Structural Studies Of Supercooled Water、Alumina And Iron Oxides Liquids By High Energy X-Ray、Neutron Scattering And Simulations

Structure determines properties.Microtructure is the basis for understanding the physical properties.Therefore,understanding the structure of materials from atomic scale is conducive to the development of high-performance material systems.The structure of liquid is very important for studying the nucleation,crystallization and glass transition process.In addition,high-temperature melts have also received much attention in geology,metallurgy and other disciplines.In this paper,three representative oxide systems were studied using synchronous high-energy X-ray combined with aerodynamic suspension and laser heating device,and the temperature and gas dependent structure of each system gases was analyzed in detail using empirical models and molecular dynamics simulation.The structure of supercooled water has been controversial.On the basis of the latest X-ray scattering data,Empirical Potential Structure Refinement was employed to simulate the structure of deeply supercooled water,providing information which agrees well with existing scattering data.From EPSR,the average O-O coordination number drops from 5.13 at 293 K to 4.85 at 244 K,within 3.5?.Triplet O-O-O bond angle distributions revealled a broad peak centered at 96.4° at 293 K which shifts to 100.0° at 244 K,indicative of the local geometry becoming increasingly tetrahedral with decreasing temperature.The number of non-bonded interstitial molecules between the first and second shells depletes upon cooling,while the number of interstitial molecules which are embedded within the hydrogen bonded tetrahedral network atθ_OOO=53°,remains constant.In addition,both-O-O-O-and hydrogen bonded-O-H-O-ring distribution was analyzed,indicating an increase of 6and 7-membered rings upon supercooling.This is concomitant with a shift and increase in intensity of peaks at r₄～8.7?and r₅～10.8?in the oxygen-oxygen pair distribution function,which in the models correspond to correlations between adjacent and next-nearest-neighbor hydrogen-bonded rings.Molten alumina is widely used in the fields of crystal growth,metallurgy,optical materials.However,the structure of liquid alumina is not fully studied,and the reason why alumina cannot form a glass is still unclear.High energy synchrotron X-ray measurements and stroboscopic neutron diffraction were performed over a temperature range of 1817≤T（K）≤2700 and 1984 K≤T（K）≤2587 K.The diffraction patterns have been fitted with EPSR models and compared with that of classical molecular dynamics（MD）simulation.Similar trends were obtained from both simulation methods,mean Al-O coordination number turns out to be around 4.4and increases under supercooling.Both EPSR and MD simulations reveal a direction of the temperature dependence of the aluminate network structure which moves further away from the glass forming ideal(n _{Al O}=3)during supercooling.The existence of a large amount of high coordination Al,three fold oxygen and edge sharing connection limits the flexibility of network and hence hinders glass forming.By comparing the structure of liquid and amorphous alumina,we declare that amorphous form likely has a larger Al-O coordination number than the liquids,consistent with the expectation for hypothetical glass.Molten iron oxides play an irreplaceable role in the fields of iron and steel metallurgy,geology,etc.,but the complex redox reaction increases the difficulty of structural research.Here we employed X-ray diffraction,EPSR and molecular dynamics simulations to study the structure of melts with compositions from 5%Fe³⁺to 92%Fe³⁺and temperatures from 1973 K to 1573 K.The EPSR model matches well with the experimental data,and shows that the average Fe-O coordination number in the molten iron oxide is between 4 and 5,and increases with the Fe³⁺content;At composition close to Fe O,Fe²⁺mainly exists in the form of tetrahedral,while at composition close to Fe₂O₃,five-fold Fe³⁺plays a dominant role.Both six-fold coordinated Fe²⁺and Fe³⁺sites exsit in the melts,the number of which increase with Fe³⁺content.Mean O-Fe coordination number ranges from 3.5 to 4.5,with three coordination relationships:three,four and five-fold,in which four-fold oxygen is the predominant form.The number of four-fold oxygen is nearly independent with Fe³⁺content.From reducing to oxidizing condition,the percentage of three-fold oxygen increases at the expense of five-fold.Expect for EPSR,a series of molecular dynamics simulations were conducted,but the results do not match the experimental data,indicating the classical reference potentials have certain shortcomings.More complicated simulation methods are needed to analyze Fe-O coordination environment and charge transfer under various redox conditions.In conclusion,the microstructure of three typical oxide liquids was studied by using a combination of experiment and theoretical calculations,various structural information such as local coordination environment,bond angle distribution,ring distribution at different temperatures and topology network were analyzed.This work is expected to help people further understand the structure and physical properties of liquids,crystallization process and the nature of glass formation.