| Tungstate materials have attracted much attention due to their diversity in morphologies and properties.The tungstate crystals have been widely used as inorganic scintillators,fluorescent substances,Raman active laser crystals,component materials in lithium-ion batteries and fuel cells,photocatalysts and solvents for growing functional crystals for a long time because of their excellent optical properties together with chemical and physical stability.However,most tungstate functional crystals are synthesized from the homogeneous melts.The preparation of functional crystals has encountered great difficulties and the development of crystal growth technology has been hindered for lacking of in-depth knowledge of the evolution of the structure units existed in melts,structure in solid liquid boundary layer.The structural diversity has clearly led to distinct Raman spectra of the crystalline tungstate compounds.DFT(Density Functional Theory)calculation of their Raman active vibration wavenumbers together with scattering activities were performed firstly.The crystallographic data of these crystalline tungstate compounds were then connected with their vibrational characteristics based on DFT calculation,which provides for analyzing the structure evolution from the perspective of in-situ HTRS(High Temperature Raman Spectroscopic)technique.The understanding and improvement of crystal growth mechanism is not based on the in-situ observation,but mainly on the post-growth analysis of the structure and performance of the crystal for a long time.In-situ HTRS technique together with DFT calculation was proposed to investigate the micro-structure evolution of tungstates in this work.In-situ Raman spectra with good signal-noise ratio of the tungstate materials were obtained by selecting excited lasers with different wavelengths,detectors and improving the crucible and heating furnace firstly.DFT and factor group analysis were then adopted to assign the major vibrational modes of tungstates in different states.The structural evolution and vibrational characteristics of A20(A = Li,Na,K)-WO3 and MgO-WO3 binary tungstate crystals and melts have been studied systematically by in-situ HTRS technique together with DFT calculation in the present work.The main conclusions are as follows:A series of new mechanisms were proposed based on experimentally measured Raman spectra and DFT calculation,by which the possible pathways for the micro-structure evolution of tungstates while being melted were illustrated successfully.The molten tungstates equilibrate to a mixture of W-O anion groups,in which certain configuration will be dominant according to the melt compositions.Different contents of WO3 result in the formation of chains of various lengths composed of[WO4]2-tetrahedra by sharing their comer oxygen atoms.When the mole ratio of A2O with WO3 in melts is 1:1,1:2,1:3 and 1:4,the corresponding anion group dominant in melts is[WO4]2-,[W2O7]2-,[W3O10]2-and[W4O13]2,respectively.The[W2O7]2-,[W3O10]2-and[W4O13]2-are made up of two,three and four[WO4]2-tetrahedra by cormer sharing,respectively.When the composition of the melt is determined,different A cations have delicate but unnegligible effect on spectral characteristics,such as the band position,relative intensity and FWHH(Full Width at Half Height)of molten tungstates.This reflects the charge transfer process,bond length and its distribution in W-0 group and has been confirmed by Mulliken overlap population analysis.The distortion of W-O group is obvious in the crystals due to the diverse connection types between W-0 groups along with the Davydov Splitting and Crystal Field effects.This leads to the independence of the symmetrical stretching vibration wavenumber of W-Onb(non bridge oxygen)on the coordination number of W6+.While the symmetrical stretching vibration wavenumber of W-Onb increases linearly with the decreasing W-Onb bond length in the crystalline tungstates.This linear relation is found suitable in the melt.In addition,the wavenumber of W-Onb symmetrical stretching vibration mode generally follows:[WO4]2->[WO5]4->[WO6]6-for melts.When the W-O group is determined,this wavenumber will increase with the increasing the number of bridging oxygen.The correlation among the micro-structure configuration of the W-O group,W-Onb bond length and the symmetrical stretching vibration wavenumand is helpful for the diagnosis and identification of the anion groups in the ber has been verified by experimental data crystalline and molten alkali tungstate compounds.RSCS(Raman Scattering Ccross Sections)is the key to quantitative analysis by Raman spectroscopy.The RSCS of the main micro-structures in the K2O-WO3 melts were determined by DFT calculation baed on cluster models constructed in the work.The content changes of the main micro-structure units in the melts were then obtained by quantitative analysis.It is believed that it is feasible to quantitatively analyse the contents of species in melts by Raman spectroscopic technique and theoretical calculation.In summary,the micro-structure evolution of tungstates during melting process can be tracked effectively by in-situ HTRS technique and DFT calculation,and the vibrational characteristics of tungstates in different states at different temperatures were then obtained.This work can enhance the understanding for the micro-mechanism of the functional crystal growth from melts,and thus provide for optimizing the conditions of crystal growth from melt and open new opportunities for rational design of tungstates functional crystals. |
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