In situ observation of the crystallization via incongruent melting and peritectic reaction in Sr(NO 3)–H 2O system

Abstract

The crystallization via incongruent melting and peritectic reaction was investigated in the Sr(NO 3) 2–H 2O system. The dissolution processes of the secondary β phase (Sr(NO 3) 2·4H 2O) and the primary α phase (Sr(NO 3) 2) in water above and below the peritectic temperature T p=29.3°C were observed in situ, respectively, by optical microscopy. Around the β phase dissolving above 61°C, α phase formed. With the depletion of the β phase, the α phase stopped growing and dissolved. On the other hand, β phase formed on the α phase dissolving below −13°C. After a while, ice crystals (γ phase) formed and enclosed the β phase. The result shows that the incongruent melting and the peritectic reaction are the sequential processes of the dissolution of the metastable solid phase and the formation of the stable solid phase in the liquid. A possible driving force for the crystallization is originated from the difference in the solubility of the two solid phases at a given temperature. The formation of γ phase on the α phase dissolving below 0°C was also observed, instead of the peritectic reaction. In this process, the α phase acted as the site for both melting and regrowth of the γ phase. This resulted in a zonal structure of the γ phase around the α phase.