YBa2Cu3O7-x240Kphase transition

YBa2Cu3O7-x ceramic samples have been investigated simultaneously by the thermal expansion and the acoustic emission methods during thermocycling through 200-300 K into liquid nitrogen steam. No jumps in the dilatation curves but change in the slope of one has been found at 228 and 234 K on heating and cooling respectively. These points are accompanied by the acoustic emission signals, a small value testifying for oxygen atoms displacement into the crystal lattice. On the basis of these measured experimental data, a thermal expansion coefficient alpha and a specific capacity c(m) have been calculated. By this c(m) value an oxygen atom activation energy DeltaE = 0.15 eV has been calculated. It is shown that an isostructural oxygen-displacement second-order phase transition takes place between certain orthorhombic phases in YBa2Cu3O7-x, near 240 K.