THE CRYSTAL-STRUCTURE OF YBA2CU4O8 AS A FUNCTION OF PRESSURE UP TO 5 GPA

The crystal structure of YBa2Cu4O8 (1-2-4) has been determined as a function of pressure up to 5 GPa at room temperature, using X-ray diffraction techniques and a single-crystal sample. The principal changes with pressure are in the (fractional) z-coordinates of Ba, Cu(2), O(1) and O(4), the shortening of the basal-plane copper to apical oxygen distance Cu(2)-O(1), the reduction in the pucker of the barium-apical oxygen layer, and the increase of the orthorhombic distortion of the unit cell. There are also a number of other Cu-O distances that change significantly in fractional rather than absolute terms, and the compressibility of different parts of the structure varies considerably. The rates of change are generally smaller than those reported in an earlier neutron-diffraction study of 1-2-4 at 1 GPa. It is now found that the rate of change of Cu(2)-O(1) with T(c) under pressure in 1-2-4 is the same within 20% as has been reported for T(c) variation with oxidation in YBa2Cu3O(x). This striking similarity, in two different materials, gives new support to the idea that Cu(2)-O(1) and T(c) are closely related. There is some marginal evidence that the structural changes follow the non-linear pressure dependence observed for T(c) above 4 GPa, but this is not yet certain.