Raman spectrum of synthetic zircon (ZrSiO4) and thorite (ThSiO4)

Synthesis and Characterization of Single Crystal Zircon-Hafnon Zr(1-x)Hf(x)SiO4 Solid Solutions and the Comparison with the Reaction Products of a TEOS-Based Hydrothermal Route

Two synthesis routes of the zircon-hafnon solid solution series were carried out. The high-temperature route uses the growth of single crystals via a flux mixture that has been cooled down slowly from 1400 °C over 4 weeks. The reaction products were colorless and idiomorphic without byproducts. The hydrothermal tetraethoxysilane (TEOS)-based route represents the low-temperature method at 200 °C for approximately 7 days. The hydrothermal route yielded a white powder and scanning electron microscopy analysis thereof did not reveal any specific idiomorphic properties. However, the synthesis also featured some byproducts besides the zircon-hafnon solid solutions. Thermogravimetric analysis coupled with differential scanning calorimetry, and mass spectroscopy indicated, that hydrothermal reaction products feature the presence of organic residues originating from the starting materials. However, a specific dependency on the hafnium content could not be observed due to the data scatter. Infrared (IR) analysis revealed the presence of Zr/Hf-oxides. The structural characterization demonstrated that properties change constantly with the hafnium amount, however, gradual variations of some properties related to the composition of the solid solution series depend in part on the synthesis route, considering the c/a ratio and IR modes. Furthermore, analyses of the single crystals by Raman spectroscopy and μXRF suggested a nonequilibrated crystal growth based on the starting composition.

Phonon softening induced phase transition of CeSiO4: a density functional theory study

Density functional theory plus Hubbard U (DFT+U) methodology was used to calculate the structures and energetic landscapes of CeSiO4, including its stetindite and scheelite phases from ambient pressure to ∼24 GPa. To ensure accurate simulations of the high-pressure structures, assessments of strain-stress methods and stress-strain methods were conducted in prior, with the former found to have a better agreement with the experimental result. From DFT calculations the equation of states (EOS) of both stetindite and scheelite were further obtained, with the fitted bulk moduli being 182(2) GPa and 190.0(12) GPa, respectively. These results were found to be consistent with the experimental values of 177(5) GPa and 222(40) GPa. Furthermore, the calculated energetics suggest that the stetindite structure is more thermodynamically stable than the scheelite structure at a pressure lower than 8.35 GPa. However, the stetindite → scheelite phase transition was observed experimentally at a much higher pressure of ∼15 GPa. A further phonon spectra investigation by the density functional perturbation theory (DFPT) indicated the Eg1 mode is being softened with pressure and becomes imaginary after 12 GPa, which is a sign of the lattice instability. Consequently, it was concluded that the stetindite → scheelite transition is predominantly initiated by the lattice instability under high-pressure.

Synthesis, characterization, mechanical and magnetic characteristics of Gd3+ /PO4 3 - substituted zircon for application in hard tissue replacements

The study reports on the use of sol-gel technique to yield zircon type [Zr(1-0.1-x) GdxTi0.1 ] [(SiO4 )1-x (PO4 )x ] solid solution. Titanium has been used as a mineralizer to trigger zircon formation while equimolar concentrations of Gd3+ and PO4 3- were added to determine their accommodation limits in the zircon structure. The crystallization of t-ZrO2 as a dominant phase alongside the crystallization of m-ZrO2 and zircon were detected at 1200°C while their further annealing revealed the formation of zircon as a major phase at 1300°C. Heat treatment at 1400°C revealed the formation of zircon-type solid solution [Zr(1-0.1-x) Gdx Ti0.1 ][(SiO4 )1-x (PO4 )x ] comprising the accommodation of 10 mol.% of Gd3+ /PO4 3- at the zircon lattice. Beyond 10 mol.% of Gd3+ /PO4 3- , the crystallization of GdPO4 as a secondary phase is noticed. Structural analysis revealed the expansion of zircon lattice due to the simultaneous occupancy of Gd3+ /PO4 3- for the corresponding Zr4+ /SiO4 4- sites. The mechanical strength of single-phase zircon solid solution was higher in comparison to that of multiphase materials, namely in the presence of GdPO4 formed as a secondary phase in samples with added equimolar Gd3+ /PO4 3- contents beyond 10 mol.%. Nevertheless, the paramagnetic behavior of the samples demonstrated a steady surge as a function of enhanced Gd3+ content.

