Resumo:
The diffusion mechanisms controlling viscous flow, structural relaxation, liquid-liquid phase separation,
crystal nucleation, and crystal growth in multicomponent glass-forming liquids are of great
interest and relevance in physics, chemistry, materials, and glass science. However, the diffusing entities
that control each of these important dynamic processes are still unknown. The main objective of
thiswork is to shed some light on this mystery, advancing the knowledge on this phenomenon. For that
matter, we measured the crystal growth rates, the viscosity, and lead diffusivities in PbSiO3 liquid and
glass in a wide temperature range. We compared our measured values with published data covering
16 orders of magnitude.We suggest that above a certain temperature range Td (1.2Tg–1.3Tg), crystal
growth and viscous flow are controlled by the diffusion of silicon and lead. Below this temperature,
crystal growth and viscous flow are more sluggish than the diffusion of silicon and lead. Therefore, Td
marks the temperature where decoupling between the (measured) cationic diffusivity and the effective
diffusivities calculated from viscosity and crystal growth rates occurs.We reasonably propose that the
nature or size of the diffusional entities controlling viscous flow and crystal growth below Td is quite
different; the slowest is the one controlling viscous flow, but both processes require cooperative movements
of some larger structural units rather than jumps of only one or a few isolated atoms
