Anomalously large oxygen-ordering contribution to the thermal expansion ofuntwinned YBa2CU3O6.95 single crystals: A glasslike transition near room temperature

We present high-resolution thermal expansion data from 5-500K of untwinned YBa2Cu3Ox (Y123) single crystals for x similar to 6.95 and x = 7.0. Large c ontributions to the thermal expansivities due to O ordering are found for x similar to 6.95, which disappear below a kinetic glasslike transition near 280 K. The kinetics at this glass transition is governed by an energy barr ier of 0.98 +/- 0.07 eV, in very good agreement with other O-ordering studi es. Using thermodynamic arguments, we show that O ordering in the Y123 syst em is particularly sensitive to uniaxial stress along the chain axis and th at the lack of well-ordered chains in Nd123 and La123 is most likely a cons equence of a chemical-pressure effect.