Phonon anomalies and structural stability in the ${\mathit{R}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_4$ system (R=Gd,Sm,Nd,Pr)

We present a comprehensive analysis of Raman spectra in the ${\mathit{R}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Ce}}_{\mathit{x}}$ ${\mathrm{CuO}}_4$ system (R=Gd,Sm,Nd,Pr) as a function of doping, temperature, rare-earth atomic radius, and Raman resonance conditions. Phonon frequencies as well as their temperature dependences behave anomalously for R=Pr: The ${\mathit{B}}_{1\mathit{g}}$ phonon, for instance, softens by as much as 11 ${\mathrm{cm}}^{\mathrm{-}1}$ when the crystal is cooled from room temperature to 10 K, while it hardens by 11 ${\mathrm{cm}}^{\mathrm{-}1}$ for R=Nd. These observations are attributed to the fact that Pr, as the largest rare-earth atom that can give rise to the T’ structure, is already close to the T’ stability limit, yielding large phonon anharmonicities in this compound. The dependence of the phonon frequencies on doping is strong only for the oxygen ${\mathit{E}}_{\mathit{g}}$ mode, which can thus be used for sample characterization. We also present and discuss phonon resonance profiles for ${\mathrm{Nd}}_{1.85}$ ${\mathrm{Ce}}_{0.15}$ ${\mathrm{CuO}}_4$ and ${\mathrm{Nd}}_2$ ${\mathrm{CuO}}_4$, which yield electronic structural information that should be compared with future calculations of the resonance profiles. Finally, we also discuss the origin of an additional large ${\mathit{A}}_{1\mathit{g}}$-symmetry peak that shows a dramatic, rare-earth-dependent resonance behavior and gives a possible explanation on the grounds of a partial T’→T transition. We always observe several well-defined, unexpected vibrational peaks that seem to be intrinsic to the ${\mathit{R}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Ce}}_{\mathit{x}}$ ${\mathrm{CuO}}_4$ system, indicating crystal distortions that have not been conclusively identified by x-ray- or neutron-diffraction experiments so far.