PHONON ANOMALIES AND STRUCTURAL STABILITY IN THE R2-XCEXCUO4 SYSTEM (R=GD,SM,ND,PR)

We present a comprehensive analysis of Raman spectra in the R(2-x)Ce(x)CuO4 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 B1g phonon, for instance, softens by as much as 11 cm-1 when the crystal is cooled from room temperature to 10 K, while it hardens by 11 cm-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 E(g) mode, which can thus be used for sample characterization. We also present and discuss phonon resonance profiles for Nd1.85Ce0.15CuO4 and Nd2CuO4, 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 A1g-symmetry peak that shows a dramatic, rare-earth-dependent resonance behavior and give 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 R(2-x)Ce(x)CuO4 system, indicating crystal distortions that have not been conclusively identified by x-ray- or neutron-diffraction experiments so far.