Covalent-like bondings and abnormal formation of ferroelectric structures in binary ionic salts

Abstract

In the Mooser-Pearson diagram, binary ionic compoundss form into nonpolar symmetrical structures with high coordination numbers, while wurtzite structures should appear in the covalent region. Their tetrahedral bonding configurations break the inversion symmetry, with polarizations almost unswitchable due to the high barriers of abrupt breaking and reformation of covalent bonds. Here, through first-principles calculations, we find some exceptional cases of highly ionic ferroelectric binary salts such as lithium halides, which may form into wurtzite structures with covalent-like sp 3 bondings, and the origin of these abnormal formations is clarified. Their high polarizations induced by symmetry breaking are switchable, with much smoother switching pathway refrained from abrupt bond breaking due to the long-range feature of Coulomb interactions. These covalent-like ionic bondings do reduce not only their ferroelectric switching barriers but also the phase transition barriers between polar and nonpolar phases, rendering high performance in applications such as nonvolatile memory and energy storage.

Theoretical designs of low‐barrier ferroelectricity

“…For example, some highly ionic binary compounds like lithium halides are exceptions, which may form into ferroelectric wurzite structures with covalent-like sp 3 bondings. 82 The energy difference between…”

“…Reprinted with permission. 82 WZ and RS phases ΔE = E(WZ) À E(RS) for a series of common binary compounds and the dependence of ΔE on their Philips ionicity 79 are plotted in Figure 7c. It seems that WZ structure is more favorable in the covalent zone with ionicity below the critical value 0.785, while the RS structure is the ground state for most binary compounds in the ionic zone with ionicity over 0.785 (the boundary is marked by the red line).…”

“…Recently, the formation of covalent‐like ionic bondings have been predicted in several systems, giving rise to highly ionic ferroelectric structures. For example, some highly ionic binary compounds like lithium halides are exceptions, which may form into ferroelectric wurzite structures with covalent‐like sp 3 bondings 82 . The energy difference between WZ and RS phases Δ E = E (WZ) − E (RS) for a series of common binary compounds and the dependence of Δ E on their Philips ionicity 79 are plotted in Figure 7c.…”

Switching it up: New mechanisms revealed in wurtzite-type ferroelectrics

“…After complete polarization inversion, Mg 3 MoN 4 exhibits the negative-polarity structure that has the same wurtzite-type structure as the positive-polarity structure but with polarization pointing in the opposite direction. Prior reports on binary compounds have reported that the nonpolar intermediate structure represents a saddle point in the MEP (11,13,23). We find that multinary compounds that follow the comparable wurtzite-hexagonal-wurtzite pathway can have a shallow local minimum at or near the nonpolar intermediate structure rather than it being a saddle point.…”

“…To address this challenge and reduce E c , there are generally two approaches. One is to further engineer known wurtzite materials like AlN and ZnO via alloying and straining (4,7,8,10,12), while the other is to explore and develop new wurtzite(-type) materials that have lower coercive fields (11,13,14). Ultimately, the goal is to identify the underlying polarization switching mechanisms and the structure-property relationships that control E c ; in other words, what deviations from the baseline wurtzite structure are involved in the transition from a positive-polarity wurtzite (wz + ) to the equivalent negative-polarity wurtzite (wz − )?…”