Effect of the Local Field and Dipole–Dipole Interaction on the Spontaneous Ordering of Dipole Moments in Bismuth Monolayers with an Orthorhombic Structure

Abstract

Using two complementary approaches, the instability of bismuth monolayers with an orthorhombic structure with respect to the emergence of spontaneous electric dipole moments at the crystal lattice sites has been studied. The first approach, based on determining the dipole moments of lattice sites through the polarizability of bismuth atoms and taking into account the local Lorentz–Lorenz field, suggests the possibility of the existence of several antiferroelectric and ferroelectric phases in orthorhombic bismuth monolayers. Using the second approach, the vibrational spectrum of a nonpolar symmetric lattice has been analyzed, and two types of soft optical modes corresponding to the ferroelectric and antiferroelectric instabilities of the monolayers have been revealed. The relationship of the Born effective charges to the polarization direction in the ferroelectric phase, as well as their role in the reduction of the frequency of the polar optical phonon, which characterizes the lattice deformation in the ferroelectric phase due to the dipole–dipole interaction, has been shown.