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

• 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 ProjectResearch Project

•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 oxideLaCoO3 is a ferroelastic oxide

Lugovy et al. Physical Review B 78, 024107 (2008)

• 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 stressFerroelastic Behavior under stress

N. Orlovskaya et al. / Acta Materialia 51 (2003)

LaCoO3 structureLaCoO3 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

Stacking Faults in untreated LaCoO3 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 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 SampleStacking 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 OrderingAtomic 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

• 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 PatternDiffraction Pattern

Reasons for OrderingReasons 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

AcknowledgementsAcknowledgements