Crystallization Mechanism of Gel-Derived SiO₂–TiO₂ Amorphous Nanobeads Elucidated by High-Temperature In Situ Experiments

Abstract

SiO2–TiO2 amorphous nanobeads with a TiO2 content up to 50 mol % were synthesized by sol–gel spray drying. Their crystallization during heat treatments was then characterized by in situ high-temperature techniques including X-ray diffraction, Raman spectroscopy, and particularly high-resolution transmission electron microscopy. Intrinsic nanoscale chemical modulations could be identified already in the as-prepared nanobeads and were shown to play a major role for the nonisochemical precipitation of TiO2 nanocrystals during heating experiments: the size and compositional contrast of such fluctuations progressively evolved and increased until the emergence of long-range ordering. The formation of TiO2 polymorphs occurred according to Ostwald’s rule of stages, with the metastable TiO2(B) phase acting as a precursor to stabilize anatase and rutile. The temporary appearance of TiO2(B) nanocrystals at early annealing stages was interpreted as the first direct experimental observation of subcritical crystalline nuclei.

Non-stoichiometric crystal nucleation in a spodumene glass containing TiO2 as seed former: Effects on the viscosity of the residual melt

“…1). Nevertheless, structural relaxation seems to be indissolubly connected to compositional rearrangements at the nanoscale, which occur almost simultaneously and initiate the phase separation of amorphous TiO2-enriched droplets, as evidenced in previous works [19,53] and inferred here by Raman spectroscopy. The driving force for chemical diffusion can be identified in the poor solubility of transition metal cations in deeply supercooled silicate melts (as recently postulated for basalts [23]).…”

Nanoscale chemical heterogeneities control magma viscosity and failure

“…In-situ heating experiments were performed at the CNRS CEMHTI in Orléans (France) using a JEOL ARM200F (JEOL Ltd.) Cold FEG microscope operating at 80 kV, mounting a double spherical aberration corrector, a Gatan Imaging Filter (GIF, Gatan Ltd.) and a OneView camera. The experimental procedure was optimized in a previous work 41 to minimize artifacts and sample damages due to highly energetic electron irradiation. The AND100 glass was crushed and grinded in an agate mortar, adding ethanol to obtain a diluted suspension; one drop of the liquid was then loaded onto an MEMS grid speci cally adapted for a Protochips Fusion double-tilt heating holder and dried in air overnight.…”