Research On The Crystal Structure And Ionic Conductivity Of Some Niobates

The solid electrolyte material is also known as a super ionic conductor,which can have a conductivity comparable to that of a liquid electrolyte when it is in a solid state.Compared with traditional liquid electrolytes,solid electrolytes have the advantages of high safety,excellent mechanical properties,and high thermal stability.In view of the current types of solid electrolytes,we have selected oxygen ion conductors and sodium ion conductors as our research objects.Combining several niobate compounds,through exploring the relationship between its structure and performance,it is hoped that it can provide certain ideas for the study of solid electrolytes.The main work of this paper includes the following aspects:1.The Ln_1-xTh_xNb O_4+x/2（x=0,0.1,0.2,0.3,Ln=Nd,Gd,Lu）series of rare earth niobate doped Th4+interstitial oxygen defect oxygen ions were synthesized by high temperature solid phase method conductor.PXRD proves that it is a monoclinic limonite structure（space group I2/c）at room temperature.Through the AC impedance test,we found that the Ln Nb O₄（Ln=Nd,Gd）matrix itself has a certain ionic conductivity,and the source of carriers may be the interstitial oxygen defect caused by the oxidation of Ln³⁺.The conductivity of Nd Nb O₄ doped with Th⁴⁺ions decreases as the doping concentration increases.This is because the increased interstitial oxygen concentration also increases the degree of defect association,which reduces the conductivity.The AC impedance test results of Gd_1-xTh_xNb O_4+x/2（x=0,0.1,0.2,0.3）show that its oxygen ion conductivity is basically not affected by the defect concentration.Combined with tests under air and argon atmospheres,we have proved that it has certain electronic conductive components.2.Na Nb_1-xGa_xO_3-x（x=0,0.1,0.2,0.3）was synthesized by high temperature solid phase method,all of which are quadruple orthogonal twisted perovskite structure,and the space group is Pbcm.The AC impedance test results show that the oxygen ion conduction characteristic appears after the doping.As the doping concentration increases,the conductivity increases.We replaced Nb⁵⁺with a part of Ta⁵⁺,and obtained the solid solution Na Nb_0.7-xTa_xGa_0.3O_0.7（x=0.1,0.2,0.3,0.4,0.5,0.6,）in the lower range of Ta⁵⁺doping concentration（x=0.1,0.2）,its crystal structure is closer to the structure type of Na Nb O₃（space group Pbcm）,and in the range of higher Ta⁵⁺doping concentration（x=0.5,0.6）,its crystal structure is closer to that of Na Ta O₃ Double twisted orthogonal perovskite structure type（space group Pnma）.Since the effective electronegativity of Nb⁵⁺is higher than that of Ta⁵⁺,irregular Nb O₆ and regular Ta O₆octahedral structures are formed.The different structures of the two octahedrons result in the layered and ordered distribution of Nb⁵⁺and Ta⁵⁺cations.This orderly distribution arrangement leads to an indirect orderly distribution of oxygen vacancies,which in turn reduces the conductivity.3.The La³⁺doped perovskite structure Na_xLa_（1-x）/3Nb O₃（x=0,0.1,0.2,0.3,0.4,0.5,0.6,0.7）was successfully synthesized by the high temperature solid phase method.When x≥0.4,La³⁺and Na⁺randomly occupy the positions of A1 and A2 in the lattice,showing a disordered distribution.When x≤0.3,the A1 layer is completely empty,La³⁺and Na⁺randomly and uniformly occupy the A2 layer,and the A-site ions present an orderly distribution.As the content of Na⁺decreases,the concentration of A-site vacancies in the structure increases,and the ion conductivity will increase significantly.In the low vacancy concentration range（x≥0.4）,the increase in vacancy concentration has a greater contribution to the conductivity.For samples with high concentration of vacancies,the structural evolution contributes more to the increase in conductivity.The orderly distribution of A-site ions forms a completely empty channel for ion migration,which greatly improves the conductivity.