TEM study of ferroelastic behavior in polycrystalline LaCoO 3

1. 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

2. 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

3. 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)

4. 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)

5. LaCoO3 structure • 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. Acknowledgements • National Science Foundation • EEC-NSF Grant # 0755115 • CMMI-NSF Grant # 0925425 • Department of Defense • Professor R.F. Klie • Dr. MihaelaTanase • Professor C. Takoudis • Professor G. Jursich • Ke-Bin Low