Distortion-dependent Raman spectra and mode mixing in RMnO3 perovskites (R=La,Pr,Nd,Sm,Eu,Gd,Tb,Dy,Ho,Y)

Abstract

The polarized Raman spectra of orthorhombic $R{\mathrm{MnO}}_3$ series $(R=\mathrm{La},\mathrm{Pr},\mathrm{Nd},\mathrm{Sm},\mathrm{Eu},\mathrm{Gd},\mathrm{Tb},\mathrm{Dy},\mathrm{Ho},\mathrm{Y})$ 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 $A_g\left(1\right)$ and bending $A_g\left(3\right)$ modes strongly mix for $R=\mathrm{Sm}$ to Tb, while for further decrease or $r_R$ $(R=\mathrm{Dy},\mathrm{Ho},\mathrm{Y})$ the $A_g\left(3\right)$ mode is observed at higher frequency than $A_g\left(1\right)$ mode; (2) similar distortion-dependent mode mixing takes place for the rotational $A_g\left(2\right)$ and $\mathrm{O}1\left(x\right)$ $\left[A_g\right(7\left)\right]$ modes. The mode mixing is particularly strong for the $R{\mathrm{MnO}}_3$ compounds with $r_R$ values close to the transition from $A$ type to incommensurate sinusoidal antiferromagnetic ordering at low temperatures. The frequency of rotational $A_g\left(2\right)$ and $A_g\left(4\right)$ modes scales to the angles of ${\mathrm{MnO}}_6$ $\left[101\right]$ and $\left[010\right]$ rotations, respectively, and could be used as a measure of their value.