Glass-forming ability and structural features of melt-quenched and gel-derived SiO2-TiO2 glasses

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

“…The direct appearance of this phase (in agreement with the VT-XRD data) is to be interpreted based on the composition of the starting material, much richer in TiO 2 than T17: the structure around Ti 4+ ions in T50 can be expected to mirror more closely that of TiO 2 crystals, not requiring an additional intermediate step (i.e., TiO 2 (B) formation) to lower the energy barrier for nucleation. A residual amorphous silicate phase, 23 evidently still containing a non-negligible amount of TiO 2 due to the intense Raman bands observable at 900 and 1100 cm −1 , could still be detected in both samples after the measurements. This is in qualitative agreement with the results of VT-XRD, according to which the samples contained only a very small amount of anatase and/or TiO 2 (B) at the temperatures then investigated by VT-Raman.…”

“…The strong difference in final crystallite sizes achieved in the two samples is likely to originate from the much higher total TiO 2 content in T50 and from the presence in T17 of a SiO 2 -enriched, viscous residual melt, acting as a barrier for diffusion and particle coalescence up to high temperatures. The formation of metastable TiO 2 polymorphs during crystal nucleation is a long-known phenomenon in glasses, typically involving anatase but recently also identified for TiO 2 (B) (see for instance ref 21,23,31). It can be related to two main factors: (i) a stabilization effect due to small crystallite sizes and high surface-to-volume ratios, as demonstrated for anatase 32,33 and previously postulated for TiO 2 (B) nanocrystals 21 and (ii) the energetic gain of gradually approaching the equilibrium configuration (rutile, the only thermodynamically stable polymorph) through several kinetically favored intermediate steps, as theorized in Ostwald's rule of stages.…”

“…It can be related to two main factors: (i) a stabilization effect due to small crystallite sizes and high surface-to-volume ratios, as demonstrated for anatase 32,33 and previously postulated for TiO 2 (B) nanocrystals 21 and (ii) the energetic gain of gradually approaching the equilibrium configuration (rutile, the only thermodynamically stable polymorph) through several kinetically favored intermediate steps, as theorized in Ostwald's rule of stages. 34 Indeed, TiO 2 (B) exhibits low density and a very distorted sixfold oxygen coordination around Ti 4+ cations, representing an ideal "missing link" between the prevailing tetrahedral coordination in SiO 2 −TiO 2 glasses 23 and the regular octahedral coordination of anatase and rutile.…”

“…As demonstrated in previous works, Raman spectroscopy is a particularly effective tool to unambiguously discern different TiO 2 polymorphs precipitating in glasses, even for crystalline fractions of only a few vol %. 21,23,31 The Raman spectra of TiO 2 (B) and anatase can be easily distinguished through several bands that are characteristic of monoclinic TiO 2 (B) but absent in tetragonal anatase, 35 e.g., at ∼125 cm −1 (invisible here due to the edge filter) and at 250 cm −1 . Note that measurements at high temperature expectedly induced a non-negligible broadening and shift of Raman features that may complicate their interpretation, as exemplified during the reheating of T17 to 850 °C (Figure 3).…”

“…21 Silanol groups trapped in the glass structure were instead far more persistent, so that full dehydration was only achievable at temperatures and timescales comparable to or higher than those necessary for crystal nucleation. 23 2.2. Variable-Temperature X-ray Diffraction.…”