Assessing the early stages of crystal nucleation and growth in a deeply supercooled glass-forming liquid

Abstract

For treatments conducted below the glass transition temperature (T_g), the extrapolated size–time dependence of micron-sized crystals often suggests a growth “induction period”, as the data fail to extrapolate back to the origin (time = zero). We refute this notion by presenting, for the first time, experimental evidence of the early stages of barium disilicate (BaSi₂O₃) crystal growth in an isochemical glass. Through electron microscopy measurements of nanometric crystal sizes, we demonstrate that the growth velocity—and, consequently, the derived effective diffusion coefficients that govern both nucleation and growth—are valid from the earliest stages of the transformation. When combined with (a fitted) interfacial energy, this growth-based diffusion coefficient enables an accurate analysis of crystal nucleation data below T_g within the framework of Classical Nucleation Theory, provided that a structural order parameter is introduced to account for structural relaxation. This refined approach (using diffusion coefficients derived from the early growth velocities) successfully describes the crystal nucleation dynamics in a BaSi₂O₃ glass, addressing limitations of previous studies that inferred the diffusion coefficient based on advanced growth stages.