Hydrostatic pressure Raman measurements have been carried out on the SmFeAsO series of oxypnictides with varying amount of doping (F substitution for O and Co for Fe) and transition temperature (T_{c}), in order to investigate lattice modifications a
nd their connection to doping and superconductivity. Synchrotron XRD data on some of these compounds indicated that at low doping the lattice constants vary smoothly with pressure, but with increasing F concentration there is a deviation from the normal equation of state and these effects are related with modifications in the superconducting FeAs4 tetrahedra. The hydrostatic pressure Raman measurements show that the A1g mode of the rare earth atom for the superconducting compounds deviates from the linear pressure dependence at the same pressures where the XRD results indicate pressure-induced lattice anomalies. A similar anomaly is found for the As phonon of same symmetry. As in cuprates, the effect is diminished in the undoped compounds and it is not related with the F substitution being present in the Sm(Fe_{1-x}Co_{x})AsO as well. The calculated Gruneisen parameter for the Sm phonon ({gamma approx}1.5) is very similar to the corresponding values of cuprates and it does not vary with doping. For the Fe mode it has higher value ({gamma approx}1.8) than for As ({gamma approx}1) indicating a more anharmonic phonon.
High resolution synchrotron X-ray powder diffraction (SXRPD) was used to study the temperature dependence of the oxygen deficient NdFeAsO$_{0.85}$ superconducting compound. By employing a dense temperature sampling we have managed to reveal unnoticed
structural modifications that start around $sim$180K, and disappear at the transition temperature. The data show minor changes of the structural characteristics in the Nd-O charge reservoir layer while in the superconducting Fe-As layer the FeAs$_{4}$ tetrahedron shows gradual modifications below $sim$180K, which suddenly disappear at T$_{rm c}$ strongly indicating a connection with superconductivity.
Combined synchrotron angle-dispersive powder diffraction and micro-Raman spectroscopy are used to investigate the pressure-induced lattice instabilities that are accompanied by T$_{rm c}$ anomalies in YBa$_{rm 2}$Cu$_{rm 4}$O$_{rm 8}$, in comparison
with the optimally doped YBa$_{rm 2}$Cu$_{rm 3}$O$_{rm 7-delta}$ and the non-superconducting PrBa$_{rm 2}$Cu$_{rm 3}$O$_{rm 6.92}$. In the first two superconducting systems there is a clear anomaly in the evolution of the lattice parameters and an increase of lattice disorder with pressure, that starts at $approx3.7 GPa$ as well as irreversibility that induces a hysteresis. On the contrary, in the Pr-compound the lattice parameters follow very well the expected equation of state (EOS) up to 7 GPa. In complete agreement with the structural data, the micro-Raman data of the superconducting compounds show that the energy and width of the A$_{rm g}$ phonons show anomalies at the same pressure range where the lattice parameters deviate from the EOS and the average Cu2-O$_{pl}$ bond length exhibits a strong contraction and correlate with the non-linear pressure dependence of T$_{rm c}$. This is not the case for the non superconducting Pr sample, clearly indicating a connection with the charge carriers. It appears that the cuprates close to optimal doping are at the edge of lattice instability.
