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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. 

Critical particle size can be determined with known surface energy. The surface enthalpy of yttrium titanate pyrochlores was determined to be 4.07 ± 0.32 J m⁻² by calorimetry, and the lower limit of critical particle size for this is around 5.0 nm.