Crystalline ZrSiO4 Research Articles

Selective addition of isobutene over ZrSiO4/SiO2-SO3H: Hydroacetoxylation versus dimerization

The acid-catalyzed Markovnikov addition of isobutene is affected by the concomitant isobutene oligomerization. Herein, a solid acid catalyst consisting of the sulfonated silica modified with crystalline ZrSiO4 was prepared, and the ZrSiO4/SiO2-SO3H catalyst went beyond conventional acid catalysts in the hydroacetoxylation of isobutene. The structure of the catalyst was well-characterized, and the role of ZrSiO4 in suppressing the dimerization of isobutene was discussed in terms of both kinetic study and in-situ DRIFT analysis. Moreover, this catalyst also showed general applicability for a broad substrate scope, further highlighting the necessity of ZrSiO4 in the solid acid catalyst.

Structural evolution of oxidized surface of zirconium-silicide under ion irradiationm

Irradiation-induced structural evolution and compositional alterations on nanometer length scales of ZrO2-SiO2 nanocomposite and crystalline ZrSiO4, produced by oxidation of ZrSi2 at 1000 °C and 1400 °C, respectively, have been investigated. Irradiations were performed with 3.9 MeV Si2+ at 305 °C up to damage levels of 60 displacements per atom (dpa), and post-irradiation characterization was conducted by cross-sectional scanning- and transmission electron microscopy (STEM and TEM) in conjunction with energy dispersive spectroscopy (EDS), as well as glancing-angle X-ray diffraction (XRD). The multilayered nanocomposite oxide showed extensive phase mixing so that the originally distinct interfaces between ZrO2 and SiO2 phases were obliterated resulting in the formation of a single amorphous Zr-Si-O phase. Crystalline ZrSiO4 grains were completely amorphized at damage levels of 5–15 dpa, as evidenced by disappearance of grain boundaries and as confirmed by electron diffraction. The irradiation-induced phase transformation for both types of oxide structures has the potential to dramatically enhance corrosion resistance in high temperature aqueous and steam environments.

Phase quantification of mullite–zirconia and zircon commercial powders using PAC and XRD techniques

The short range technique of the Perturbed Angular Correlation (PAC) and x-ray diffraction (Rietveld) methods have been employed to determine the phase content in commercial mullite–zirconia and zircon raw materials that are ordinarily used to produce ceramic materials. The PAC technique, which probes zirconium-containing compounds at nanoscopic level, showed that zircon contains crystalline ZrSiO4 and an important amount of a structurally distorted zircon, which is also observed accompanying monoclinic zirconia in mullite–zirconia. This particular zircon phase was not detected by the long range x-ray diffraction–Rietveld technique. After an annealing treatment, important changes in crystalline contents of the powders allow confirming, by the x-ray diffraction–Rietveld method, the preexistence of this particular zircon phase. This fact must be taken into account when preparing multicomposites based on the present raw materials.

Unified interatomic potential for zircon, zirconia and silica systems

We report the development of a unified force field with high transferability and reliability to model both structures and mechanical properties of ZrSiO4, ZrO2 and SiO2 based on the success of the BKS potential for SiO2 (van Beest et al., Phys. Rev. Lett., 1990, 64, 1955). The thermal expansion, relative stability and phase transition properties are consistent with experimental data and DFT calculations. The zircon to reidite transition pressure is 6.7 GPa. Amorphization of zircon results in volume expansion of 11% and decrease in the bulk modulus of 60%. Si polymerization and Zr under-coordination were observed in amorphous ZrSiO4 and in the damage produced by a 10 keV Zr recoil in crystalline ZrSiO4. The upper limit of stored energy in amorphous ZrSiO4 is 140 kJ/mol.

Calculation of the effect of intrinsic point defects and volume swelling in the nuclear magnetic resonance spectra of ZrSiO4

A variety of computational tools have been used to study the chemical properties of point defects in the crystalline phase of ZrSiO4, and their effect in its lattice parameters. The experimental evidence of a large anisotropic volume swelling in natural and artificially irradiated samples of ZrSiO4 was used to select the subset of defects that give similar lattice swelling for the concentrations studied, namely interstitials of O and Si, and the anti-site Zr Si . Using the relaxed atomic structures, the nuclear magnetic resonance spectra were calculated by means of first principles density functional methods, in order to find additional evidence for the presence of high concentrations of some of these defects in irradiated zircon. The results obtained for the defects singled out before are still compatible with available experimental information. Our calculations also show that volume swelling in crystalline ZrSiO4 would produce a considerable displacement of the 17O and 91Zr chemical shifts towards higher values, whereas the 29Si spectra would be largely independent of the defect-induced swelling.

