  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.
