SYNTHESIS AND STRUCTURAL CHARACTERIZATION OF YSR2CU3-XMXO7+/-Y(M=FE,TI,AL,CO,GA,PB-0.5-LESS-THAN-X-LESS-THAN-1), AND THE NONEXISTENCE OF THE PARENT PHASE YSR2CU3O7

The possibility of synthesising phases of general composition YSr2Cu3-xMxO7 +/- y has been examined for a wide variety of metals M. Single phase tetragonal products (typically a = 3.83 angstrom, c = 11.5 angstrom) can be synthesised provided that the concentration of M is sufficiently large (x > 0.5), and M has a preference for the Cu(1) sites (the four-coordinate sites) of the YBa2Cu3O7, structure. Neutron diffraction has demonstrated that a high degree of disorder exists in the (001) planes containing the M cations and, in particular, significant oxygen displacements are found. The non-existence of the unsubstituted variant YSr2Cu3O7 is explained in terms of the high degree of compressive stress on the Cu(1) sites which could exist in such a material.