Raman Spectroscopy And Quantum Chemical Calculation Of MF-ZrF₄（M=Li,Na,K）

Molten salt reactor(MSR)is one of the candidate reactor types for the fourth generation nuclear reactor.It used fluoride molten salt as a nuclear fuel carrier and coolant.The excellent thermal physical and chemical performance of molten fluoride brings MSR unparalleled advantage in many aspects.ZrF4 is the molten salt commonly used in MSR.However,because ZrF4 has a high melting point,it is not suitable to be used directly as a coolant.One or more alkali metal fluoride salts are usually added to ZrF4 to make a mixed molten salt that as a coolant.The law of its physical and chemical properties change is determined by the molten salt ionic structures.Therefore,the structure of molten salt containing ZrF4 mixed with alkali metal fluoride has become an important factor for the smooth operation of molten salt in the molten salt reactor circuit.Based on the above analysis,the density functional theory B3LYP method coulped with Lanl2dz basis set was used in the quantum chemical simulation calculation software Gaussian 09W to study the ionic structure of MnZrF(4+n)(M=Li,Na,K,n=4-8).These simulation calculations were performed in Gaussian 09W software.The ionic structures of MnZrF(4+n)group clusters were optimized and the Raman spectra of the clusters were caculated.First,the phase analysis was performed on the prepared MF-ZrF4(M=Li,Na,K)and FLiNaK-ZrF4 samples by X-ray diffraction technology,and Raman spectra of MF-ZrF4(M=Li,Na,K)and FLiNaK-ZrF4 systems were studied based on the Northeast University high temperature Raman spectrum test platform.The ion groups in the MF-ZrF4(M=Li,Na,K)system was discussed.The law of mutual transformation between ionic structures and the change of the structural properties of ionic groups in molten salt with temperature and composition of molten salt was discussed.The results show that according to the Raman spectra of the MnZrF(4+n)(M=Li,Na,K,n=4-8)clusters,the characteristic peak frequency of the clusters gradually decreases as the Zr4+ coordination number increases in all alkali metal fluorozirconates.The characteristic peak positions influenced by three different cations(Li+,Na+,K+)are not significantly different.The larger the cation radius,the lower the characteristic peak frequency.With the increase of Zr coordination number,the difference between the Raman characteristic peak frequencies of anions corresponding to different cations decreases.There are four types of ZrF84-,ZrF73-,ZrF62-and ZrF5-complexes in the MF-ZrF4(M=Li,Na,K)and FLiNaK-ZrF4 systems,the complexes of Zr2F113-,Zr2F135-and Zr3F208-appear in the systems at high temperature.As the temperature increases,ZrF73with a chain-like "fluor bridge" is formed,the relative ammount of ZrF73-decrease,and the further increase in temperature causes the "fluor bridge" to break,forming ZrF84-,ZrF62-and ZrF5-complex ion groups,so that the relative content of ZrF84-,ZrF62-and ZrF5-complex ion groups increased.In the MF-ZrF4(M=Li,Na,K)system,when the ZrF4 content is constant,the structures of four complex ion groups ZrF84-,ZrF73-,ZrF62-,and ZrF5-become loose as the temperature increases.As the content of ZrF4 increases,the four complex ion groups ZrF84-,ZrF73-,ZrF62-,and ZrF5-become unstable at the same temperature.The Raman shifts of the characteristic peaks decrease in order of Li+,Na+,and K+.The research results of this paper have guiding significance for the study on the physicochemical properties of MF-ZrF4(M=Li,Na,K)system,and provide a reference for the subsequent design of molten salt reactors.