[Paper Review] Symmetry changes at the ferroelectric transition in the multiferroic YMnO3

Gwilherm Nénert, Yang Ren+2 more|arXiv (Cornell University)|Apr 21, 2005

Multiferroics and related materials3 citations

TL;DR

This study identifies the symmetry change at the ferroelectric transition in multiferroic YMnO3 using single-crystal synchrotron diffraction and group theory, proving it is a proper ferroelectric with P63/mcm symmetry in the intermediate phase. The ferroelectricity arises from correlated tilting of MnO5 polyhedra along the (100), (110), and (010) directions, resolving prior ambiguity about its improper nature.

ABSTRACT

We have identified, for the first time, the change in symmetry at the ferroelectric transition TFE near 1023K of the ferroelectromagnet YMnO3. This transition takes place 300K below the transition to the centrosymmetric state at TIP. Single crystal synchrotron diffraction coupled to a group theoretical analysis show that the paraelectric intermediate phase between TIP and TFE has P63/mcm symmetry. This proves that YMnO3 is a proper ferroelectric and not an improper ferroelectric, as suggested by a previous group theoretical assignment. The origin of the ferroelectricity is caused by a correlated tilting of the MnO5 polyhedra along the (100), (110) and (010) directions

Motivation & Objective

• To determine the crystal symmetry of the intermediate phase between the centrosymmetric and ferroelectric states in YMnO3.
• To resolve the long-standing debate on whether YMnO3 exhibits proper or improper ferroelectricity.
• To identify the microscopic origin of ferroelectricity in this multiferroic material through symmetry analysis.
• To provide definitive experimental evidence using high-resolution diffraction and group theoretical analysis.

Proposed method

• Single-crystal synchrotron X-ray diffraction to probe the crystal structure across phase transitions.
• Group theoretical analysis to identify the irreducible representations associated with the symmetry-lowering transition.
• Comparison of observed diffraction patterns with predicted patterns for candidate space groups.
• Identification of the intermediate phase as having P63/mcm symmetry based on systematic extinctions and intensity analysis.
• Analysis of atomic displacements to link structural distortions to ferroelectric polarization.
• Correlation of MnO5 polyhedral tilting patterns with the observed symmetry and polarization.

Experimental results

Research questions

• RQ1 What is the crystal symmetry of the paraelectric intermediate phase in YMnO3 between TIP and TFE?
• RQ2 Is the ferroelectric transition in YMnO3 a proper or improper ferroelectric transition?
• RQ3 What is the microscopic origin of the ferroelectric polarization in YMnO3?
• RQ4 How do the MnO5 polyhedral distortions contribute to the symmetry change at TFE?
• RQ5 Can experimental diffraction data definitively rule out previous group theoretical assignments of improper ferroelectricity?

Key findings

• The intermediate phase between TIP ≈ 1323 K and TFE ≈ 1023 K has P63/mcm symmetry, confirming a proper ferroelectric transition.
• The ferroelectric transition is driven by a correlated tilting of MnO5 polyhedra along the (100), (110), and (010) directions.
• The observed symmetry change rules out the previously proposed improper ferroelectric mechanism for YMnO3.
• The polarization arises from a collective distortion involving off-center displacements linked to the tilting mode.
• The group theoretical analysis confirms that the order parameter transforms as a specific irreducible representation consistent with proper ferroelectricity.
• The results resolve a key ambiguity in the multiferroic mechanism of YMnO3, establishing it as a proper ferroelectric with a well-defined symmetry breaking.