Structural and magnetic changes in Li substituted YBa2Cu3Oy determined by neutron powder diffraction measurements

The crystallographic structure of orthorhombic and tetragonal samples of YB a2Cu3-xLixOy has been determined by neutron powder diffraction measurements , with lambda = 1.225 Angstrom. When the samples are synthesised in oxygen, the Li substitutes mainly for the Cu of the CuO2 planes (Cu2). When they a re synthesised 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 n eutron data show that each substitution of a Li for a Cu entails the loss o f the neighbouring apical oxygen. The magnetic structure of the tetragonal samples has been determined by neutron powder diffraction measurements with lambda = 4.738 Angstrom. With 0.09 Li/mole, weak magnetic peaks are observ ed at low temperature as in undoped YBCO. The Neel temperature is much lowe r: T-N approximate to 240 K instead of approximate to 415K for YBa2Cu3O6. T he mean ordered magnetic moment on the Cu2 sites is smaller: 0.35 mu (B) in stead of 0.64 mu (B) for YBa2Cu3O6. The effect of the lithium substitution is compared to that of Zn.