The Internal Friction Peaks in YBa2Cu3O6+x Above Room temperature Mark the Phase Transformations of the System

the YBa2Cu306+, system exhibits internal friction peaks above room temperature. These peaks are thermally activated. When the system is fully oxygenated (x=l), one peak only is detected. At a lower oxygen content, x<0.8, this peak change in shape and position and a new one can be contemporarily detected. These peaks are reasonably linked with the crystal phases of the system. We present a sequence of measures taken on the same sample, where the evolution of these peaks is shown. The YBa2C~306+x system exhibits several Internal Friction peaks, thermally activated, with energies ranging from 0.11 to 1.5 eV '. Most of them are attributed to the oxygen atom motion in the YBa2C~306+x basal plane2, which are proved to be the most mobile atoms in this compound by means of neutron and xray diffraction data 5. In this article, we will mainly discuss data taken above room temperature in the 1 Hz to 5 kHz range on policrystalline samples. In these conditions 2 Internal friction peaks are measured. One of them has an activation energy of (1.54+0.09) eV, measured on samples with an oxygen stoichiometry x>0.8 . The other one has an activation energy of (1.3~kO.l) eV, measured on samples with 0.40.9), what is usually observed is the presence of the so called orthorhombic I phase ( and of the tetragonal phase at temperatures above 700 K, depending on the sample and the exact oxygen content). At lower oxygen contents becomes stable, in the low temperature range, the other orthorhombic phase ", which differs from the previous one for the oxygen atoms in the basal plane are aligned in every other line along the b