Grain-boundary diffusion in pure and Ca-doped samples was investigated at temperatures ranging from 430 to 560C. The Arrhenius plot (figure 26) of the grain-boundary diffusivity parameter exhibited a marked dip at about 510C. It was suggested that this dip was due to a phase transformation which occurred at the grain boundary; possibly Ca segregation.

Anion Grain−Boundary Diffusion in Sodium Chloride. K.S.Sbharwal, J.Mimkes, M.Wuttig: Journal of Applied Physics, 1975, 46[4], 1839-40