Unconventional Ferroelectricity in Violation with Neumann's Principle

TL;DR

This paper introduces a new class of ferroelectric materials called UFVNP that violate Neumann's principle, expanding the understanding of symmetry and polarization in crystals with theoretical and computational evidence.

Contribution

It identifies symmorphic space groups capable of UFVNP and demonstrates the phenomenon in specific materials through group theory and first-principles calculations.

Findings

33 symmorphic space groups can exhibit UFVNP

Polarization in UFVNP materials is quantized

UFVNP explains in-plane polarization in monolayer {\

Abstract

The physical properties of crystals are governed by their symmetry according to Neumann's principle. However, we present a case that contradicts this principle wherein the polarization is not invariant under its symmetry. We term this phenomenon as unconventional ferroelectricity in violation of Neumann's principle (UFVNP). Our group theory analysis reveals that 33 symmorphic space groups have the potential for UFVNP, with 26 of these symmorphic space groups belonging to non-polar groups. Notably, the polarization component in UFVNP materials is quantized. Our theory can explain the experimentally proven in-plane polarization of the monolayer {\alpha}-In2Se3, which has C3v symmetry. Additionally, we employ first-principles calculations to demonstrate the existence of UFVNP in Td phase AgBr, which was not initially anticipated to exhibit polarization. Thus, UFVNP plays an integral role in characterizing and exploring the possible applications of ferroelectrics, significantly expanding the range of available materials for study.