Distortion-dependent Raman spectra and mode mixing in R MnO3 perovskites (R=La,Pr,Nd,Sm,Eu,Gd,Tb,Dy,Ho,Y)
The polarized Raman spectra of orthorhombic RMnO3 series (R=La,Pr,Nd,Sm,Eu,Gd,Tb,Dy,Ho,Y) were studied at room temperature. The variation of phonon frequencies with R ionic radius rR as a whole confirms the commonly accepted Raman line assignments with two noticeable exceptions: (1) with decreasing rR the stretching Ag(1) and bending Ag(3) modes strongly mix for R=Sm to Tb, while for further decrease or rR (R=Dy,Ho,Y) the Ag(3) mode is observed at higher frequency than Ag(1) mode; (2) similar distortion-dependent mode mixing takes place for the rotational Ag(2) and O1(x) [Ag(7)] modes. The mode mixing is particularly strong for the RMnO3 compounds with rR values close to the transition from A type to incommensurate sinusoidal antiferromagnetic ordering at low temperatures. The frequency of rotational Ag(2) and Ag(4) modes scales to the angles of MnO6  and  rotations, respectively, and could be used as a measure of their value.