| Our project team found a new method for generating urea-ammonium chloride crystals—the solid phase microchannel method.Under the same conditions,we tried to synthesize urea-ammonium bromide and urea-ammonium fluoride crystals and found that these two urea co-crystals were not easily generated.Then we tried to synthesize urea ammonium halide eutectic in laboratory using slow evaporation method.It was found that urea-ammonium chloride was easy to generate,and it was still difficult to synthesize urea-ammonium bromide and urea-ammonium fluoride.In this paper,the possible reasons for the difficulty in the synthesis of urea ammonium halide co-crystal is discussed by theoretically combining experimental methods,and provides a theoretical reference for the experimental work of preparing urea ammonium halide co-crystal.Firstly,using the density functional theory method,based on the unit cell structure of urea-ammonium chloride,the Cl-in the unit cell structure was replaced by F-and Br-,respectively,and the stability of these three urea eutectic structures was compared and discussed.At the same time,the effect of water as a solvent on the halide ion?X-=F-,Cl-,Br-?was also calculated,and the possible reason why urea-ammonium bromide and urea-ammonium fluoride crystals cannot be easily synthesized was investigated.The discovery of a large Br-radius will enlarge the stable internal cavity structure of the urea unit cell,and the bromide ion hydration cluster will also be stable,which will hinder the entry of Br-into the urea channel.For urea-ammonium fluoride,it was found that the intramolecular hydrogen bond formed in the urea unit cell due to the F-strong negative electrode destroys the stable structure of urea itself.The stable structure of the F-?H2O?5 hydrate clusters will also seriously hinder F-successfully entry into the urea crystal cell.Secondly,through the previous chapter,we found that the water solvent effect is one of the possible reasons that affect the urea ammonium eutectic synthesis.Based on this,we used high-temperature melting method to fully mix urea and ammonium halide,and quickly solidified the cooled product of urea and ammonium halide.It was found that halogen ions?X-=F-,Cl-,Br-?in the ammonium halide form hydrogen bonds with the-NH-of urea to form a clathrate.We also discussed the main factors affecting the urea-ammonium bromide,urea-ammonium fluoride eutectic synthesis,and provided feasible ideas for synthetic experiments.Finally,based on the monoclinic urea-ammonium bromide unit cell found in the CCDC database,the Br-in the unit cell structure was replaced with F-and Cl-.The stability of the three isomorphous unit cell structures was compared and discussed.At the level of non-isomorphous cell structure,we investigated the stability and orthotropicity of orthorhombic and monoclinic urea-ammonium fluoride,urea-ammonium chloride,and urea-ammonium bromide respectively.The monoclinic urea-ammonium bromide is easily synthesized,which is related to the size of the urea unit cell in different crystal forms.The orthorhombic urea-ammonium chloride eutectic is the easiest to synthesize,but for urea-ammonium fluoride crystals,these two crystal forms are unstable.This has a great relationship with the electronegativity of F-. |
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