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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. 

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. 

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. 

Although ZrSiO 4 is the most well-known compound in the zircon-structured family (space group I41/amd), the experimental conditions for preparing pure and well-crystallized phases which are doped with tetravalent element via hydrothermal synthesis has never been clearly discussed in the literature. With the aim to answer this question, the experimental conditions of preparation of ZrSiO 4 and (Zr,Ce)SiO 4 were investigated in order to synthesize well-crystallized and pure phases. A multiparametric study has been carried out using soft hydrothermal conditions, with variables including reactant concentration, initial pH of the reactive medium, and duration of the hydrothermal treatment. Pure ZrSiO 4 was obtained through hydrothermal treatment for 7 days at 250°C, within a large acidity range (1.0 ≤ pH ≤ 9.0) and starting from C Si ≈ C Zr ≥ 0.2 mol·L -1 . As hydrothermally prepared zircon structured phases can be both hydrated and hydroxylated, its annealed form was also studied after heating to 1000°C. Based on these results, the synthesis of (Zr,Ce)SiO 4 solid solutions were also investigated. The optimal hydrothermal conditions to acquire pure and crystallized phases were obtained in 7 days at 250°C with initial pH = 1 and concentration of the reactants equal to 0.2 mol·L -1 . This led to (Zr,Ce)SiO 4 solid solutions with the incorporated Ce content up to 40 mol.%. Samples were characterized by multiple methods, including lab and synchrotron PXRD, IR and Raman spectroscopies, SEM, and TGA. Moreover, it was found that these phases were thermally stable in air up to at least 1000°C.