  Using resonant X-ray diffraction and Raman spectroscopy, we study charge
correlations and lattice dynamics in two model cuprates,
HgBa$_{2}$CuO$_{4+\delta}$ and HgBa$_{2}$CaCu$_{2}$O$_{6+\delta}$. We observe a
maximum of the characteristic charge order temperature around the same hole
concentration ($p \approx 0.09$) in both compounds, and concomitant pronounced
anomalies in the lattice dynamics that involve the motion of atoms in and/or
adjacent to the CuO$_2$ layers. These anomalies are already present at room
temperature, and therefore precede the formation of the static charge
correlations, and we attribute them to an instability of the CuO$_2$ layers.
Our finding implies that the charge order in the cuprates is an emergent
phenomenon, driven by a fundamental variation in both lattice and electronic
properties as a function of doping.
