Extending Stability of 2D Polymorphs through Oriented Stress

Van der Waals (vdW) bonds are more compliant than covalent or ionic bonds and more easily accommodate applied stresses. Applied stresses can therefore cause certain materials to reduce their dimensionality, forming soft inter-layer vdW bonds in favor of straining the ambient-pressure structure. Hull and Keen [1] demonstrated this effect by applying increasing hydrostatic pressure to AgI, which transitions between cubic (3D ionic, sg: F4̅3m), tetragonal (2D vdW, sg: P4/nmm), and cubic (3D ionic, sg: Fm3̅m) polymorphs. However, the 2D vdW polymorph is only stable over a narrow pressure range of 2-4 kbar. Here we simulate oriented stress (uniaxial, shear) on each AgI polymorph using density functional theory and show that the layered phase is stabilized in comparison to the other polymorphs by isolating compression along its vdW bonds. The effects of oriented stress on finite temperature stability are estimated by calculating vibrational entropies. This work may enable the discovery of new 2D materials though oriented stress.

[1] Keen, D. A.; Hull, S. A Powder Neutron Diffraction Study of the Pressure-Induced Phase Transitions within Silver Iodide. J. Phys.: Condens. Matter 1993, 5 (1), 23–32. https://doi.org/10.1088/0953-8984/5/1/005.