Origin of Ferroelectricity in Orthorhombic LuFeO$_3$

TL;DR

This study reveals that orthorhombic LuFeO$_3$ exhibits weak ferroelectricity at its magnetic transition temperature due to magnetic structure and bond covalency, supported by experimental and theoretical evidence.

Contribution

It demonstrates the emergence of ferroelectricity in orthorhombic LuFeO$_3$ linked to magnetic order and covalency, with theoretical prediction of a polar phase below $T_N$.

Findings

Weak ferroelectric polarization observed at $T_N$.

Enhanced bond covalency contributes to polarization.

Electric field induces piezostriction and domain contrast.

Abstract

We demonstrate that small but finite ferroelectric polarization ($\sim$0.01 $\mu$C/cm$^2$) emerges in orthorhombic LuFeO$_3$ ($Pnma$) at $T_N$ ($\sim$600 K) because of commensurate (k = 0) and collinear magnetic structure. The synchrotron x-ray and neutron diffraction data suggest that the polarization could originate from enhanced bond covalency together with subtle contribution from lattice. The theoretical calculations indicate enhancement of bond covalency as well as the possibility of structural transition to the polar $Pna2_1$ phase below $T_N$. The $Pna2_1$ phase, in fact, is found to be energetically favorable below $T_N$ in orthorhombic LuFeO$_3$ ($albeit$ with very small energy difference) than in isostructural and nonferroelectric LaFeO$_3$ or NdFeO$_3$. Application of electric field induces finite piezostriction in LuFeO$_3$ via electrostriction resulting in clear domain contrast images in piezoresponse force microscopy.