High resolution neutron powder diffraction investigation of temperature and pressure effects on the structure of the high-T c superconductor Y 2Ba 4Cu 7O 15

Abstract

We have investigated the effect of temperature and 10 kbar pressure on the structure of the new high-temperature superconductor Y 2Ba 4Cu 7O 15 ( T c=68 K) in an attempt to relate the pressure dependence of T c 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 (YBa 2Cu 3O 7) and double CuO-chain (YBa 2Cu 4O 8) blocks. The thermal expansion is anisotropic, with the b-axis, parallel to the single and double Cu-O chains, remaining approximately constant for T< T c. As in YBa 2Cu 4O 8, Cu-O distances bridging chains and planes, as well as Ba-O distancesare significantly changed by pressure. However, in Y 2Ba 4Cu 7O 15 these changes are different for the single and double CuO-chain layers. A correlation with the pressure dependence of T c is proposed.