Raman Spectroscopic And Theoretical Studies Of Metal Fluorides And Oxyfluorides In Fluoride Molten Salts

Molten salt containing transition metal,lanthanide and actinide metal fluorides is the main medium in electrorefining of costly metal,pyroprocessing technology and operation of molten salt reactor.The impurities with oxygen will be inevitably introduced into these processes.They will lead to the formation of oxyfluoride even oxide precipitation which seriously affect the above process.In order to determine and regulate the effect of oxygen on the fluoride molten salts,it is crucial to study the structure of metal fluorides and oxyfluorides,and to understand the mechanism of the conversion of fluoride to oxyfluoride and even to oxides.However,no systematic studies have been conducted in related fields.Herein,we combined with Raman spectroscopy and quantum chemistry calculation for studying the existing forms of the transition metals,lanthanide and actinide fluoride molten salts.The influence of O^2-on the speciation of metal fluorides and the mechanism from oxyfluoride to oxide are investigated by addition of Li₂O in molten salts.The main results are as follows:(1)For transition metal fluoride,there are two different species in alkali fluoride(FLiNaK,KF,LiF,etc)melts,which are in the form of MF_n^2-and MF_n^3-(M=Sc,Y,Zr,Hf,Nb,Ta)anions.Their relative contents depend on the variation of the free F^-content.In molten FLiBe,a different species may be present in the melt according to the bond energies of M-F and Be-F.As to zirconium fluoride,Zr mainly exists in the form of ZrF₆^2-and ZrF₇^3-,and Be mainly exists in the form of dimer Be₂F₇^3-,because the bond energy of Zr-F bond is larger than that of Be-F bond.For tantalum fluoride,due to same bond energies of Ta-F and Be-F,Ta mainly exists in the form of TaF₆^-and TaF₇^2-,while Be mainly exists in the form of BeF₄^2-anion with a small number of Be₂F₇^3-.Li₂O is employed as the O^2-source and added into the fluoride melt,caused the reaction of producing a series of oxyfluoride,which finally leads to unfused oxide precipitates.For the group 3 fluorometallates,the mono-bridged oxygen structure(Sc₂OF₆^2-/Y₂OF₆^2-anion)is generated after the addition of Li₂O in the melt.Further reaction occurs to form a new oxyfluoride structure Sc₂O₂F₆^4-/Y₂O₂F₆^4-with adding more Li₂O,which contains a rhomboid ring of double-bridged oxygen.For the group4 fluorometallates,O^2-is more inclined to react with ZrF₇^3-/YF₇^3-to form a ZrOF₅^3-/HfOF₅^3-structure which processes terminal oxygen.This is due to the higher average formation energy of ZrF₇^3-/YF₇^3-than that of ZrF₆^2-/YF₆^2-.As to the group 5fluorometallates,TaOF₅^2-/NbOF₅^2-and TaOF₆^3-/NbOF₆^3-are formed after the addition of Li₂O.The new oxyfluorides TaO₂F₂^-/NbO₂F₂^-and TaO₂F₃^2-/NbO₂F₃^2-are then formed with further adding of O^2-.(2)For the actinide and lanthanide fluorides,MF_n^2-and MF_n^3-anions exist in molten alkali metal fluorides.However,in molten FLiNaK-CeF₃,the Raman band of CeF₅^2-anion is too weak to be observed due to the low Raman activity.In the FLiBe-ThF₄ melt,ThF₆^2-is the dominant species which is similar to the situation of high concentration of ThF₄ in molten FLiNaK.Be mainly exists in the form of polymer,which is mainly due to the Th-F bond energy greater than that of Be-F bond.With the addition of O^2-ion,various oxyfluorides with different structures are formed in the melt.In the case of FLiNaK/FLiBe-ThF₄ melt,ThF₆^2-and ThF₇^3-anions react with O^2-to form the oxyfluoride Th₂OF₁₀^4-,and the released free F^-combines with Be to form the beryllium fluoride anion.For FLiNaK-UF₄ melt,the UOF₅^3-anion with terminal oxygen is formed by the addition of O^2-.As to the lanthanide oxyfluorides,Lu₂OF₈^4-,Ce₂OF₁₀^6-and Ce₂OF₈^4-are formed respectively,and then combined with more O^2-to form double-bridged oxygen structures.Combined with Raman spectroscopy and quantum chemistry calculations,the structure and reaction mechanism of metal fluoride and oxyfluoride in molten salts are obtained.The relationship between the coordination structure of key elements and reaction mechanism is established,which provids a scientific basis for the development of related technology.