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Abstract Synthetic thorite and huttonite, two polymorphs of ThSiO4, were investigated by a combination of in situ high-pressure synchrotron X-ray powder diffraction and in situ high pressure Raman spectroscopy. The average onset pressure of the thorite-to-huttonite transition was determined to be 6.6 ± 0.2 GPa, using both techniques. The bulk moduli of thorite and huttonite were determined to be 139(9) and 246(11) GPa, respectively, by fitting their unit-cell volume data to a second order Birch-Murnaghan equation of state (EOS). Based on its bulk modulus, thorite is the most compressible zircon-structured orthosilicate, as it has the largest unit cell volume among tetravalent metal orthosilicates. The pressure derivatives of the vibrational modes of thorite were found to be consistent with those previously reported for other orthosilicates (e.g., zircon, hafnon, stetindite, and coffinite), while having the smallest Grüneisen parameter. A new P-T phase diagram for ThSiO4 is proposed, where the boundary of the thorite → huttonite transition is: P(T) = (7.8 ± 0.9 GPa) − (0.006 ± 0.002 GPa/K)T. Based on the new P-T phase diagram, we further estimated the enthalpy of formation of huttonite, ΔHf,ox, to be 0.6 ± 6.0 kJ/mol, suggesting its metastability and rare locality in nature. 

Abstract Zircon-class ternary oxide compounds have an ideal chemical formula of ATO4, where A is commonly a lanthanide and an actinide, with T = As, P, Si, or V. Their structure (I41/amd) accommodates a diverse chemistry on both A- and T-sites, giving rise to more than 17 mineral end-members of five different mineral groups, and in excess of 45 synthetic end-members. Because of their diverse chemical and physical properties, the zircon structure-type materials are of interest to a wide variety of fields and may be used as ceramic nuclear waste forms and as aeronautical environmental barrier coatings, to name a couple. To support advancement of their applications, many studies have been dedicated to the understanding of their structural and thermodynamic properties. The emphasis in this review will be on recent advances in the structural and thermodynamic studies of zircon structure-type ceramics, including pure end-members [e.g., zircon (ZrSiO4), xenotime (YPO4)] and solid solutions [e.g., ErxTh1–x(PO4)x(SiO4)1–x]. Specifically, we provide an overview on the crystal structure, its variations and transformations in response to non-ambient stimuli (temperature, pressure, and radiation), and its correlation to thermophysical and thermochemical properties. 

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. 

We used DFT+Uto explore high-P structures and energetics of CeSiO₄, and found the stetindite → scheelite transition at ∼15 GPa (>8.4 GPa predicted by enthalpy) is driven by lattice instability, due to softening and imaginary state of the E_g¹mode.