Structural and magnetic changes in Li substituted YBa sub(2)Cu sub(3)O sub(y) determined by neutron powder diffraction measurements

The crystallographic structure of orthorhombic and tetragonal samples of YBa sub(2)Cu sub(3-x)Li sub(x)O sub(y) has been determined by neutron powder diffraction measurements, with lambda identical with 1.225 angstrom. When the samples are synthesised in oxygen, the Li substitutes mainly for the Cu of the CuO sub(2) planes (Cu2). When they are synthesized in air or annealed in argon at high temperature, a fraction of the Li substitutes for the Cu of the Cu-O- chains (Cu1). Moreover, the neutron data show that each substitution of a Li for a Cu entails the loss of the neighbouring apical oxygen. The magnetic structure of the tetragonal samples has been determined by neutron powder diffraction measurements with lambda identical with 4.738 angstrom. With 0.09 Li /mole, weak magnetic peaks are observed at low temperature as in undoped YBCO. The Neel temperature is much lower: T sub(N) approximately 240 K instead of approximately 415K for YBa sub(2)Cu sub(3)O sub(6). The mean ordered magnetic moment on the Cu2 sites is smaller: 0.35 mu sub(B) instead of 0.64 mu sub(B) for YBa sub(2)Cu sub(3)O sub(6). The effect of the lithium substitution is compared to that of Zn.