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

We present extended x-ray-absorption fine structure 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 0.005–0.01 Å for all atoms, which eliminates the possibility of moderate distortions 0.02 Å 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 6 10−4 Å, 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 6 10−4–5 10−3 Å.