Pressure-induced structural changes in the superconductor (Nd_1-xSr_x (Nd_1-yCe_y) CuO_4-z. A neutron diffraction study

The crystal structure of (Nd_0.59Sr_0.41) (Nd_0.73Ce_0.27)CuO_4-z (z≈0.4) was investigated by means of TOF neutron powder diffraction at six hydrostatic pressure up to 0.637 GPa with a He gas pressure cell. The superconducting transition temperature of this oxide has a positive pressure coefficient, dT_c/dP, of 4.0 K GPa^-1. As the pressure is increased, the a and c lattice parameters of its tetragonal cell both decrease linearly, which leads to a similar decrease in the length of a Cu-O (1) bond within a CuO₂ sheet. Increasing pressure also decrease the distance Cu and axial O(2) atoms at a similar rate. Unfortunately, larger uncertainties for the Cu-O(2) bond length make a quantitative comparison difficult. It is clear, however, that no pronounced changes are observed in individual Cu-O bond lengths when compared with the overall cell compression. We conclude that an understanding of the pressure effect on T_c must be gained by considering pressure-induced changes in the overall structure rather than focussing attention on a single Cu-O bond.