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

Orthosilicates adopt the zircon structure types (I4₁/amd), consisting of isolated SiO₄ tetrahedra joined by A-site metal cations, such as Ce and U. They are of significant interest in the fields of geochemistry, mineralogy, nuclear waste form development, and material science. Stetindite (CeSiO₄) and coffinite (USiO₄) can be formed under hydrothermal conditions despite both being thermodynamically metastable. Water has been hypothesized to play a significant role in stabilizing and forming these orthosilicate phases, though little experimental evidence exists. To understand the effects of hydration or hydroxylation on these orthosilicates, in situ high-temperature synchrotron and laboratory-based X-ray diffraction was conducted from 25 to ∼850 °C. Stetindite maintains its I4₁/amd symmetry with increasing temperature but exhibits a discontinuous expansion along the a-axis during heating, presumably due to the removal of water confined in the [001] channels, which shrink against thermal expansion along the a-axis. Additional in situ high-temperature Raman and Fourier transform infrared spectroscopy also confirmed the presence of the confined water. Coffinite was also found to expand nonlinearly up to 600 °C and then thermally decompose into a mixture of UO₂ and SiO₂. A combination of dehydration and dehydroxylation is proposed for explaining the thermal behavior of coffinite synthesized hydrothermally. Additionally, we investigated high-temperature structures of two coffinite-thorite solid solutions, uranothorite (U_xTh_1-xSiO₄), which displayed complex variations in composition during heating that was attributed to the negative enthalpy of mixing. Lastly, for the first time, the coefficients of thermal expansion of CeSiO₄, USiO₄, U_0.46Th_0.54SiO₄, and U_0.9Th_0.1SiO₄ were determined to be α_V = 14.49 × 10^-6, 14.29 × 10^-6, 17.21 × 10^-6, and 17.23 × 10^-6 °C^-1, respectively.

The formation of PuSiO4 under hydrothermal conditions

Attempts to synthesize plutonium(iv) silicate, PuSiO4, have been made on the basis of results recently reported in the literature for CeSiO4, ThSiO4, and USiO4 under hydrothermal conditions. Although it was not possible to prepare PuSiO4via applying the conditions reported for thorium and uranium, an efficient method of PuSiO4 synthesis was established by applying the conditions optimized for the CeSiO4 system. This method was based on the slow oxidation of plutonium(iii) silicate reactants under hydrothermal conditions at 150 °C in hydrochloric acid (pH = 3-4). These results shed new light on the potential behavior of plutonium in reductive environments, highlighting the representative nature of cerium surrogates when studying plutonium under such conditions and providing some important pieces of information regarding plutonium chemistry in silicate solutions.

Challenges to rutile-based geoscientific tools: low-temperature polymorphic TiO2 transformations and corresponding reactive pathways

Rutile, a common accessory mineral in a wide variety of rocks, is the most stable naturally occurring TiO₂ polymorph. The relationship between its trace element composition and formation conditions has provided geoscientists with discriminant tools for fingerprinting geological processes, such as magmatic evolution and subduction zone metamorphism, alongside applications to the study of sediment provenance. In the present work, volcaniclastic rock samples belonging to Fara and Saiq Formations, outcropping in Jebel Akhdar mountains, Oman, are studied with Raman spectroscopy and Electron Microprobe (EMP) aiming: of (i) the identification of different naturally-occurring TiO₂ polymorphs, (ii) the evaluation of their trace element contents in relation with hydrothermal alteration features, and (iii) the analysis of the mineral reactive pathways behind the observed textural relationships. Raman investigations demonstrated that interstitial, fine-grained TiO₂ corresponds to anatase, whereas rutile occurs as isolated single grains. EMP determinations further revealed that an identified Nb-enrichment in anatase is coupled with a corresponding Nb-depletion in rutile. The combination of the obtained results with petrographic observations enabled unravelling the TiO₂ reactive pathways affecting the studied samples. Thus, a coupled polymorphic dissolution-precipitation reaction assisting rutile-to-anatase conversion has been defined, together with the role of Nb in further stabilizing the structure of the lower temperature polymorph. Semi-quantitative thermometric considerations suggest that rutile substrates are likely of magmatic origin, whereas anatase formation is clearly associated with a lower temperature aqueous environment. The gathered results raise fundamental questions concerning the application of commonly used rutile-based geochemical and thermometric tools.

Preparation of CeSiO4 from aqueous precursors under soft hydrothermal conditions

Even though CeSiO₄ was synthesized one time through a hydrothermal treatment, the conditions leading to its formation remain largely unknown. In order to define the optimized conditions of synthesis, a multiparametric study was developed by varying the pH of the solution, the temperature, and the nature of the reactants and of the complexing ions in solution. This study highlighted that CeSiO₄ could not be obtained starting from Ce(iv) reactants. An optimal set of conditions was defined to prepare single phase samples. Pure CeSiO₄ was obtained through a hydrothermal treatment at 150 °C using a starting mixture of 1 mol L^-1 Ce(iii) nitrate and Na₂SiO₃ solutions and by adjusting the initial pH to 8. The chemical limitations observed during the synthesis of CeSiO₄ suggested that the formation of this phase may result from the slow in situ oxidation of a Ce(iii) silicate complex during the hydrothermal treatment.

