Solid state synthesis of monazite type compounds LnPO4 (Ln = La to Gd)

International audienceThis work is devoted to the synthesis of monazite-type compounds LnPO4 (with Ln ¼ La, Ce, Pr, Nd, Sm, Eu and Gd) by solidesolid reaction between a lanthanide oxide and a phosphate precursor NH4H2PO4. Starting mixtures and resulting powders were characterized by coupling different techniques, in particular thermal analysis, X-ray diffraction and MAS 31P NMR. Results are presented according to the valence state of the lanthanide element in its oxide form. The intermediate chemical reactions occurring during the firing of starting reagents are described for the first time in the case of monazite with one or several cations. It has been highlighted that the solid-state route is an efficient way in order to obtain very pure and very well crystallized monazite powder. Optimum synthesis conditions are 1350 Ce2 h. The synthesis of monazite powders containing several lanthanides appears to be more difficult, because all the lanthanides do not react at the same temperature, leading to the formation of heterogeneous powders

31P MAS NMR and DFT study of crystalline phosphate matrices

We present the study of the phosphorus local environment by using 31P MAS NMR in a series of seven double monophosphates MIIMIV(PO4)2 (MII and MIV being divalent and tetravalent cations, respectively) of yavapaiite and low-yavapaiite type crystal structures. Solid-state and cluster DFT calculations were found to be efficient for predicting the 31P isotropic chemical shift and chemical shift anisotropy. To achieve this performance, however, a proper computational optimisation of the experimental structural data was required. From the three optimisation methods tested, the full optimisation provided the best reference structure for the calculation of the NMR parameters of the studied phosphates. Also, a better prediction of the chemical shifts was possible by using a correction to the GIPAW calculated shieldingJRC.G.I.5-Advanced Nuclear Knowledg

Caractérisation expérimentale des propriétés physiques et mécaniques de FGPM Ni/BaTiO3_3

International audienceThis study focuses on the experimental characterization of the physical and mechanical properties of FGM/FGPM. These materials present a controlled variation of the properties at the thickness level. Ni/BaTiO$_3$ samples have been developed by the SPS sintering technique. The variation of thermal conductivity across the thickness was determined using the transient planar source method. However, the small size of the samples does not allow the use of standard techniques to measure the elastic constants. An alternative protocol based on the use of ultrasonic waves has been proposed to overcome this difficultyCette étude porte sur la caractérisation expérimentale des propriétés physiques et mécaniques de matériaux FGM /FGPM. Ces matériaux présentent une variation contrôlée des propriétés au niveau de l'épaisseur. Des échantillons Ni/BaTiO3 ont ainsi été élaborés par la technique originale de frittage SPS. La variation de la conductivité thermique dans l'épaisseur a été déterminée à l'aide de la méthode de la source plane transitoire. Cependant, la petite taille des échantillons ne permet pas d'utiliser les techniques standard pour mesurer les constantes élastiques. Un protocole alternatif basé sur l'utilisation des ondes ultrasons est donc proposé pour palier à cette difficulté

Crystal Structure and Thermal Expansion of the Low- and High- temperature Forms of BaMIV(PO4)2 Compounds (M ¼ Ti, Zr,HfandSn)

The crystal structure of b-BaZr(PO4)2, archetype of the high-temperature forms of BaM(PO4)2 phosphates (with M=Ti, Zr, Hf and Sn), has been solved ab initio by Rietveld analysis from synchrotron X-ray powder diffraction data. The phase transition appears as a topotactic modification of the monoclinic (S.G. C2/m) lamellar a-structure into a trigonal one(S.G. P3m1) through a simple mechanism involving the unfolding of the [Zr(PO4)2]n2- layers. The thermal expansion is very anisotropic (e.g., -4.1 < ai < 34.0 x 10-6 K-1 in the case of a-BaZr(PO4)2) and quite different in the two forms, as a consequence of symmetry. It stems from a complex combination of several mechanisms, involving bridging oxygen rocking in M¿O¿P linkages, and ¿¿bond thermal expansion¿¿.JRC.E.6-Actinides researc

The chemistry of the phosphates of barium and tetravalent cations in the 1 : 1 stoichiometry

International audienc

SrNp(PO4)2: an Original Ordered Modification of Cheralite

The new compound SrNp(PO4)2 (orthorhombic Cmca) has been synthesized by solid state reaction and studied by Rietveld analysis. Though chemically and structurally related to cheralite CaTh(PO4)2, it shows alternate layers of SrO10 and NpO8 polyhedra instead of a disordered array of MO9 units. This unknown structural model allows explaining the narrow limits of the cheralite domain in terms of cations size and suggests that transuranium-loaded compounds could also be ordered derivates of the archetype. The validity of thorium-designed models for approaching transuranian structural chemistry is discussedJRC.E.3-Materials researc