TEM study of ferroelastic behavior in polycrystalline LaCoO3 S. Kell, M. Tanase and R.F. Klie Nanoscale Physics Group Department of Physics University of Illinois at Chicago 1

Research Project 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

LaCoO3 is a ferroelastic oxide 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 Lugovy et al. Physical Review B 78, 024107 (2008)

Ferroelastic Behavior under stress 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 N. Orlovskaya et al. / Acta Materialia 51 (2003)

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 La Co O

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

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

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

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

Diffraction Pattern 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

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

Acknowledgements Department of Defense Professor R.F. Klie 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