Mechanical Spectroscopy Studies Of Complex Phase Transition In Perovskite-type Ferrites

In recent years,the perovskite-type materials have been favored by many researchers.As a typical strong correlation system,the perovskite-type transition metal oxides have complex coupling among the charge,spin,orbit and lattice degrees.Many novel physical phenomena have been founded,such as metal insulator transition,high temperature superconductivity,and giant magnetoresistance effect.Among them,rare earth iron oxide,R1-xSrxFeO3,has attracted wide attention due to its wide application in the fields of oxygen separation membranes and the charge disproportionation transition.There are many problems to be unsolved,for example,the existence of the charge disproportionation transition at low doping level in La1-xSrxFeO3(x<0.4),the magnetic phase diagram in La1-xSrxFeO3(0<x<1).It is well known that mechanical spectroscopy is a very effective experimental method for studying phase transitions and relaxations of microstructures.In this thesis,we focus on the microstructural relaxations,the charge ordering transition,the structural and magnetic phase transitions in perovskite-type iron oxides.The main works are as follows:1.A-site ordering effect on the octahedral tilting transition and the relaxations of ferroelastic domain walls in La0.75Sr0.2sFeO3.The octahedral tilting transition(R3c?Pnma)in La0.75Sr0.25FeO3 was detected by mechanical spectroscopy,where a dip of reduced modulus was observed and accompanied by a sharp internal friction peak.The modulus softening is due to the strain/order parameters coupling at this improper ferroelastic transition.The tilting transition shifts toward lower temperature with reducing sintering temperature in La0.7sSr0.25FeO3-? samples,which is due to the detriment of A-site ordering in the ABO3 perovskite.A higher degree of A-site ordering favors more homogenous structural modulation,which stabilizes the low temperature Pnma phase and results in the shift of the transition to higher temperature.Another internal friction peak just below the tilting transition was observed,which is attributed to domain wall motions.A correlation between the mobility of ferroelastic domain walls in Pnma perovskite and the existence of high temperature R3c phase was suggested.2.The structural and magnetic phase diagram of La1-xSrxFeO3 samples.The octahedral tilting transitions were observed(R3c?Pnma)in 0.25 ? x ? 0.38 samples.The transition temperature decreases with the increase of Sr doping,and finally the transition cannot be observed in x>0.38 samples.A usual magnetic transition accompanies with the tilting transition.The magnetic phase transition is argued to be a spin reorientation transition.Another magnetic transition was observed at low doping La1-xSrxFeO3 samples,which follows the charge freezing indicated by an internal friction peak.The temperature of the magnetic transition has a little dependence on Sr doping.In addition,a paramagnetic-antiferromagnetic phase transition was observed above these magnetic phase transitions.The antiferromagnetic transition moves toward lower temperature with increasing Sr doping and finally merges with the charge ordering transition near 200K in x=0.67 samples.At last,the magnetic phase diagram of Lai-xSrxFeO3 samples is proposed.3.The dynamics of the ferroelastic domain walls and oxygen vacancies and A-site ordering effect on the charge ordering transition and tilting transition in La1/3Sr2/3FeO3.It was found that the oxygen vacancies suppress the growth of ferroelastic domains and inhibit the movement of domain walls in the direction normal to the walls.The decrease of A-site ordering leads to the suppression of the charge ordering transition and a smaller modulus change at the tilting transition.It shows that A-site ordering has a profound effect on the physical properties of the perovskite-type iron oxide.4.The charge ordering transition in Ndi/3Sr2/3FeO3.The comparison studies of Ndi1/3Sr2/3FeO3 and Lai1/3Sr2/3FeO3,Pr1/3Sr2/3FeO3 were made on the low temperature magnetic properties,thermal analysis,resistance and mechanical properties.A similar magnetic,thermal and electrical behavior was observed near the charge ordering transition in these samples.However,a great difference appears in the mechanical spectroscopy.It shows a less lattice effect on the stability of the charge ordering in Nd1/3Sr2/3FeO3.