Large amplitudes and (critical) slowing-down of the order parameter fluctuations near the second-order tetragonal-to-orthorhombic phase transformation in YBa2Cu3Ox

YBa2Cu3Ox exhibits a second-order tetragonal-to-orthorhombic transformation, at least in wide temperature ranges. The order parameter is proportional to the orthorhombic strain or to the O population difference between (1/2 00) and (0 1/2 O) sites in the Cu(1) plane. Quasi-static anelasticity experiments were performed at 100 and 120 degrees C on YBa2Cu3Ox samples with O concentrations x between 6.29 and 6.9. The experiments show a drastic increase of both relaxation strength and relaxation time near the tetragonal-to-orthorhombic transformation at x similar or equal to 6.4, thus demonstrating i) large amplitudes and ii) a (critical) slowing-down of the order parameter fluctuations.