Structural changes of superconducting YBa 2Cu 4O 8 under high pressure

Abstract

Structural changes in YBa 2Cu 4O 8 have been studied as a function of pressure using TOF neutron powder diffraction with a sample in a helium-gas pressure cell. Its structure contains double chains of edge-sharing [CuO 4] squares and two-dimensional CuO 2 planes. The distance between an apical oxygen atom and an in-plane copper atom decreases with increasing pressure. The distance between barium and oxygen atoms in the chain and between in-plane copper atoms located on both sides of a Y layer also change significantly with pressure. All of these movements of atoms are closely related to charge transfer from the CuO chain to the CuO 2 plane. Calculation of Madelung energies based on our structural parameters shows that the concentration of holes in the conducting CuO 2 plane increases appreciably with increasing pressure, which is in good agreement with the observed rise in T c under high pressure.