Infrared Signature Of Ion Displacement In The Noncollinear Spin State Of Orthorhombic Ymno3

We have measured far-infrared (IR) phonon spectra of orthorhombic YMnO3 as a function of temperature. In the ferroelectric phase below T-FE=31 K, a new absorption peak appears at 620 cm(-1) and grows with decreasing T. Supplementary Raman phonon measurement shows that the IR peak is derived from B-2g(1) Raman phonon. The result indicates that an inversion symmetry breaking structural change takes place in the ferroelectric state. Possible origin of the change is discussed in the context of ion displacement induced by the noncollinear spin alignment.