High-resolution thermal-expansion experiments on (i) insulating ${\mathrm{La}}_2\mathrm{Cu}{\mathrm{O}}_{4+\delta }$, (ii) its Sr- and Ba-doped metallic counterparts with reduced $T_c$ values, and (iii) ${\mathrm{Nd}}_{1.85}{\mathrm{Ce}}_{0.15}\mathrm{Cu}{\mathrm{O}}_{4-\delta }$ ($T_c\simeq 24$ K) reveal distinct lattice instabilities at $T_1=32\mathrm{}\pm 1$ K and $T_2=36\mathrm{}\pm 1$ K. $T_2$ coincides with $T_c^{max}$ found upon 15 at.% Sr doping. For ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.5}$ ($T_c=49\mathrm{}$ K), a lattice instability occurs at $T=90\mathrm{}\pm 1$ K, i.e., again close to $T_c^{max}=92\mathrm{}$ K.

• Received 15 July 1991

DOI:https://doi.org/10.1103/PhysRevLett.69.482