Tridymite

About: Tridymite is a research topic. Over the lifetime, 840 publications have been published within this topic receiving 14831 citations.

High-temperature transformation of tridymite single crystals to cristobalite*

Abstract: Etude de la transformation reconstructive, a haute temperature, de la tridymite en cristobalite, en utilisant des cristaux de haute qualite et non macles. Mise en evidence de relations d'orientation entre la tridymite et la cristobalite formee. Preservation des unites structurales par la transformation, due aux relations structurales etroites existant entre tridymite et cristobalite

THE STRUCTURE OF “ORTHORHOMBIC” KAlSiO4-O1: EVIDENCE FOR Al–Si ORDER FROM MAS NMR DATA COMBINED WITH RIETVELD REFINEMENT AND ELECTRON MICROSCOPY

Abstract: Dry synthesis of KAlSiO4 at temperatures from 900 to 1500°C yielded products with slightly different powder X-ray-diffraction patterns. “Orthorhombic” K x Al x Si2− x O4- O1 with x ≈ 1 was obtained as a substantially single phase after heating at 1000°C for one day; we refined its crystal structure from powder X-ray-diffraction data in space group P 1211 [MM = 158.17 g/mol, a 15.669(2), b 9.057(1), c 8.621(1) A, β 90.16(1)°, V 1223.5 A3, Z = 12, Dx = 2.57 g cm−3, RB = 0.080]. It is composed of a relatively open [AlSiO4] framework that is a topological variant of tridymite (t) having the supercell (s) metric a s ≈ 3 a t, b s ≈ a t + 2 b t, c s ≈ c t. The space group P 1211 allows for Al–Si ordering, and refinement of distance-least-squares restrained models, although problematic owing to the pronounced pseudosymmetry, indicates preference for an ordered pattern where Al and Si are distributed on alternating tetrahedra [ d Si–O/ d Al–O = 1.628(1)/1.719(1) A], so that every SiO4 tetrahedron is coordinated to four AlO4 tetrahedra and vice versa . The alternating distribution was independently inferred from 29Si and 27Al MAS NMR spectroscopic data, and the framework model obtained from Rietveld refinement with Si on tetrahedron T 1 could be used to successfully simulate the observed Si(Al4) doublet peak in the 29Si spectrum. Electron diffraction showed that triple twinning with a rotation of 120° around c of the metrically almost hexagonal P 1211 cell is ubiquitous and enhances, in the diffraction experiment, the pseudosymmetry inherited from the tridymite subcell. Furthermore, the diffraction aspect of single individuals ( P *21*) confirms that the screw axes 21-- and --21 of the orthorhombic supergroup P 212121 are only approximated.

Temperature Stability and Photocatalytic Activity of Nanocrystalline Cristobalite Powders with Cu Dopant

Abstract: The SiO2 nanoparticles doped by 10 % mol Cu were prepared via a sol-gel method under process control. The effects of doping and calcination temperature on the structural and photo-catalytic properties of SiO2 nanopowders have been studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and UV-Vis absorption spectroscopy. Cristobalite and tridymite crystalline phases were found at a calcinations temperature range of 900∼1200 °C and amorphous phase was formed at a temperature of 800 °C for doped SiO2. The photocatalyst activity was evaluated by photocatalytic degradation kinetics of aqueous methyl orange (MO) under visible radiation. The results show that the photocatalytic activity of the 10 % mol Cu doped SiO2 nanopowders have a larger degradation efficiency than pure SiO2 under visible light at 900 °C temperature.