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Rhabdophane is a hydrous phosphate that commonly replaces monazite as a weathering product in critical mineral deposits during the alteration of rare earth elements (REE) bearing carbonatites and alkaline igneous complexes. It is an important host to the light (L)REE (i.e., La to Gd) but the stability and structure of binary solid solutions between the Ce and the other LREE endmembers have not yet been determined experimentally. Here we present room temperature calorimetric experiments that were used to measure the enthalpy of precipitation of rhabdophane (Ce1−xREExPO4·nH2O; REE = La, Pr, Nd, Sm, Eu, and Gd). The solids were characterized using X-ray diffraction, scanning electron microscopy, Raman spectroscopy, and the role of water in the rhabdophane structure was further determined using thermogravimetric analysis coupled with differential scanning calorimetry. The calorimetric experiments indicate a non-ideal behavior for all of the binary solid solutions investigated with an excess enthalpy of mixing (ΔHex) described by a 2- to 3-term Guggenheim parameters equation. The solid solutions were categorized into three groups: (1) binary Ce-La and Ce-Pr which display positive ΔHex values with a slight asymmetry; (2) binary Ce-Nd and Ce-Sm which display negative ΔHex values with a nearly symmetric shape; (3) Ce-Eu and Ce-Gd which display both negative and positive ΔHex values with nearly symmetric shape. The excess Gibbs energy (ΔGex) of the solid solutions was further investigated using a thermodynamic analysis approach of aqueous-solid solution equilibria and the optimization programs GEMS and GEMSFITS. The resulting ΔGex values combined with the calorimetric ΔHex values indicate that there is likely an excess entropy contribution implying important short-range structural modifications in the solid solutions dependent on the deviation of the REE ionic radii from the size of Ce3+. These observations corroborate with the trends in the Raman v1 stretching bands of the PO4-site. The excess molar volumes determined from X-ray diffraction analysis further indicate an overall asymmetric behavior in all of the studied binary solid solutions, which becomes increasingly important from La to Gd. The pronounced short-range order–disorder occurring in groups 2 and 3 solid solutions mimics some of the behavior observed from previous studies in anhydrous monazite solid solutions. This study highlights the potential to use the chemistry and the structural modifications of rhabdophane as potential indicators of formation conditions in geologic systems and permits improving our modeling capabilities of REE partitioning in critical minerals systems.
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A review of defect structure and chemistry in ceria and its solid solutions
Ceria and its solid solutions play a vital role in several industrial processes and devices. These include solar energy-to-fuel conversion, solid oxide fuel and electrolyzer cells, memristors, chemical looping combustion, automotive 3-way catalysts, catalytic surface coatings, supercapacitors and recently, electrostrictive devices. An attractive feature of ceria is the possibility of tuning defect-chemistry to increase the effectiveness of the materials in application areas. Years of study have revealed many features of the long-range, macroscopic characteristics of ceria and its derivatives. In this review we focus on an area of ceria defect chemistry which has received comparatively little attention – defect-induced local distortions and short-range associates. These features are non-periodic in nature and hence not readily detected by conventional X-ray powder diffraction. We compile the relevant literature data obtained by thermodynamic analysis, Raman spectroscopy, and X-ray absorption fine structure (XAFS) spectroscopy. Each of these techniques provides insight into material behavior without reliance on long-range periodic symmetry. From thermodynamic analyses, association of defects is inferred. From XAFS, an element-specific probe, local structure around selected atomic species is obtained, whereas from Raman spectroscopy, local symmetry breaking and vibrational changes in bonding patterns is detected. We note that, for undoped ceria and its solid solutions, the relationship between short range order and cation–oxygen-vacancy coordination remains a subject of active debate. Beyond collating the sometimes contradictory data in the literature, we strengthen this review by reporting new spectroscopy results and analysis. We contribute to this debate by introducing additional data and analysis, with the expectation that increasing our fundamental understanding of this relationship will lead to an ability to predict and tailor the defect-chemistry of ceria-based materials for practical applications.
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- PAR ID:
- 10142673
- Date Published:
- Journal Name:
- Chemical Society Reviews
- Volume:
- 49
- Issue:
- 2
- ISSN:
- 0306-0012
- Page Range / eLocation ID:
- 554 to 592
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/c9cs00588a