Image-Based Chemical Structure Determination

Chemical imaging is a powerful tool for understanding the chemical composition and nature of heterogeneous samples. Recent developments in elemental, vibrational, and mass-spectrometric chemical imaging with high spatial resolution (50–200 nm) and reasonable timescale (a few hours) are capable of providing complementary chemical information about various samples. However, a single technique is insufficient to provide a comprehensive understanding of chemically complex materials. For bulk samples, the combination of different analytical methods and the application of statistical methods for extracting correlated information across different techniques is a well-established and powerful concept. However, combined multivariate analytics of chemical images obtained via different imaging techniques is still in its infancy, hampered by a lack of analytical methodologies for data fusion and analysis. This study demonstrates the application of multivariate statistics to chemical images taken from the same sample via various methods to assist in chemical structure determination.

Incorporation of Thorium in the Zircon Structure Type through the Th1-xErx(SiO4)1-x(PO4)x Thorite-Xenotime Solid Solution

Pure powdered compounds with a general formula Th_1-xEr_x(SiO₄)_1-x(PO₄)_x belonging to the zircon-xenotime family were successfully synthesized under hydrothermal conditions (250 °C, 7 days) as recently reported for the preparation of coffinite. Therefore, a thorough, combined PXRD, EDX, EXAFS, Raman, and FTIR analysis showed the formation of a solid solution in agreement with Vegard's law. Moreover, the examination of the local structure shows that the Th-O distances remain close to those found in ThSiO₄, whereas the Er-O distances show a significant decrease from 2.38(14) to 2.34(7) Å when increasing the erbium content from x = 0.2 to x = 1. The variation of the local structure also affects the PO₄^3- groups that are surely distorted in the structure.

Luminescence properties and crystal structure of α′-Sr2Si3x/4O2Nx:Eu2+ phosphors with different concentrations of N3− ions

This picture is used for graphical abstract because it highlight the change of PL intensity in disorder α′-SSON:Eu²⁺.

In situ spectroscopic studies of decomposition of ZrSiO4during alkali fusion process using various hydroxides

Spectra analysis shows that zircon decomposes into ZrO₂and silane, and then silane polymerizes to form phyllosilicate in an alkali fusion reaction. The structure of alkali fusion products is relative to the electrons-afford ability of alkali in the reaction.

Modification of the crystal structure of Sr2−xBaxSi(O,N)4: Eu2+ phosphors to improve their luminescence properties

A series of Sr_1.98−xBa_xSi(O,N)₄: 0.02Eu²⁺ (0 ≤ x ≤ 0.5) phosphors were synthesized. We anticipate that these materials can be used as green or red phosphors in white light emitting diodes.

From thorite to coffinite: a spectroscopic study of Th(1-x)U(x)SiO4 solid solutions

Coffinite (USiO4), along with Th(1-x)U(x)SiO4 uranothorite solid solutions, are frequently present in reduced economically exploitable uranium ores. They could also control the concentration of uranium in the environment in the case of accidental release from underground radwaste repository. This paper reports for the first time a thorough FTIR and Raman study relative to the Th(1-x)U(x)SiO4 system, including synthetic analogues of thorite and coffinite end-members. Both sets of spectra confirmed the formulation of the samples and allowed to rule out the presence of structural water molecules and/or hydroxyl groups in the coffinite. Also, no characteristic signal of UO2(2+) uranyl ion was recorded, ensuring that uranium was fully incorporated under its tetravalent oxidation state. The variation of the positions corresponding to SiO4 internal vibration modes was then followed versus the chemical composition of the samples. If the FTIR spectra did not revealed any significant shift in the bands position, several Raman modes followed a linear trend as a function of the uranium incorporation rate. On this basis, Raman spectroscopy could be considered as a promising tool for the semi-quantitative determination of chemical composition of uranothorite samples, particularly for those coming from mineral ores. Finally, the data collected for the coffinite end-member, as the first to be obtained on pure synthetic samples, allowed a review of the results previously reported in the literature for this compound.

Raman spectroscopy and scanning electron microscopy characterizations of fission track method datable zircon grains

Spectroscopic and morphological studies, designed to improve our understanding of the physicochemical phenomena that occur during zircon crystallization, are presented. The zircon fission track method (ZFTM) is used routinely in various laboratories around the world; however, there are some methodological difficulties needing attention. Depending on the surface fission track density observed under an optical microscope, the zircon grain surfaces are classified as homogeneous, heterogeneous, hybrid, or anomalous. In this study, zircon grain surfaces are characterized using complementary techniques such as optical microscopy (OM), micro-Raman spectroscopy, and scanning electron microscopy (SEM), both before and after chemical etching. Our results suggest that anomalous grains have subfamilies and that etching anisotropy related to heterogeneous grains is due to different crystallographic faces within the same polished surface that cannot be observed under an optical microscope. The improved methodology was used to determine the zircon fission track ages of samples collected from the Bauru Group located in the north of Paraná Basin, Brazil. A total of 514 zircon grains were analyzed, consisting of 10% homogeneous, about 10% heterogeneous, about 20% hybrid, and 60% anomalous grains. These results show that the age distributions obtained for homogeneous, heterogeneous, and hybrid grains are both statistically and geologically compatible.