Mechanical Spectroscopy Studies Of Perovskite-type Ferrites With Mixed Valence

In the past few decades,the perovskite-type 3d transition-metal oxides as strong correlated electron systems have been extensively studied.The physics of these oxides is governed by the intricate interplay and competition among the charge,spin,orbit and lattice degrees of freedom which may result in occurrence of an interesting variety of physical phenomena such as high temperature superconductivity,metal-insulator transition,orbital ordering,phase separation,etc.These physical phenomenon is not only provides a good platform for the study of condensed matter theory,but also has great potential prospect.Recently,the mixed valence perovskite-type ferrites have received much attention due to their peculiar charge disproportionation phenomena.Particularly in La1/3Sr2/3FeO3 the charge ordering transition occurs accompanied by charge disproportionation.In these mixed valence ferrites,some physical phenomena related to lattice distortion,magnetic and electric properties become complicated by the influence of the irons with different valence state.Thus,there is still some issues with dispute up to now.It is well known that mechanical spectroscopy is a very effective probe for the study of phase transitions and relaxations of relaxing entities in solids.Mechanical spectroscopy possesses the unique advantage that other experimental methods do not possess because of its high sensitivity.In this thesis,lattice distortion and charge disproportionation in three representative perovskite-type ferrites series has been systematically studied by mechanical spectroscopy.The whole thesis is organized as follows:In the chapter 1,a brief introduction to the perovskite structure,related octahedral distortion and mixed valence oxides is made.And we review research progress of some mixed valence perovskite ferrites.In the chapter 2,we introduce the basic principles of internal friction and mechanical spectroscopy and the theoretical basis of two different measurement methods.And internal friction measurement system,its functions and works are elucidated in detail.In the chapter 3,we first investigate the effect of substitution on charge ordering in(La1-xPrx)1/3Sr2/3FeO3-?.By substitution of smaller ion Pr for La,it is shown that the transition temperature shifts towards lower temperature at the lower Pr doping level And the charge ordering transition disappears in Pr1/3Sr2/3FeO3-? sample.The results of electric transport and magnetic properties show that metal-insulator transition and magnetic phase transition occurs accompanied with the charge ordering transition.In the mechanical loss spectrum of La1/3Sr2/3FeO3-?,a broad internal friction peak above charge transition temperature(Tco)was observed,which results from the slow hopping of Jahn-Teller polarons.At the same time,the reduced Young's modulus shows a continuous softening with the decrease in temperature toward Tco.This can be ascribed to the Jahn-Teller distortion of FeO6 octahedral.Around Tco,a step increase of the modulus and accompanied step decrease of internal friction was observed.This behavior in the mechanical spectrum indicates that the charge disproportionation transition take place and the Jahn-Teller distortion related to Fe4+ is interrupted/freezed.Below Tco,due to the breathing-type distortion of the FeO6 octahedral the modulus softening take place again.However,no step like change of mechanical spectrum was observed in charge ordered Pr1/3Sr2/3FeO3-?,which shows the absence of dynamic Jahn-Teller distortions in Pr1/3Sr2/3FeO3-?.In addition,a comparative study of the samples with different oxygen nonstoichiometric was done.The results obtained show that charge ordering transition was strongly suppressed by oxygen vacancy in these samples.In the chapter 4,we present the results of kilohertz mechanical spectrum of perovskite(La1-xPrx)1/3Sr2/3FeO3-? samples above room temperature.The reduced modulus shows a step-like decrease with the decrease in temperature.This is ascribed to an octahedral tilting Pm-3m?R-3c transition.The tilting transition temperature increases with Pr substitution except in Pr1/3Sr2/3FeO3-?.An internal friction peak at around 420K due to domain walls freezing was observed below the tilting transition.And,in Pr1/3Sr2/3FeO3-? sample after immersion in 30wt.%H2O2 solution,another internal friction peak was observed just below the structure transiton temperature,which was ascribed to the anelastic relaxation of domain walls.In addition,in oxygen nonstoichiometric La1/3Sr2/3FeO3-? and Pr1/3Sr2/3FeO3-? samples,the step like decrease of the reduced modulus was suppressed.It suggests the suppression of the tilting transition by oxygen vacancies.In the chapter 5,we investigate the charge disproportionation in orthorhombic ferrites La1-xCaxFeO3-? by kilohertz mechanical spectroscopy.An internal friction peak around 145K was observed in La1-xCaxFeO3-?,which is similar to that in La0.5Sr0.5FeO3-?.This peak is ascribed to an elastic manifestation of charge freezing.Unlike orthorhombic La1-xCaxFeO3-?,the modulus softening with decrease in temperature was observed in rhombohedral La0.5Sr0.5FeO3-? above charge disproportionation.This is related to orbital lattice coupling of Fe4+.In orthorhombic perovskite the eg orbitals in Fe4+ are degenerated by the intrinsic distortion,as a result,in La1-xCaxFeO3-? orbital effect is absent.In the chapter 6,We present the results of kilohertz mechanical spectrum of La0.5(Ca1-xBax)0.5FeO3-? samples.An internal friction peak around 140K was observed in La0.5(Ca1-xBax)0.5FeO3-?,which is suggested occurrence of charge freezing transition.After vacuum annealing the charge freezing transition is suppressed in varying degrees.In addition,for their mechanical spectroscopy above room temperature,we have performed the measurements in both heating and cooling and in different flexural vibration modes.In the samples with orthorombic Imma structure,an internal friction peak was observed below the tilting transition,which is resulted from domain wall freezing due to the pinning of oxygen vacancies.In the samples with rhombohedral and cubic structure,the internal friction peak observed above room temperature in both heating and cooling is induced by the relaxation of oxygen vacancy.And we have observed the modulus softening with the decrease in temperature,which is related to the Jahn-teller lattice distortion of Fe4+ above CD transition.Combined with x-ray diffraction data,these results obtained by mechanical spectroscopy could help us effectively determine the structure of La0.5(Ca1-xBax)0.5FeO3-? samples.The seventh chapter is devoted to the conclusion and perspectives to the future work.