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