The diffusion coefficient controlling crystal growth in a silicate glass-former

One of the most relevant open issues in glass science refers to our ignorance concerning the nature of the diffusing entities that control crystal nucleation and growth in non-crystalline materials. This information is very relevant because all the existing nucleation and growth equations account for the diffusion coefficient (D-U) of these unknown entities. In this article, we measured the shear viscosity () and the crystal growth rates of a supercooled diopside liquid (CaMgSi2O6) in a wide temperature range. The well-known decoupling of viscosity and crystal growth rates at deep supercoolings was detected. We tested and analyzed 4 different approaches to compute D-U, three existing and one proposed here. As expected, the classical approach (D-U similar to (-1)) and the fractional viscosity approach (D-U similar to (-epsilon)) were not able to describe the crystal growth rates near the glass transition temperature. However, our proposed expression to calculate D(U)gradually changing from a viscosity-controlled to an Arrhenian-controlled processwas able to describe the available data in the whole temperature range and yielded the lowest uncertainty for the adjustable parameters. Our results suggest that viscous flow ceases to control the crystal growth process below the so-called decoupling temperature, corroborating some previous studies.