Molecular Dynamics Simulation And First Principle Study On Anion Conducting Materials Based On Tetrahedra

Solid ionic conducting materials have a wide range of applications in energy storage and conversion,clean production and other fields,such as fuel cells,solar cells,and sensors.It can be divided into cationic?Na⁺,Li⁺,Ag⁺,etc.?conductor and anion(O^2-,F^-,etc.)conductor.In this paper,we focus on anion conducting materials mainly about oxide-ion conducting materials based on tetrahedra which often have high ionic conductivity due to the better flexibility of tetrahedral structure.However,it is difficult to explain the stable existence and migration of intersitial or vacancy oxygen defects in the material experimentally.So we use static lattice and molecular dynamics simulation based on classical mechanics and first principles density functional theory?DFT?calculation based on quantum mechanics to do theoretical simulation for the related materials.It provides a reliable theoretical support for corroborating the experimental results and developing new ion conducting materials.The specific research content is as follows:1.Detailed molecular dynamics simulations are done for the scheelite based CeNbO_4+x?x=0,0.08,0.25 and 0.33?oxide-ion conducting materials.The results showed the migration of oxide-ions occurred only in the components containing excess intersitial oxygen and the similar migration machanisms were observed for CeNbO_4.08.08 and CeNbO_4.25.For the component CeNbO4.08,we summarized two different migration pathway for oxide-ions and firstly indicated the breaking and reforming of Nb2O9 dimer when ignore the longer Nb-O bonds.However,for CeNbO_4.33,because intersitial oxide-ions have formed a ordered structure to hinder conduction of oxide-ions,the oxygen-ions migration was only reflected in the very high temperature simulation and mainly along the Nb6O26 chain structure.2.Static lattice simulation and molecular dynamics simulation were carried out for monazite based La_1-xCa_xPO_4-x/2 materials.The results of static lattice simulation showed that the defect formation energy was the maximum when O1 was removed in the PO4 tetrahedron,that is,the O1 position was a relatively stable position.Similar formation energy of O2,O3 and O4 in the PO4 tetrahedron indicates that they are all likely to form oxygen vacancies.The molecular dynamics simulation showed that oxide-ion migration is mainly realized through the breaking and reforming of P₂O₇ dimer assisted by the rotation and deformation of PO₄tetrahedron at high temperature.3.La_1-xNa_xPO_4-x materials were prepared by conventional solid-state reaction method.A narrow solid solution?0?x?0.1?was formed by combining X-ray power diffraction,scanning electron microscopy,structure refinement and DFT calculation.Thermogravimetric and infrared spectroscopy tests demonstrated that the oxygen vacancy was generated in the doped structure and that it could react under humid conditions to lead to the formation of protons.Ac and dc impedance tests indicate that the material does indeed conduct oxide-ions in a dry atmosphere and the oxide-ions conduction contributes about 10%of the total conductivity.Finally,the molecular dynamics simulation of the parent and the doped structure was carried out with the appropriate potential parameters,and it was found that the oxide-ions conduction was derived from the continuous breaking and reforming of P₂O₇ dimer at high temperature.4.The first-principles DFT calculation and molecular dynamics simulation were carried out respectively for the La_1+xSr_1-xGa₃O_7+0.5?x-??F?mixed anion conducting materials.The DFT energy calculation showed that the center site of the five-membered ring is the most favorable for the F^-ion.The electron structure calculation showed that the reason why O^2-ion and F^-ions can be stabilized is mainly due to the cooperative relaxation of the local structure of the five-membered ring and the formation of their covalent bond with a Ga2 atom which has a terminal oxygen in the five-membered ring.The molecular dynamics simulation showed that the migration of oxygen ions comes from the collision exchange between the intersitial and the skeleton oxide-ions and is restricted to the Ga₃O₇ layer.The migration of intersitial F^-ions is more intense and contains interlayer migration.