Limitations on the extent of off-center displacements in TbMnO3 from EXAFS measurements

TL;DR

This study uses EXAFS measurements to establish upper limits on atomic displacements in TbMnO3, constraining possible structural distortions and linking them to observed electrical polarization.

Contribution

The paper provides the first quantitative limits on atomic displacements in TbMnO3 using EXAFS, clarifying the relationship between structural distortions and polarization.

Findings

Atomic displacements are less than 0.005-0.01 Å for all atoms.

Displacements of O2 atoms along the c-axis are at least 6×10^{-4} Å.

Displacements likely range from 6×10^{-4} to 5×10^{-3} Å.

Abstract

We present EXAFS data at the Mn K and Tb L3 edges that provide upper limits on the possible displacements of any atoms in TbMnO3. The displacements must be less than 0.005-0.01A for all atoms which eliminates the possibility of moderate distortions (0.02A) with a small c-axis component, but for which the displacements in the ab plane average to zero. Assuming the polarization arises from a displacement of the O2 atoms along the c-axis, the measured polarization then leads to an O2 displacement that is at least 6X10^{-4}A, well below our experimental limit. Thus a combination of the EXAFS and the measured electrical polarization indicate that the atomic displacements likely lie in the range 6X10^{-4} - 5X10^{-3}A.