Precursor phenomenon on ferroelectric transition in multiferroic YMn2O5

Series of RMn2O5 (R = Sm-Lu, Bi, Y) are simultaneously ferroelectric and antiferromagnetic at low temperatures. The ferroelectric Curie temperature (TCE = 25–40 K) is slightly lower than the antiferromagnetic Néel temperature (TN = ∼45 K), implying that the ferroelectricity is induced with the antiferromagnetic long-range order in RMn2O5. Previously, we have investigated the thermal expansion anomaly in YMn2O5 at higher temperatures than TN = 45 K, which would be the precursor phenomenon relating to the ferroelectric transition. This study tries to investigate a further detailed precursor phenomenon to make clear the origin of the ferroelectricity in YMn2O5. Particularly, we focus on the temperature factors of each cation in the temperature range from 50 K to 170 K. The single crystal X-ray diffraction of YMn2O5 was integrated at 298 K and 112 K by using a single crystal diffractometer with an imaging plate. The structural parameters were well refined as Pbam at both temperatures. In the refined result, it was found that the thermal ellipsoid became more anisotropic for the Mn or the Y ions with decreasing temperature from 298 K to 112 K. In addition, the long principal axis of the thermal ellipsoids rotated in the a–b plane with decrease of temperature. At 112 K, the long principal axis of Mn3+ thermal ellipsoid in a–b plane is almost parallel to the direction from the apex to the base of the pyramid square. The direction agrees with a possible ionic displacement speculated from the irreducible representation analysis.