Klomp, Arne Jan (2022)
Atomistic Computer Simulations of Dislocations in Strontium Titanate Single Crystals.
Technische Universität Darmstadt
doi: 10.26083/tuprints-00022467
Dissertation, Erstveröffentlichung, Verlagsversion

Kurzbeschreibung (Abstract)

Thermo-mechanical, functional, as well as processing related properties of ceramics are typically tailored via defect engineering. While zero-, two-, and three-dimensional defects are widely studied, the domain of one-dimensional defects, i.e., dislocations has hardly been exploited to tailor ceramics. The reason is a lack of understanding of dislocations in ceramic crystal structures. Furthermore, effective means of introducing and controlling dislocations in brittle ceramics are not well explored. We expect that once dislocations in ceramics are properly understood, they can also be controlled and finally employed as an engineering framework to enhance mechanical properties such as crack resilience as well as functional properties, such as ferro- and piezoelectricity. However, this requires a fundamental understanding how dislocations work on the microscopic level. Here, we use atomistic computer simulations to shed light on the microscopic structure and properties of dislocations in SrTiO3 which is a prototype member of the important perovskite family.

In three consecutive sections we explore (I) the equilibrium structure and low temperature configuration of dislocations that can be controlled by external mechanical stress; (II) the behavior of these dislocations under applied load, their tendency to glide, and the imperfections occurring during dislocation glide; (III) the implications of complex dislocation arrangements for the macroscopic plastic behavior of SrTiO3.

(I) First, we classify the dislocations reported in literature into five groups and model each of these types explicitly. We find that only dislocations with Burgers vector <110> and glide plane {1-10} can dissociate into partial dislocations. With the help of analytic estimates we show that this glide dissociation makes them good candidates for easy dislocation glide. However, their structure is very sensitive to the dislocation line orientation as well as the oxygen ion stoichiometry at the dislocation core.

(II) Second, stress is applied to the dislocation configurations identified in (I). We determine the Peierls stress as a function of the dislocation core configuration and stoichiometry and observe that the emission of defects from a gliding dislocation is a prominent feature for certain dislocation configurations.

(III) Third, we study multiple dislocations and their interactions. In doing so, we combine in silico observations of different dislocation arrangements with the results of ex situ electron microscopic characterization. We determine that points of preferential dislocation nucleation and opportunities for dislocation multiplication enable the plasticity of SrTiO3. This understanding paves the way to design methods for creating and controlling dislocations.

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