Structural Studies Of Molybdate Crystals And Their Melts By In-situhigh Temperature Raman Spectroscopy And First Principles Simulation

By observing the changes of microstructure during the crystal melting process and the crystallization process in real-time,to understand the microscopic mechanism of the crystallization process and crystal growth is the basis for obtaining or discovering the crystal growth mechanism,and has great scientific significance for the optimization and design of new functional materials.Molybdate material has a series of unique physichemical properties,and has a wide range of applications in science and technology fields.However,the study of the structure of high temperature melt is still in the initial qualitative stage of the alkali metal molybdate series.The structure units and the existing form of the corresponded clusters structure,as well as the quantitative characterization of different structure units in the molten state have yet to be further studied.Therefore,in-situ high temperature Raman spectroscopy and first principles simulation in this paper were combined together to carry out the related basic and applied researches on alkali metal molybdate functional crystals and their melts,in order to explore the melt structure units and their existing forms,and try to convert the quantitative characterization of structure units into a description of the macroscopic properties of the melt.Further deepen the investigation on the transition and rare earth molybdates.The main contents of this thesis are as follows:We investigated the structural state of molybdenum ions and their correlation with the crystals structure,as well as the existing forms of the clusters structure in the melt of the alkali metal molybdate K₂Mo_nO_3n+1?n=1,2,3?compounds.The Mo⁶⁺presented almost four-coordinated morphology with high abundance.As the basicity decreases,the melt structure units changes from monomer to dimer and trimer.The variation and the fracture of the chemical bonds during the melting process are discussed in detail.The mechanisms of the microstructure evolution of K₂Mo_nO_3n+1?n=1,2,3?from crystalline to the molten state are revealed.The qualitative and quantitative studies of the melt structure in molten binary alkali metal polymolybdate?1-x?K₂MoO₄-xMoO₃?x=0,19,39,50,67,75,83,87.5mol%?were carried out.All possible species of Q_i^jklm and H_i^jklmno in the melt were proposed and studied.The presence of[MoO₆]^6-anionic groups were proposed and confirmed to be present in the K₂MoO₄-MoO₃ melts.The melt structure exhibit the characteristic of coexistence of a variety of structure units.The Raman scattering cross sections of different species of 2H₁¹,Q₀,Q₁¹,Q₁²,Q₂¹¹ and Q₂¹² which existed in the melt were determined by quantum chemistry ab initio calculations.Raman quantitative characterization of these species was studied.Moreover,the quantitative results were correlated with the published results of the high temperature viscosity and density of binary molybdate melts,thereby verifying the reliability of the analysis and quantification of the melt structure units.The ternary molybdate with coexisting of alkali metal and simple trivalent cations of KAl?MoO₄?₂ crystal,glass,and melt have been investigated by in-situ high temperature Raman spectroscopy and ²⁷Al MAS NMR.The crystal transformation between crystalline and quenched KAl?MoO₄?₂,as well as the coordinated species of Al³⁺were analysed.The effect of K⁺,from ordered arrangement in the crystal to the homogenously random distribution in the melt,on the local chemical environment of Al³⁺was also revealed.The distribution and quantitative analysis of different Al³⁺coordination subspecies of KAl?MoO₄?₂ glass samples with different thermal histories were discussed in-depth.The results demonstrate that Al³⁺is present predominantly in[AlO₆]^9-octahedra in both KAl?MoO₄?₂ glass and melt,with the tetrahedrally coordinated Al³⁺being minor approximately at 2.7%under the fast quenched conditions in this work.This study opens the door and provides further insights into the role of trivalent cations in the crystalline and molten state of ternary molybdate MRE?MoO₄?₂?M=alkali metal,RE=Y,Bi,Ln?family coexisting with alkali metal and transition?or rare earth?cations.The room temperature and molten Raman spectra of the ternary dimolybdate MRE?MoO₄?₂?M=Li,K;RE=Y,Gd,Bi,Eu?coexisting with alkali metal and transition?or rare earth?cations were investigated.The changes and characteristics of the spectra were observed and summarized.And further exploring the cation ion size,charge,electronegativity,mass and other factors might influence the spectra,as well as carrying on the possible mechanism analysis.Aspects of the above investigations can not only improve the understanding of the mechanisms of microstructure evolution during the melt of molybdate crystals,and provide an experimental and theoretical basis for profound understanding the nature of the melt behavior,but also supply more experimental data for the prediction of the structure and properties of related molybdates under extreme conditions.Furthermore,it is of crucial theoretical and practical significance for the synthesis,optimization,design and development of molybdate functional materials.And further provide reference and theoretical guidance for the improvement of high quality crystal growth process and technology.