[Paper Review] Is hexagonal InMnO3 ferroelectric?

Fei Huang, X. Wang+5 more|arXiv (Cornell University)|Mar 22, 2013

Multiferroics and related materials21 citations

TL;DR

This study resolves the long-standing debate on hexagonal InMnO3 by demonstrating through advanced electron microscopy and polarization measurements that its ground state is ferroelectric with P6₃cm symmetry, exhibiting topological vortex domains. The high-temperature phase is a non-ferroelectric P̄3c1 state that can be quenched to room temperature, revealing a sluggish transition between phases due to kinetic barriers.

ABSTRACT

The presence of ferroelectricity in hexagonal (h-)InMnO3 has been highly under debate. The results of our comprehensive experiments of low-temperature (T) polarization, TEM and HAADF-STEM on well-controlled h-InMnO3 reveal that the ground state is ferroelectric with P6_3cm symmetry, but a non-ferroelectric P-3c1 state exists at high T, and can be quenched to room T. We also found that the ferroelectric P6_3cm state of h-InMnO3 exhibits the domain configuration of topological vortices, as has been observed in h-REMnO3 (RE=rare earths).

Motivation & Objective

• To resolve the longstanding controversy over whether hexagonal InMnO3 is ferroelectric.
• To identify the true crystallographic ground state of InMnO3 using well-controlled single-phase specimens.
• To investigate the structural and ferroelectric phase transitions in InMnO3 under varying thermal histories.
• To determine the role of atomic distortions and symmetry breaking in driving improper ferroelectricity in InMnO3.
• To clarify the nature of the non-ferroelectric P̄3c1 phase and its relationship to the ferroelectric ground state.

Proposed method

• Performed low-temperature polarization measurements to detect ferroelectric hysteresis loops.
• Used dark-field transmission electron microscopy (DF-TEM) to image domain structures and probe inversion symmetry breaking.
• Employed high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) with atomic resolution to visualize In and Mn ion displacements.
• Conducted structural analysis on polycrystalline and single-crystal InMnO3 samples prepared under different thermal treatments (slow-cooled, furnace-cooled, quenched).
• Applied Friedel’s law analysis to distinguish non-centrosymmetric (ferroelectric) from centrosymmetric (non-ferroelectric) phases via diffraction intensity anomalies.
• Used STEM-EDX to confirm elemental assignments and validate atomic column positions in HAADF-STEM images.

Experimental results

Research questions

• RQ1 Is hexagonal InMnO3 ferroelectric, and what is its true ground-state crystal symmetry?
• RQ2 What is the origin of the conflicting reports on ferroelectricity in InMnO3, and how do sample preparation conditions affect the observed phase?
• RQ3 Does the non-ferroelectric P̄3c1 phase represent a stable high-temperature phase that can be kinetically trapped at room temperature?
• RQ4 Are topological vortex domains present in the ferroelectric phase of InMnO3, similar to those in h-REMnO3?
• RQ5 What is the nature of the phase transition between the P̄3c1 and P6₃cm phases, and why is it unusually sluggish?

Key findings

• The ground state of InMnO3 is ferroelectric with P6₃cm symmetry, confirmed by saturation polarization in P(E) hysteresis loops at 100 K.
• Topological vortex domains are observed in the ferroelectric phase, analogous to those in h-REMnO3, indicating complex ferroelectric ordering.
• A non-ferroelectric P̄3c1 phase with 'down-no-up' In ion distortions exists at high temperatures and can be quenched to room temperature.
• Nanoscale PUA (partially un-distorted antiferroelectric) islands with 'down-no-up' In configurations are embedded in a ferroelectric matrix in furnace-cooled samples.
• HAADF-STEM imaging confirms off-center In displacements in the 'up-up-down' configuration, while Mn ions remain centered, validating the ferroelectric distortion mechanism.
• The transition from high-temperature P̄3c1 to low-temperature P6₃cm is sluggish and kinetically hindered, explaining the persistence of non-ferroelectric phases in some samples.