THE EFFECT OF HYDROSTATIC-PRESSURE ON THE AMBIENT-TEMPERATURE STRUCTURE OF CUO

Room temperature, time-of-flight, powder neutron diffraction measurements on cupric oxide, CuO, at ambient and 22(1) kbar hydrostatic pressure have provided the pressure dependence of the monoclinic lattice parameters and the single positional parameter, namely the y parameter of oxygen. The percentage changes in the cell parameters are DELTA-a/a = 0.68(4), DELTA-b/b = -2.02(2), DELTA-c/c = -0.38(2), DELTA-beta/beta = 1.02(2), so the volume of the cell upsilon changes by DELTA-upsilon/upsilon = -2.02(4)%. The value of y is altered by -0.0185(14). The effect of these changes is to reduce the distortion of the square-planar Cu2+ coordination by equalizing the bond lengths to the two oxygen pairs and making the O-Cu-O angle closer to 90-degrees. Although there is a sizeable reduction in the bond length to the apical, next-nearest oxygen pair, this distance remains greater than the shortest O-O bond. These observations are related to previous ambient temperature studies of the pressure dependence of the Raman spectra and of the lattice parameters by synchrotron radiation, which did not determine the y parameter.