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.
