Structure and properties of YBaSrCu 3O y ( y=6.2-7.3)

Sintered YBaSrCu ₃O _y was treated at 300-950°C under various oxygen pressures of P_{O 2}=0.2-5×10 ⁴ atm. The structure as observed by powder X-ray diffraction (XRD) changed with increasing oxygen content from tetragonal ( y⩽6.6) to orthorhombic ( y⩽7.0) to tetragonal ( y⩾7.0). The maximum orthorhombic distortion and the highest superconducting transition temperature where reached at y=6.90-6.95. The orthorhombic structure for y=6.95 refined by a Reitveld analysis of the XRD data was essentially the same as that of YBa ₂Cu ₃O _6.9. On the other hand, transmission electron microscopic studies of the latter tetragonal structure strongly suggested ordering of Cu(1)-O chains in microdomains with sizes of 50×50 Ų, the XRD “tetragonal” symmetry being only an averaged one. The transition temperature, T_c, was 82 K at y=6.90, while 80 K at y=7.20. The concentration of carrier holes as estimated from the Hall effect was 4.9×10 ²¹ cm ^-3 at y=6.90 and 3.9×10 ²¹ cm ^-3 at y=7.20 at 100 K. Excess oxygen has been supposed to be concentrated in microdomain boundaries where Cu ions get octahedrally coordinated, leading to the reduction of the carrier concentration.