Titanium oxides and silicates as high‐κ dielectrics: A first‐principles investigation

AbstractUsing density functional theory, we investigate the structural, vibrational, and dielectric properties of titanium oxides and silicates, which have attracted considerable attention in the framework of the quest for alternative high‐κ materials. For the oxides, three crystalline phases of titanium dioxide are considered. The first two are hypothetical; they are obtained by similarity with the cubic and tetragonal structure of zirconia ZrO2 or hafnia HfO2. The third is the rutile, a crystal that occurs naturally. For the silicates, we analyze a hypothetical TiSiO4 structure constructed by analogy with crystalline ZrSiO4 and HfSiO4 (zircon and hafnon). © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Intrinsic point defects and volume swelling inZrSiO4under irradiation

The effects of high concentration of point defects in crystalline ZrSiO4 as originated by exposure to radiation, have been simulated using first principles density functional calculations. Structural relaxation and vibrational studies were performed for a catalogue of intrinsic point defects, with different charge states and concentrations. The experimental evidence of a large anisotropic volume swelling in natural and artificially irradiated samples is used to select the subset of defects that give similar lattice swelling for the concentrations studied, namely interstitials of O and Si, and the anti-site Zr(Si), Calculated vibrational spectra for the interstitials show additional evidence for the presence of high concentrations of some of these defects in irradiated zircon.

Annealing of -decay damage in zircon: a Raman spectroscopic study

Recrystallization and structural recovery in -decay damage in zircon samples have been studied using Raman spectroscopy. Fifteen zircon samples with different degrees of radiation damage have been thermally annealed between 600 K and 1800 K for up to 28 days and 8 hours. The experimental results from this study reveal that recrystallization in the damaged zircon samples is a multi-stage process that depends on the degree of initial damage of the samples. In partially damaged samples the lattice recovery of damaged crystalline ZrSiO4 takes place at temperatures as low as about 700 K, as shown by a remarkable band-sharpening and a significant increase in the frequencies of 1 and 3 Si-O stretching vibrations together with the external band near 357 cm-1 with increasing temperature. A dramatic increase of Raman scattering intensity of ZrSiO4 occurs in partially damaged samples near 1000 K due to a recrystallization process involving epitaxial growth. Heavily damaged samples tend to decompose into ZrO2 and SiO2 at high temperatures. Tetragonal ZrO2 has been observed under annealing between 1125 K and about 1600 K in heavily damaged samples while monoclinic ZrO2 appears above 1600 K. Weak signals from ZrSiO4 were detected at 1125 K in highly metamict zircon although the main recrystallization appears to occur near 1500 K accompanied by a decrease of the volumes of ZrO2 as well as SiO2. This suggests that this recrystallization is associated with the reaction of ZrO2 with SiO2 to form ZrSiO4. A possible intermediate phase has been observed, for the first time, by Raman spectroscopy in damaged zircons annealed at temperatures between 800 K and 1400 K. This phase is characterized by strong, broad Raman signals near 670, 798 and 1175 cm-1. Prolonged isothermal annealing at 1050 K results in a decrease of these characteristic bands and eventually the disappearance of this intermediate phase.

Atomic ordering in xZrO2·(1−x) SiO2 xerogels (x=0.3, 0.5) by X-ray diffraction and reverse Monte Carlo simulations

Atomic distribution functions for xZrO2·(1 − x) SiO2 xerogels (x = 0.3, 0.5) have been obtained by X-ray diffraction experiments. Three dimensional structure models that closely reproduce the experimental structural data have been constructed by reverse Monte Carlo simulations. The model results suggest that the investigated xerogels do not exhibit any characteristics of a phase segregation and have a homogeneous structure at the atomic scale. In particular, the local atomic ordering in 0.5ZrO2·0.5 SiO2 xerogels heat treated to 770 K has been found to be similar to that in crystalline ZrSiO4. The question as to why a phase segregation occurs on the crystallization of xZrO2·(1 − x) SiO2 xerogels (x − 0.3, 0.5) has been addressed.

Silicate particles engulfed in human alveolar macrophages: An analytical electron microscopy study

The chemical composition and crystalline structure of silicate particles engulfed in human alveolar macrophages were investigated by transmission analytical electron microscopy. A crystalline ZrSiO4 and an amorphous Si2AlO4 particle were identified. Small crystalline Fe microparticles (2–4 nm), which are likely ferritin molecules, were found concentrated mainly at the amorphous particle-tissue interface.