The Study Of Oxygen Vacancies Regulation And Topotactic Structure Phase Transitions Of LaCoO_x Films

As one of the common defects in complex transition-metal oxides,oxygen vacancies(V_O)not only change the volume of unit cell,bond angle and length,but also change the valence state of the transition-metal ions.Oxygen vacancies can not only change the filling of the d orbital by electron doping,but also affect the degenerate of d orbital by changing the crystal field splitting,and then change the spin state and Fermi energy level of the transition metal ions,so as to induce many novel physical properties.Some oxides composed of variable valence transition metal ions not only exist in the perovskite(ABO₃)phase,but also(A_nB_nO_3n-1)phases with the low valance states.Due to the changes of structure and valance states,the physical properties of ABO₃ and A_nB_nO_3n-1 phases are significantly different,such as magnetic,electrical,optical and other properties.Moreover,since A_nB_nO_3n-1 phase can be regarded as perovskite ABO₃structure with long ordered distribution of Vo,precise control of V_O concentration and lattice occupancy in the oxide can induce topotactic phase transition of this crystal structures,thus realizing effective regulation of various physical properties in single crystal materials.In cobaltite,there are many valence states of Co ions and different spin states caused by different electron arrangement,which affect their magnetic and electrical properties.Therefore,various macroscopic properties can be regulated by changing the electron filling and arrangement of Co ions.In this dissertation,we mainly used pulse laser deposition and specially designed film surface heating technology,combined with the characterization of valence state and structure,and transport properties test,to study the topotactic phase transitions of LaCoO₃ films grown on Sr Ti O₃(STO)substrates,including the perovskite phase LaCoO₃,oxygen vacancy ordered phases LaCoO_2.67,LaCoO_2.5 and Ruddlesden-Popper(RP)phase La₂Co O₄,and the change of physical properties.Due to the metastability of V_O-ordered LaCoO_x and the injection of oxygen ions from STO substrate to LaCoO_x films during the film growth,all the as-grown LaCoO_xfilms are perovskite phase,even in ultra-high vacuum.Therefore,we designed a surface heating equipment for annealing the heteroepitaxial films in vacuum.It can produce a large temperature gradient from surface to substrate,which can inhibit the injection of oxygen ions from STO substrate to LaCoO_x films,and facilitate surface desorption of oxygen ions from LaCoO_x films to vacuum.By optimizing the annealing temperature and time,we obtained the large-scale monophased V_O-ordered LaCoO_2.67 and LaCoO_2.5films and characterized their macroscopic properties.We also found that RP phase La₂Co O₄ can be obtained by annealing V_O-ordered LaCoO_2.5 films.In addition,we achieved the topotactic phase transitions of perovskite LaCoO₃ films grown on STO substrate with different orientations,including[001],[110]and[111],thus obtained the LaCoO_2.67 and LaCoO_2.5 films with different orientations of V_O-ordered channels.These are helpful for the development of LaCoO_x films for catalytic and electrochemical applications related to the orientation of ionic conductive channels.The perovskite LaCoO₃ films grown on STO substrates exhibits ferromagnetism,but the concentration of V_O([V_O])has a significant inhibiting effect on its ferromagnetism.Hence,it can be regulated by controlling the distribution and concentration of V_O.Although it is difficult to reduce the[V_O]of LaCoO₃ films by direct annealing in oxygen atmosphere or applying ionic liquid gating voltage,we achieved the regulation of V_O in LaCoO₃ films by controlling the reversible phase transitions of perovskite and brownmillerite phases,including quantitative concentration and precise spatial distribution.The results of first-principles calculations,we found that after a cycle of reversible phase tractions,2/3 of V_O in the LaCoO₃ films can be eliminated,and the residual V_O can be confined in specific lattice sites of LaCoO₃ films,which lead to the ferromagnetic enhancement.By constructing the field effect structure of ionic liquid as the top gate,the oxygen ions in the ionic liquid can be injected into LaCoO_2.5 films by applying negative gating voltage,so as to realize the phase transitions from brownmillerite to perovskite driven by gating voltages at room temperature.Due to the lack of direct detection methods,the magnetism of LaCoO_2.67,LaCoO_2.5and La₂Co O₄ films remain mysterious.We combine ferromagnetic La_0.67Sr_0.33Mn O₃(LSMO)films with different cobaltite to heterojunctions to study the magnetism of the different cobaltite.The LaCoO_2.67/LSMO shows the in-plane exchange bias,demonstrating the antiferromagnetic properties of LaCoO_2.67 film.The La₂Co O₄/LSMO shows the out-of-plane exchange bias,indicating the special antiferromagnetic configuration of La₂Co O₄ films.They are all consistent with the corresponding in-plane antiferromagnetic configuration of bulk LaCoO_2.67 and the out-of-plane configuration of bulk La₂Co O₄.Moreover,with the topotactic phase transitions of LaCoO_x,from LaCoO₃/LSMO to LaCoO_2.67/LSMO,LaCoO_2.5/LSMO and La₂Co O₄/LSMO,the in-plane coercive field increases gradually,while the out-of-plane coercive field decrease gradually.It suggests the easy axis of LSMO films rotate from in-plane to out-of-plane under the different antiferromagnetic configurations.This provides us with a new way to regulate the magnetic properties of thin films.In conclusion,we have realized the four different phase transitions between LaCoO₃,LaCoO_2.67,LaCoO_2.5 and La₂Co O₄.The reversible phase transitions of perovskite and brownmillerite have been used to tune V_O in LaCoO₃ films,and regulate their ferromagnetism.By combing with ferromagnetic LSMO films,the antiferromagnetism of LaCoO_2.67 and La₂Co O₄ films have been directly identified experimentally.This method can be extended to the topological phase transitions of other oxides by controlling the migration of oxygen ions,thus realizing the regulation of multifunctionalities of the rich transition metal oxide family.