Effect of structural instability between 80 and 300 K on superconductivity of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathrm{x}}$

The lattice parameters of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{\mathrm{x}}$ have been carefully measured by x-ray polycrystalline diffraction between 80 and 300 K. As a function of temperature, they reveal a large variation above $T_c$, which is mainly attributed to a few abrupt changes near certain temperatures rather than thermal expansion. Such structure instability, accompanied with the appearance of internal friction peaks and elastic softening, is related to oxygen content x. The instability near 120 K was found to be related to the superconductivity at 90 K.