High-Pressure High-Temperature Stability and Thermal Equation of State of Zircon-Type Erbium Vanadate

Enhancing the zircon yield through the addition of calcium phosphates into ZrO2-SiO2 binary systems: synthesis and structural, morphological, mechanical and in vitro analysis

The crystallization of ZrSiO4 is generally accomplished by the addition of mineralizers into ZrO2-SiO2 binary oxides. The current investigation aimed to investigate the effect of adding calcium phosphates into ZrO2-SiO2 binary oxides on the yield of ZrSiO4. The concentration of calcium phosphate additions were varied to obtain ZrSiO4 that fetches improved mechanical and biological properties for application in hard tissue replacements. The findings highlight the significant role of Ca2+ and P5+ in triggering the ZrSiO4 formation via their accommodation at the Zr4+ and Si4+ sites. Especially, calcium phosphate additions trigger the t- → m-ZrO2 transition beyond 1000 °C, which consequently reacts with SiO2 to promote ZrSiO4 formation. Calcium phosphates are accommodated at the lattice sites of ZrSiO4 with a maximum limit of 20 mol%, beyond which the crystallization of β-Ca3(PO4)2 is noticed. The optimum amount of 20 mol% of calcium phosphates displayed a better strength than that of all the investigated specimens. More than 80% of cell viability in MG-63 cells was invariably determined in all the calcium phosphate-added ZrSiO4 systems.

Pressure-induced phase transitions in TmVO4 investigated by Raman spectroscopy

The structural behavior of TmVO₄ at pressures up to ∼ 55 GPa was investigated by Raman spectroscopy. The changes in Raman spectra suggest the existence of three phase transitions upon compression. The first phase transition appeared at ∼ 7.7 GPa, which was an irreversible phase transition from the ambient-pressure zircon phase to the scheelite phase, confirming previous X-ray measurements. Subsequently, the second reversible phase transition from the scheelite phase to the fergusonite phase occurred at ∼ 23 GPa. Additional changes in the Raman spectra were observed at ∼ 37 GPa, validating the third phase transition. Based on a comparison to related rare earth orthovanadates, we assumed that the post-fergusonite of TmVO₄ has an orthorhombic structure described by space group Cmca. The wavenumbers of the Raman modes and their pressure coefficients for all four phases of TmVO₄ are reported. Our study provides the vibrational difference in various polymorphs of TmVO₄, which will refine our understanding of the structural behavior of rare-earth orthovanadates.

High-pressure phase transformations in NdVO4 under hydrostatic, conditions: a structural powder x-ray diffraction study

Room temperature angle dispersive powder x-ray diffraction experiments on zircon-type NdVO₄ were performed for the first time under quasi-hydrostatic conditions up to 24.5 GPa. The sample undergoes two phase transitions at 6.4 and 19.9 GPa. Our results show that the first transition is a zircon-to-scheelite-type phase transition, which has not been reported before, and contradicts previous non-hydrostatic experiments. In the second transition, NdVO₄ transforms into a fergusonite-type structure, which is a monoclinic distortion of scheelite-type. The compressibility and axial anisotropy of the different polymorphs of NdVO₄ are reported. A direct comparison of our results with former experimental and theoretical studies on other rare-earth orthovanadates found in literature highlights the importance of the role played by non-hydrostatic stresses in their high-pressure structural behavior.

High-Pressure Phase Transition of Micro- and Nanoscale HoVO4 and High-Pressure Phase Diagram of REVO4 with RE Ionic Radius

In situ Raman spectra of HoVO₄ micro- and nanocrystals were obtained at high pressures up to 25.4 and 18.0 GPa at room temperature, respectively. The appearance of new peaks in the Raman spectra and the discontinuities of the Raman-mode shift provided powerful evidence for an irreversible zircon-to-scheelite structure transformation for HoVO₄ microcrystals at 7.2 GPa and for HoVO₄ nanocrystals at 8.7 GPa. The lattice contraction caused by the size effect was thought to be responsible for the different phase-transition pressures. Also, the higher stability of HoVO₄ nanocrystals compared with the microcrystals was also confirmed using the Raman frequencies and pressure coefficients. The results of the phase transition of HoVO₄ were compared with previously reported rare-earth orthovanadates, and the phase diagram of REVO₄ with RE ionic radius at different pressures was presented.