Anomalous behavior of the pressure dependence of lattice constants in Tl2Ba2CuO6+x

The crystal structure of Tl2Ba2CuO6+x has been investigated as a function of pressure and temperature under hydrostatic conditions using neutron powder diffraction. The crystal structure at 0.609 GPa and 60 K depends systematically on the path in pressure-temperature space by which these conditions are achieved. Applying pressure at room temperature and then cooling produces different changes in the structure than cooling first and then applying pressure. Our results support those of Sieburger and Schilling who reported that the Tc at high pressure for this compound depends markedly on whether the sample is pressurized at room temperature or low temperature. We speculate that this unusual behavior may be associated with differences in the mobility of interstitial oxygen defects between room temperature and 60 K and that applying pressure at room temperature changes the state of the sample through such processes as defect ordering, while changing the pressure at low temperature does not. (orig.)