisms of thermally stimulated luminescence in Ca9Ln(PO4)7 (Ln=lanthanide)

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Effect of the sintering method on microstructure and thermal and mechanical properties of zirconium oxophosphate ceramics Zr 2 O(PO 4 ) 2

International audienceDue to an ultra-low thermal expansion, Zr2O(PO4)2 could find many applications as a thermal shock resistant material. To this end, ceramic processing is a key step in order to reach best properties. In this work, Zr2O(PO4)2 was sintered by conventional sintering and by the spark plasma sintering technique (SPS) with and without additive. Samples made by conventional sintering with ZnO as sintering aid have a maximum relative density of around 92 %. Microstructure is composed of large grains and microcracks can be observed. When doped 2 with 5 wt. % of MgO, samples can be densified by SPS up to 99.6 % of the relative density and the grain size maintained between 0.5 and 1.5 μm. Thermal conductivity and Vickers microhardness were investigated as a function of the microstructure. Best values were obtained for the ceramic doped with 5 wt.% MgO and sintered by SPS, thanks to a fine microstructure and a small amount of residual microcracks