A crystal is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The ability to comprehend and control the mechanical properties of crystalline solids holds great promise for new advancements in crystal engineering and material design. Among other classes, the organic materials offer especially wide range of stress response. For instance, the “organosuperplasticity” effect, defined as superplastic deformation without heating, was found recently in an organic crystal. Another advantage of organic materials is the opportunity for systematic improvement. Recent publications have further demonstrated how density functional theory (DFT) in conjunction with global minimization methods for the lattice energy may serve as powerful tools in crystal structure predictions (CSP). In particular, the latter include evolutionary algorithms that help to avoid computationally expensive exhaustive enumeration of all the local minima and direct the search toward lower energy structures by retaining and combining energetically favorable structural features. However, while CSP methods have been widespread in recent years, developments of computational tools for predicting mechanical properties have received less attention.
Many organic compounds form two or more solid phases that differ only in the packing of molecules in the unit cell. These phases are known as polymorphs and can have dissimilar physical properties. Therefore, there is the need to develop a methodology that can discern various polymorphs. In this regard, University of Central Florida and South Ural State University researchers led by Professors Artem Masunov and Ekaterina Bartashevich developed a novel methodology for computational predictions of the mechanical properties for single crystals. The proposed methodology was based on constrained optimization using dispersion-corrected density functional theory level, and can be dubbed the virtual tensile test. Their work is currently published in the research journal, Crystal Growth & Design.
This approach was tested on 4-bromophenyl 4-bromobenzoate (BPBB), which has been shown to possess different mechanical properties in its three known polymorphs: form I, form II, and form III. Each one of these polymorphic crystal structures was stretched stepwise along each crystallographic axis, while the remaining lattice parameters and atomic coordinates were relaxed. The geometrical properties of halogen bonds and the other noncovalent interactions were monitored at each step to understand the nature of mechanical response. Lastly, the unit cell volumes and lattice energies were plotted as functions of the stretching parameter, and these curves were analyzed in terms of mechanical properties of the brittle, plastic, and elastic polymorphs.
The authors reported that when elastic form I was stretched along the c axis, the local cavities between molecules in the crystal were formed, which explained the increase in cell volume. At the 40% stretch, the cavity volume was seen to reach its maximum. With further increase in strain, the cavities “collapsed” and Br atoms formed new short contacts, resulting in decrease of lattice energy into a new local minimum. For brittle form II, even under small deformations along all cell axes, the lattice energy spiked. The researchers also reported that when the plastic form III was stretched along the c axis, the layers of molecules slipped along each other.
In summary, the study reported on the development of a novel methodology for computational predictions of the mechanical properties for single crystals. The approach described here can be called the virtual tensile test. Remarkably, it seems to be quite universal and applicable to a wide range of crystals. In a statement to Advances in Engineering, Professor Artem Masunov explained: “Combining crystal structure prediction techniques with our virtual tensile test holds a great promise for crystal engineering. We envision no limits on the type and composition of a crystal, since only the stepwise elongation of one lattice parameter followed by its freezing in the relaxation of the altered structure is required in our approach.”
Artem E. Masunov, Meryl Wiratmo, Alexander A. Dyakov, Yury V. Matveychuk, Ekaterina V. Bartashevich. Virtual Tensile Test for Brittle, Plastic, and Elastic Polymorphs of 4‑Bromophenyl 4‑Bromobenzoate. Crystal Growth & Design 2020: volume 20, page 6093−6100.