In-situ observation and analysis of solid-state diffusion and liquid migration in a crystal growth system: A segregation-driven diffusion couple

Energy-resolved neutron imaging is employed for in-situ measurements of dopant transport in a simple experiment performed before the crystal growth of the scintillator BaBrCl:5%Eu via a vertical gradient freeze technique. During a stabilization period preceding growth, we observed the diffusion of Eu from the solid phase into the melt over a period of approximately 4 h. Comparing the measured centerline concentration profile with a mathematical model for the system, we estimate the solid-state diffusivity of Eu in BaBrCl as D₁=1.9×10⁻¹⁰ m²/s and an upper limit for the liquid-phase diffusivity of Eu in the melt as D₂ ^*=2.5×10⁻¹⁰ m²/s, at temperatures near the melting point. We compare this experiment, where diffusion is driven by a concentration discontinuity arising from segregation, to the classical diffusion couple technique. Suggestions are offered on how this segregation-driven couple might be improved as a tool for measuring diffusion coefficients, and we draw attention to the great promise of neutron imaging for in-situ measurements of the distribution of elements, with sufficiently high neutron attenuation coefficients, in difficult environments.