Some unusual crystallization phenomena in frozen aqueous solutions

When a 30 weight percent ferrous chloride solution is quick cooled to 78°K, it forms a transparent glass. When this is slowly warmed, both differential thermal analysis and Mössbauer measurements distinguish four temperatures, which correspond to (1) melting from glass to undercooled liquid, (2) crystallization of salt-free ice, (3) crystallization of FeCl₂·9H₂O, and (4) melting of both crystals. Our recent differential-thermal-analysis measurements show that the third temperature increases by 22°K as the fraction of D₂O in the water increases from zero to 100 percent, while the other three remain nearly constant. The cause of the large isotope effect is not known.