Distortion-dependent Raman spectra and mode mixing in RMnO3 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 r(R) as a whole confirms the commonly accepted Raman line assignments with two noticeable exceptions: (1) with decreasing r(R) the stretching A(g)(1) and bending A(g)(3) modes strongly mix for R=Sm to Tb, while for further decrease or r(R) (R=Dy,Ho,Y) the A(g)(3) mode is observed at higher frequency than A(g)(1) mode; (2) similar distortion-dependent mode mixing takes place for the rotational A(g)(2) and O1(x) [A(g)(7)] modes. The mode mixing is particularly strong for the RMnO3 compounds with r(R) values close to the transition from A type to incommensurate sinusoidal antiferromagnetic ordering at low temperatures. The frequency of rotational A(g)(2) and A(g)(4) modes scales to the angles of MnO6 [101] and [010] rotations, respectively, and could be used as a measure of their value.