Giant pressure effect in oxygen deficient YBa2Cu3Ox

The pressure effect on T-c of polycrystalline and single crystalline YBa2Cu3Ox has been investigated as a function of oxygen content x by ac-susceptibility measurements under helium pressure. In the overdoped region x > 6.93 the single crystals show a negative dT(c)/dp, as expected from the charge transfer model. For optimally doped samples with x = 6.93 we find dT(c)/dp = 0.4 K/GPa which points to pressure effects on T-c aside from charge transfer. In the underdoped region x < 6.93 the dT(c)/dp values obtained from the experiment depend strongly on the storage temperature of the sample during the experiment. When the samples are stored at temperatures well below 240 K throughout the entire experiment including pressure application and pressure release, dT(c)/dp increases to approx. 7 K/GPa at x = 6.7 but with a further decrease of the oxygen content the dT(c)/dp drops to approx. 2 K/GPa at x = 6.4. These effects are intrinsic to the YBa2Cu3Ox structure and can be explained by considering the anisotropic structure of YBa2Cu3Ox. The decrease of the c-axis lattice parameter results in a charge transfer to the CuO2-planes mainly [1], whereas the compression of the a- and b-axis lattice parameter is known to produce different pressure effects which are responsible for the peak in dT(c)/dp at x = 6.7 [2]. When pressure is changed at room temperature oxygen ordering effects occur which cause a relaxation of T-c to the equilibrium value T-c(p) at this pressure with a time constant depending on the oxygen content x. A decrease of x results in a peak effect in dT(c)/dp at x = 6.7 again, which is enhanced to approx. 12 K/GPa. If the oxygen content is decreased further, dT(c)/dp first drops to 5 K/GPa at x = 6.6, but then increases to values of more than 20 K/GPa for x < 6.42. These giant pressure effects at low oxygen contents are mainly caused by a reversible T-c increase (dT(c)/dp)(0) due to pressure induced oxygen ordering via oxygen motion between unit cells.