TEM study of ferroelastic behavior in polycrystalline LaCoO 3
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Transcript of TEM study of ferroelastic behavior in polycrystalline LaCoO 3
REU Presentation
July 30, 2009UICPhysics
TEM study of ferroelastic behavior in polycrystalline LaCoO3
S. Kell, M. Tanase and R.F. Klie
Nanoscale Physics GroupDepartment of Physics
University of Illinois at Chicago
REU Presentation
July 30, 2009UICPhysics
• Relate the microstructure of LaCoO3 to ferroelastic strain to applying load
• Using Transmission Electron Microscopy to study microstructure
• Comparison of sample with no load applied with one subjected to 110 MPa of stress
Research Project
REU Presentation
July 30, 2009UICPhysics
• A ferroelastic material exhibits spontaneous strain after application of stress
• Under applied stress a material is
deformed
• Ferroelastic materials respond to stress by twinning or phase change
• LaCoO3 exhibits ferroelastic behavior at room temperature
LaCoO3 is a ferroelastic oxide
Lugovy et al. Physical Review B 78, 024107 (2008)
REU Presentation
July 30, 2009UICPhysics
• Twins are a crystallographic shear
deformation• Twins are the dominant structural feature
of LaCoO3 at room temperature
• Each twin is a ferroelastic domain• Stress can cause twinning or detwinning
(collapse of one twin into the other)• The behavior of a ferroelastic material
under stress is repeatable
Ferroelastic Behavior under stress
N. Orlovskaya et al. / Acta Materialia 51 (2003)
REU Presentation
July 30, 2009UICPhysics
LaCoO3 structure
La Co O
• LaCoO3 is a perovskite oxide
• Due to a slight distortion LaCoO3
is rhombohedral but can be thought of as pseudo-cubic
• Distortion is created by a tilting of the CoO6 octahedra
REU Presentation
July 30, 2009UICPhysics
Stacking Faults in untreated LaCoO3
• Stacking faults can be seen
in untreated material
• They often form closed loop
structures
• Such defects have been
reported previously and are
expected
REU Presentation
July 30, 2009UICPhysics
Twinning in untreated LaCoO3
• Deformation twinning is
present in the material
• Similar to what has been
previously reported
• The electron diffraction pattern
shows the presence of twinning
in splitting of spots
REU Presentation
July 30, 2009UICPhysics
Stacking Faults in Treated Sample
• Stacking faults can be seen in
treated material
• Extent of stacking faults
appears to be greater in treated
material than in untreated
• This form of deformation is
expected
REU Presentation
July 30, 2009UICPhysics
Atomic Scale Ordering
• Bright lines appear under high
resolution TEM• Lines are periodic with three
lattice parameters between
each one• Perpendicular defects in the
[100] and [010] directions• Defects of both directions are
interwoven
REU Presentation
July 30, 2009UICPhysics
• Periodicity of defects leads to
superstructure in diffraction
pattern• A cubic structured can clearly
be inferred from main spots in
pattern • Extra spots occur at three times
the frequency of the main spots• Superlattice reflection show up
more clearly in one direction
Diffraction Pattern
REU Presentation
July 30, 2009UICPhysics
Reasons for Ordering
• Stress induces distortion in CoO6
octahedra• This leads to shift in the position
of Co and possibly oxygen
vacancies
• A similar phenomenon in LCO
was attributed to monoclinic
domains (Holmestad et. al, 2007)• In this case, the defects seen are
atomic scale twins and are
important in ferroelastic behavior
REU Presentation
July 30, 2009UICPhysics
Acknowledgements
• National Science Foundation• EEC-NSF Grant # 0755115
• CMMI-NSF Grant # 0925425
• Department of Defense
• Professor R.F. Klie
• Dr. Mihaela Tanase
• Professor C. Takoudis
• Professor G. Jursich
• Ke-Bin Low