Screw Dislocations
in Complex, Low Symmetry Oxides: Core Structures, Energetics, and
Impact on Crystal Growth
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Abstract
Determining the atomic structure
and the influence of defects on properties of low symmetry oxides
have long been an engineering pursuit. Here, we focus on five thermodynamically
reversible monoclinic and orthorhombic polymorphs of dicalcium silicates
(Ca<sub>2</sub>SiO<sub>3</sub>)a key cement constituentas
a model system and use atomistic simulations to unravel the interplay
between the screw dislocation core energies, nonplanar core structures,
and Peierls stresses along different crystallographic planes. Among
different polymorphs, we found that the α polymorphs (α-C<sub>2</sub>S) has the largest Peierls stress, corresponding to the most
brittle polymorph, which make it attractive for grinding processes.
Interestingly, our analyses indicate that this polymorphs has the
lowest dislocation core energy, making it ideal for reactivity and
crystal growth. Generally, we identified the following order in terms
of grinding efficiency based on screw dislocation analysis, α-C<sub>2</sub>S > α<sub>H</sub>-C<sub>2</sub>S > α<sub>L</sub>-C<sub>2</sub>S > β-C<sub>2</sub>S > γ-C<sub>2</sub>S, and the following order in term of reactivity, α
-C<sub>2</sub>S > α<sub>L</sub>-C<sub>2</sub>S > γ-C<sub>2</sub>S > α<sub>H</sub>-C<sub>2</sub>S > β-C<sub>2</sub>S. This information, combined with other deformation-based
mechanisms, such as twinning and edge dislocation, can provide crucial
insights and guiding hypotheses for experimentalists to tune the cement
grinding mechanisms and reactivity processes for an overall optimum
solution with regard to both energy consumption and performance. Our
findings significantly broaden the spectrum of strategies for leveraging
both crystallographic directions and crystal symmetry to concurrently
modulate mechanics and crystal growth processes within an identical
chemical composition