The order-disorder phase transition in the Nd x Zr 1-x O 2-0.5x system is studied by complementary techniques which include wet chemical synthesis of a series of compositions with various Nd/Zr ratios with the final annealing at 1873 K, X-ray diffraction, oxide melt solution calorimetry and ab initio thermodynamic modeling.Our structural data indicate the transition from ordered to disordered pyrochlore at x ~ 0.31at a temperature of1873 K. Our calorimetric data show a transition enthalpy of ~30 kJ/mol, which corresponds to an entropy of disordering of ~16 J/K/mol. The latter value is significantly smaller than the configurational entropy of transition computed under the
Bimetallic Au-Ni supported on TiO 2 were prepared using co-depositionprecipitation with urea and tested on the selective hydrogenation of 1,3butadiene. The catalyst containing the lower amount of Ni (atomic Au:Ni 1:0.08) showed a satisfying compromise between activity provided by Ni and selectivity to butenes provided by Au for this selective hydrogenation reaction. Even though Au and Ni exhibit limited miscibility in the bulk, characterizations by TPR and UV-vis spectroscopy showed evidence of interaction between Au and Ni in nanoparticles. STEM-XEDS study confirmed the formation of bimetallic nanoparticles.
Radiation damage in minerals is caused by the α-decay of incorporated radionuclides, such as U and Th and their decay products. The effect of thermal annealing (400-1000 K) on radiation-damaged pyrochlores has been investigated by Raman scattering, X-ray powder diffraction (XRD), and combined differential scanning calorimetry/thermogravimetry (DSC/TG). The analysis of three natural radiation-damaged pyrochlore samples from Miass/Russia [6.4 wt% Th, 23.1 · 10 18 α-decay events per gram (dpg)], Panda Hill/Tanzania (1.6 wt% Th, 1.6 · 10 18 dpg), and Blue River/Canada (10.5 wt% U, 115.4 · 10 18 dpg), are compared with a crystalline reference pyrochlore from Schelingen (Germany). The type of structural recovery depends on the initial degree of radiation damage (Panda Hill 28 %, Blue River 85 % and Miass 100 % according to XRD), as the recrystallization temperature increases with increasing degree of amorphization. Raman spectra indicate reordering on the local scale during annealing-induced recrystallization. As Raman modes around 800 cm −1 are sensitive to radiation damage . The most radiation damaged pyrochlore (Miass) shows an abrupt recovery of both, its short-(Raman) and long-range order (X-ray) between 800 and 850 K, while the weakly damaged pyrochlore (Panda Hill) begins to recover at considerably lower temperatures (near 500 K), extending over a temperature range of ca. 300 K, up to 800 K (Raman). The pyrochlore from Blue River shows in its initial state an amorphous X-ray diffraction pattern superimposed by weak Bragg-maxima that indicates the existence of ordered regions in a damaged matrix. In contrast to the other studied pyrochlores, Raman spectra of the Blue River sample show the appearance of local modes above 560 K between 700 and 800 cm −1 resulting from its high content of U and Ta impurities. DSC measurements confirmed the observed structural recovery upon annealing. While the annealing-induced ordering of Panda Hill begins at a lower temperature (ca. 500 K) the recovery of the highly-damaged pyrochlore from Miass occurs at 800 K. The Blue-River pyrochlore shows a multi-step recovery which is similarly seen by XRD. Thermogravimetry showed a continuous mass loss on heating for all radiation-damaged pyrochlores (Panda Hill ca. 1 %, Blue River ca. 1.5 %, Miass ca. 2.9 %).
Rare earth phosphates comprise a large family of compounds proposed as possible nuclear waste disposal forms. We report structural and thermodynamic properties of a series of rare earth rhabdophanes and monazites. The water content of the rhabdophanes, including both adsorbed and structural water, decreases linearly with increase in ionic radius of the rare earth. The energetics of the transformation of rhabdophane to monazite plus water and the enthalpy of formation of rhabdophane from the constituent oxides was determined by high temperature drop solution calorimetry. The former varies linearly with the ionic radius of the lanthanide, except for cerium. By combining the enthalpy of formation determined by high temperature drop solution calorimetry and the free energy of formation determined previously by solubility experiments, a complete set of thermodynamic data was derived for the rhabdophanes. They are thermodynamically metastable with respect to the corresponding monazites plus water at all temperatures under ambient pressure conditions. This conclusion strengthens the case for monazites being an excellent nuclear waste form.
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