Synthesis, Crystal Structure, and Enthalpies of Formation of Churchite-type REPO₄·2H₂O (RE = Gd to Lu) Materials

Abstract

HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

The Role of Water and Hydroxyl Groups in the Structures of Stetindite and Coffinite, MSiO4 (M = Ce, U)

“…[22][23][24] This conclusion can be generalized to other natural metastable minerals having hydrothermal origins, such as the dichotomy pairs of monazite-rhabdophane or xenotimechurchite. 3,9,109,110 For this reason, in order to compare high temperature structural data of stetindite and coffinite with other metal orthosilicates, such as zircon, hafnon, and thorite reported in studies of Mursic et al 111 , Ding et al 112 and Knyazev et al 89 (Table 2), 89,[111][112][113][114][115] we used the XRD data obtained during cooling where samples were fully dehydrated or dehydroxylated. In addition, we normalized all the data by taking the relative change in unit cell parameters (a/a0 and c/c0) as a function of temperature (Figure 14).…”

Investigation of Nd3+incorporation in Ce‐rhabdophane: Insight from structural flexibility and occupation mechanism

“…Nano‐sized plutonium iron phosphate hydroxide structurally related to the rhabdophane group minerals was found in Hanford site sediments including Pu waste in the 216‐Z9 crib sediment in 2014 27 . Importantly, rhabdophane is usually considered as the low‐temperature precursors for monazite or a “metastable” phase in a geochemical environment, and can be converted to monazite by high‐temperature treatment (>800°C) 22,28,29 . Therefore, selecting rhabdophane to precipitate and enrich actinides can effectively reduce the waste volume of HLLW, which will be great significance to the sustainable development to nuclear energy.…”

“…Before 2014, hydrated rhabdophane with a general formula LnPO 4 ·nH 2 O (Ln = La ‐ Dy) has been considered to crystallize in the hexagonal system (P6222 or P3121) 28,30 . Intriguingly, a recent report associated with the re‐examination of the location of water molecules in rhabdophane by the use of synchrotron powder data evidenced LnPO 4 ·0.667H 2 O rhabdophane led to a structure crystallizing in the monoclinic C2 space group, which results in a new study on the placeholder mechanism of doping elements in rhabdophane 30,31 .…”

“…27 Importantly, rhabdophane is usually considered as the low-temperature precursors for monazite or a "metastable" phase in a geochemical environment, and can be converted to monazite by high-temperature treatment (>800 • C). 22,28,29 Therefore, selecting rhabdophane to precipitate and enrich actinides can effectively reduce the waste volume of HLLW, which will be great significance to the sustainable development to nuclear energy.…”

Exploring the relationship between Ln leaching and Ln–O binding energy in monazite (Nd, Sm, Eu)

“…10 Fortuitously, the priority formation of rhabdophane precipitations on monazite surface prevents the further degradation of monazite in leachates. [12][13][14][15][16] Recent dissolution experiments reveal the chemical stability of Ce 1−x Gd x PO 4 monazite will be weaken with increasing the concentration of Gd, and Gd is more likely to be dissolved compared to Ce. 17 Similar phenomenon has been observed in Ce 0.5 Eu 0.5 PO 4 monazite, associated with the leaching rate of Eu is half lower than that of Ce, 18 implying lanthanides have different stability in monazite.…”