Investigations On The Structures Of Three Melts With Typical Composition In Li₂O-GeO₂ Binary System

Melt structure is an important basic issue in science of crystal growth.It not only determines the macroscopic physical properties of the melt such as viscosity,surface tension,density and diffusion,but also the microscopic process of the crystal growth.The germanate crystals are important functional crystals with various structures and wide applications.Most of the germanate crystals currently investigated are composed of[GeO2(?)2]n((?)= bridge oxygens),Ge2O7 or GeO4 groups.The Li2O-rich side in Li2O-GeO2 binary system contains three representative compounds:Li2GeO3,Li6Ge2O7 and Li4GeO4,which are composed of the above three groups.In this paper,in situ high temperature Raman spectroscopy,density functional theory(DFT)calculations and factor group analysis methods were implemented to investigate the melt structures of Li2GeO3,Li6Ge2O7 and Li4GeO4.1.The melt structure of Li2GeO3:(1)The Li2GeO3 vibrational modes were fully analyzed by the factor group analysis theory.Li2GeO3 crystal has 36 vibrational modes(10A1+8A2+8B1+10B2),except for three acoustic modes,the rest vibrational modes are all Raman active.The DFT calculations confirmed the factor group analysis results and assigned all of the experimental Raman bands.The results show that the Raman bands in the high-frequency region mainly originated from the symmetric stretching vibrations of the O-Ge-O bonds,and the Raman bands in the mid-frequency region mainly originated from the Li+motions and the symmetric bending vibrations of the Ge-O-Ge bonds.(2)There are two characteristic Raman bands in the experimental spectrum:The 495 cm-1 band(mid-frequency region)is attributed to the symmetric bending vibrations of the Ge-O-Ge bonds,and the 814 cm-1 band(high-frequency region)arises from the symmetric stretching vibrations of the O-Ge-O bonds.When the crystal melted,anomalously blue shift occurs in the mid-frequency region and normally red shift occurs in the high-frequency region.(3)Two possible Li2GeO3 melt structure models were established:one consisting of the[GeO2(?)2]n chain and the other consisting of the[Ge3O9]ring.DFT method was employed to calculate the Raman spectra of above two melts,the results show that the Li2GeO3 melt is composed of[GeO202]n chains and Li+ ions.The anomalous blue shift of the 495 cm-1 Raman band in the mid-frequency region is related to the shrinkage of the[GeO2(?)2]n chain.On the basis of the melt structure,the high viscosity of the Li2GeO3 melt and the growth phenomena of the Li2GeO3 crystal were explained.2.The melt structures of Li6Ge2O7 and Li4GeO4:(1)The lattice vibrational modes of Li6Ge2O7 and Li4GeOa crystals were analyzed by factor group analysis theory.The Li6Ge2O7 crystal has 180 vibrational modes(45Ag + 45Au+ 45Bg+45Bu),including three acoustic modes,90 Raman active modes(45Ag+45Bg)and 87 IR active modes(44Au?43Bu).The Li4GeO4 crystal has 54 vibrational modes(8Ag+4Au+8Big+7Biu+5B2g+9B2u+6B3g+7Bi?),including three acoustic modes,27 Raman active modes(8Ag+8+Mg+5B2g+6B3g)and 20 IR active modes(6B1u+8B2u+6B3u).The DFT calculations confirmed the factor group analysis results and assigned all of the experimental Raman bands.The results show that:The Li6Ge2O7 Raman bands in the high-frequency region mainly originate from the symmetric stretching vibrations of O-Ge-O bonds;the Raman bands in the mid-frequency region originate from Li+ motions,bending vibrations of Ge-O-Ge bonds and weak bending vibrations of O-Ge-O bonds.The Li4GeO4 Raman bands in the high-frequency region mainly originate from the symmetric stretching vibrations of the O-Ge-O bonds;the Raman bands in the mid-frequency region originate from the Li+ motions and the weak bending vibrations of the O-Ge-O bonds.(2)According to Raman spectra changes before and after the melting of Li6Ge207 and Li4GeO4,it can be obtained that:Li6Ge2O7 melt is composed of the Ge2O7 groups and Li+ions,Li4GeO4 melt is composed of the GeO4 groups and Li+ions.(3)With the addition of Li2O into Li2GeO3 melt,the[GeO2O2]n chains first changed into Ge2O7 groups,and then further into GeO4 groups.According to the melt structures of Li2GeO3,Li6Ge2O7 and LiaGeO4,the congruent melting behaviors of the three compounds were explained,and the relationship between the crystalline regions of these three compounds and their melt structures was explained.