Ultrasonic Studies Of Charge Ordering In Perovskite-type Iron Oxides

The perovskite type transition-metal oxides are the typical strongly correlated systems due to their strong coupling among the degrees of freedom of charge, spin, lattice and orbit. Many related physical phenomena such as high temperature superconductivity, CMR effect, metal-insulator transition, charge ordering, and phase separation have been found in these compounds. The perovskite type ions oxides La_1-xSr_xFeO₃ have recently received much attention due to their complex charge ordering (CO) transition and charge disproportionation (CD) phenomena. This system characteristically possesses a rich variety of interesting features originating from the interplay among the different valence states of iron. And there are still many unresolved problems in this system. For example, the main driving force of the CO state in La_1/3Sr_2/3FeO₃ is still under discussion, and the direct evidence for the local CO state in the low-doping regime (x≈1/3) has not been obtained. Thus it is necessary to further investigate the correlations between the charge, spin, and orbit in La_1-xSr_xFeO₃.In this dissertation, we systematically studied the ultrasonic sound velocity and attenuation properties in La_1-xSr_xFeO₃, La_1/3Sr_2/3Fe_1-xM_xO₃ (M=Co, Cr, Mn) and R_1/3Sr_2/3FeO₃(R=La, Pr, Gd). A series of dramatic ultrasonic anomalies are observed in these compounds, which are attributed to the electron-phonon coupling via the Jahn-Teller effect of Fe⁴⁺ and the breathing-type distortion of Fe-0 octahedron. These results suggest that the stability of Fe⁴⁺ and Fe⁵⁺ plays an important role for the charge-ordered state in La_1-xSr_xFeO₃.The dissertation consists of six chapters. The arrangement of the chapters and the main content in each chapter are presented as follows:In chapter 1, we make a brief introduction to the research history, the crystal structure, the dynamic, static and cooperative Jahn-Teller effect, the magnetic ordering, the electronic structure, and the charge ordering transition.In chapter 2, the experimental methods are briefly introduced.In chapter 3, the longitudinal ultrasonic velocity(V₁), attenuation, magnetization, and resistivity of polycrystalline La_1-xSr_xFeO₃( 1/3≤x≤0.9) have been measured to investigate its complex charge-ordered state. For La_1/3Sr_2/3FeO₃, upon cooling from 300K, the V₁ softens dramatically and stiffens abruptly below the T_CO, which is attributed to the dynamic Jahn-Teller effect. Below the CO transition, the V₁ undergoes an another softening transition. This feature is attributed to the breathing-type distortion of Fe-0 octahedron. With increasing Sr content, the T_CO shifts to lower temperature and the resistivity jump at T_CO becomes smoothed. When x>0.8, the CO transition disappears and only the antiferromagnetic ordering exists. In the low-doping regime (x≈1/3), a dramatic stiffening below 210K coincided with a big attenuation peak are observed, while no electric or magnetic anomalies are found in the vicinity of this temperature. These results give new experimental evidence for the local charge-ordered state in these compounds. Combined with the transport and magnetic measurements, we present a phase diagram for La_1-xSr_xFeO₃(1/3≤x≤0.9).In chapter 4, the effect of M (M=Co, Cr, Mn) doping on the CO state in La_1/3Sr_2/3Fe_1-xM_xO₃ was investigated by the resistivity and ultrasonic velocity measurements. It is found that the CO state in La_1/3Sr_2/3FeO₃ is partially suppressed by the M-doping and the softening of V₁ below T_CO weakens. This behavior is due to the enhanced delocalization of the e_g electrons induced by the Co substitution and the lattice distortion induced by the Cr and Mn doping. For La_1/3Sr_2/3Fe_1-xM_xO₃ (M=Cr, Mn), the abnormal elastic softening above T_CO can be described well by the mean-field theory, which indicates that the Jahn-Teller effect plays an important role for the CO state in La_1/3Sr_2/3FeO₃.In chapter 5, the resistivity, magnetic and ultrasonic properties of R_1/3Sr_2/3FeO₃(R=La, Pr, Gd) were studied from 20K to 300K. When the R-site ion is changed from R=La to smaller-size R ion towards R=Gd, T_CO is decreased and finally the CO transition disappears for R=Gd. Dramatic anomalies in ultrasonic velocity for the R=La, Pr compounds are observed near the T_CO, which gives clear evidence for the electron-phonon coupling. Below T_CO, another unexpected softening in V| is discerned in La_1/3Sr_2/3FeO₃. This feature is attributed to the breathing-type distortion of Fe-0 octahedron, which is weakened in Pr_1/3Sr_2/3FeO₃. These results suggest that the high-valence state Fe⁵⁺ be obviously stabilized not only via the coupling with the breathing-type distortion, but also via the strong p-d hybridization.Finally, in chapter 6, we summarized the achievements and the deficiency in this dissertation. Some prospects of the possible future research are suggested.