Thesis Title: "Static and Dynamic Defects in Ferroic Materials: Pathways to Functional Responses"

Name of the Student: Mr. Shubham Kumar Parate

Degree Registered: Ph.D. Engineering 

Advisor: Prof. Pavan Nukala, CeNSE

Date: 17th April 2026, (Friday), 4 PM

Venue: CeNSE Seminar Hall

Abstract:

Crystalline materials are often described as periodic atomic arrangements, yet real solids invariably 
contain defects spanning multiple length scales, from point vacancies to extended structural 
heterogeneities. Rather than acting only as imperfections, such defects can reshape the energy 
landscape of functional materials, enabling enhanced responses and stabilizing new structural states. 
In ferroic materials, where coupled order parameters such as polarization and strain govern functional 
behavior, defects can strongly influence switching dynamics, electromechanical response, and phase 
stability. This thesis investigates how defects serve as a unifying thread linking property enhancement, 
mesoscale transformations, and long-range phase evolution in ferroic oxides and layered chalcogenides, 
with particular emphasis on the contrast between static defectlandscapes and dynamically evolving 
defect networks.

The first part of this work focuses on defect-engineered BaTiO3, where predominantly static vacancy 
configurations modify lattice anharmonicity and electromechanical coupling. It is shown that 
non-stoichiometric BaTiO₃ can exhibit a giant electromechanical response even in the absence of 
classical ferroelectric switching. By tuning defect concentration and distribution, electric fields 
generate unusually large reversible strain, establishing defect-mediated electrostriction as a pathway 
to strong functionality in nominally non-ferroelectric systems. Complementary in-situ bias transmission 
electron microscopy studies further attempt to directly visualize polarization switching in BaTiO₃, 
revealing how static defect structures guide domain nucleation and propagation.

The thesis then shifts to layered In2Se3 nanowires, where defects are not static but dynamically 
reorganize under external stimuli and are explored through combined ex-situ and in-situ transmission 
electron microscopy.Electrical bias induces coupled interactions among polarization, interlayer sliding, 
and strain localization, while evolving defect intersections act as nucleation sites for structural 
instability. This results in cascading amorphization events that propagate over long distances, 
demonstrating how dynamic defect networks convert local perturbations into collective structural collapse.

The subsequent chapters further investigate electrically (reversible), optically and thermally 
induced transformations in polymorphs of In₂Se₃. These studies show that defect redistribution 
and mesoscale strain fields govern transformation pathways and reversibility, with electric bias 
and heat acting as controllable parameters for tuning phase stability.

Together, these studies establish a unified framework in which both static and dynamic defects 
function as active structural elements that mediate electromechanical coupling, nucleate phase 
transitions, and control switching pathways.This work provides design principles for energy-efficient 
functional materials based on metastability and defect-engineered energy landscapes.