YBa₂Cu₃O7_1-x 240 K Phase Transition

YBa₂Cu₃O_7-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 α and a specific capacity c_m have been calculated. By this c_m value an oxygen atom activation energy ΔE = 0.15 eV has been calculated. It is shown that an isostructural oxygendisplacement second-order phase transition takes place between certain orthorhombic phases in YBa₂Cu₃O_7-x near 240 K.