Pressure dependence of the oxygen ordering process in RBa2Cu3Ox

In YBa2Cu3Ox (x < 7.0), above 225 K a oxygen ordering in the CuO chains is visible as a glass-like transition in thermal-expansion measurements. The l arge contribution of this process to the thermal expansion and the T-c chan ges observed under application of external hydrostatic pressure point to a strong dependence of the oxygen ordering and its activation energy E-a on t he unit-cell volume V. If, however, V is changed chemically by replacing Y with other rare earth ions, R, the values of E-a reported in the literature show only a slight dependence on V. We have measured the thermal expansion of a YBa2Cu3O6.94 Single crystal under hydrostatic pressure up to 0.5 GPa. The activation energy at ambient pressure was determined to be 1.0+/-0.1 e V with a pressure dependence dE(a)/dp = 78 meV/GPa. The volume dependence d E(a)/dV = -48 meV/<Angstrom>(3) obtained from these pressure experiments is clearly higher than dE(a)/dV = -5.4 meV/Angstrom (3) from R-ion substituti on. Thus we conclude that a non-uni form compression of interatomic distanc es within the unit cell has to be considered to explain the difference betw een dE(a)/dV(p) and dE(a)/dV(R). The contribution, Delta alpha (V), of the ordering process to the thermal-expansion coefficient, alpha (V), increases with pressure, indicating that more and more oxygen atoms take part in the process. From an extrapolation to negative pressure via R-ion substitution we expect a vanishing (R=Nd) or inverted (R=La) pressure effect on oxygen ordering.