HIGH-RESOLUTION NEUTRON POWDER DIFFRACTION INVESTIGATION OF TEMPERATURE AND PRESSURE EFFECTS ON THE STRUCTURE OF THE HIGH-TC SUPERCONDUCTOR Y2BA4CU7O15

We have investigated the effect of temperature and 10 kbar pressure on the structure of the new high-temperature superconductor Y2Ba4Cu7O15 (Tc=68 K) in an attempt to relate the pressure dependence of Tc in such materials to changes in certain Cu-O distances. The orthorhombic Ammm X-ray model of Bordet et al., with lattice constant c≈50.3 Å, has been confirmed by our high-resolution neutron powder diffraction measurements. The structure can be described as alternate layers of single CuO-chain (YBa2Cu3O7) and double CuO-chain (YBa2Cu4O8) blocks. The thermal expansion is anisotropic, with the b-axis, parallel to the single and double Cu-O chains, remaining approximately constant for T<Tc. As in YBa2Cu4O8, Cu-O distances bridging chains and planes, as well as Ba-O distancesare significantly changed by pressure. However, in Y2Ba4Cu7O15 these changes are different for the single and double CuO-chain layers. A correlation with the pressure dependence of Tc is proposed. © 1990.