Lattice distortion at ferroelectric transition of YMn2O5

We conducted X-ray diffraction study of YMn2O5 single crystals with an orbital-radiation X-ray source to determine the lattice distortions related to the ferroelectric phase transition. At present, no anomaly of temperature dependence of lattice parameters has been detected around the ferroelectric transition point T-CE = 39 K within the accuracy of lattice parameters, +/- 1 x 10(-4) Angstrom. Moreover neither splitting of Bragg peaks nor appearance of a super lattice reflection were noticed below T-CE. These facts indicate that ionic displacement at T-CE is much smaller in YMn2O5 than in typical ferroelectrics. On the basis of symmetry consideration, we discuss a possible ionic displacements at the ferroelectric transition.