  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 [101] and [010] rotations, respectively, and could
be used as a measure of their value.
