Ferroelastic LaCoO₃-based polycrystalline ceramics : a transmission electron microscopy and X-ray diffraction study
Doctoral thesis
Åpne
Permanent lenke
http://hdl.handle.net/11250/229018Utgivelsesdato
2005Metadata
Vis full innførselSamlinger
- Institutt for fysikk [2698]
Sammendrag
In the present work rhombohedrally distorted LaCoO3-based materials have been studied using transmission electron microscopy, in situ synchrotron X-ray diffraction during uniaxial compression/decompression, and ex situ X-ray diffraction after compression. In addition, macroscopic stress-strain behavior has been measured. Focus has been on the understanding of the ferroelastic behavior as a function of stress, chemical composition and temperature, from the atomic to the macroscopic scale. In La1-x(Ca,Sr)xCoO3- with low degrees of substitution (x 0.2) transmission electron microscopy revealed (100) and (110) twins. In addition, microdomains with a onedimensional superstructure corresponding to a tripling of the pseudo cubic lattice parameter, ac, were found in some grains. The superstructure vanished over time and was sensitive to external mechanical stress. In highly substituted La1-x(Ca,Sr)xCoO3-(0.3 x 0.5) materials no normal twinning was observed, but microdomains with a one-dimensional superstructure corresponding to a doubling of ac were observed. The superstructures have been interpreted as a periodic miniaturization of the ferroelastic twin structures. The microdomains consisted of two alternating rhombohedral domain states, separated by (100) twin walls, which gave the observed superstructures. A theory of an adaptive ferroelectric phase has previously been developed to describe similar miniaturized domains in ferroelectric materials and intermetallic phases with martensitic transitions. Here the theoretical framework for the formation of miniaturized domains due to an adaptive phase in ferroelastic rhombohedral perovskite materials is derived. The stability of the adaptive phase is discussed in terms of conceptual phase diagrams based on present and previous experimental findings. Mechanical stress–strain behavior of La1-xCaxCoO3 (x = 0, 0.2, 0.3) was studied under compression at 25°C and 300°C. A hysteresis in the stress–strain relationship due to reorientation of the ferroelastic domains was observed, and a remnant strain was measured after unloading. The coercive stress increased with substitution of Ca for La and decreased with increasing temperature. LaCoO3 can be regarded as a soft ferroelastic material while the 30 % Ca substituted material is a hard ferroelastic. The hysteresis of the stress–strain relationship was clearly dependent on both composition and temperature. The effects of uniaxial stress during compression/decompression have been studied in situ in La0.8Ca0.2CoO3 using synchrotron X-ray diffraction. X-ray diffractograms were simultaneously detected, parallel and perpendicular to the stress axis. With increasing stress, reorientation of the four rhombohedral domains was demonstrated. The fraction of domains with the unique hexagonal c-axis parallel to the stress axis increased at the expense of domains with c-axis perpendicular to the stress axis. The crystal structure evolved inhomogeneously in the materials with increasing stress. In favorable domains, with c-axis parallel to the stress axis, the rhombohedral distortion increased. In unfavorable domains, with c-axis perpendicular to the stress axis, the structure became closer to cubic. The ferroelastic behavior of successive cycling to higher and higher loads was compared to the behavior of a single compression/decompression cycle and to the mechanical stress-strain behavior.
Finally, secondary phases and grain boundaries in LaCoO3, La0.7Sr0.3CoO3-? and La0.5Sr0.5Fe0.5Co0.5O3- ceramics have been studied by transmission electron microscopy. The materials contained small amounts of grains with Co3O4 as a secondary phase. These grains were an agglomerate of several smaller grains, which had a face centered cubic spinel structure. High resolution electron microscopy combined with annular dark field scanning transmission electron microscopy and electron energy loss spectroscopy spectrum imaging were used to analyze grain boundaries in LaCoO3 and La0.7Sr0.3CoO3-? weresharp with ? constant content of La, Co and O across the boundaries. In La0.7Sr0.3CoO3-,a 1-2 nm thick layer between the grains was observed. This layer was rich in O and Co and deficient in Sr and La, compared to the nominal composition of the material.