SYNTHESIS AND SUPERCONDUCTIVITY OF OXYCARBONATES OF THE TL-1201 PHASE

TlBaxSr4-xCu2(CO3)Oy oxycarbonates with the Tl-1201 structure were syn thesized over a wide range of x (1.0 less-than-or-equal-to x less-than -or-equal-to 3.2) and examined by X-ray powder diffraction, electron d iffraction, high-resolution electron microscopy and magnetometry metho ds. New phases with the Tl-1201 structure having 6 and 7 times periodi cities along the b-axis were isolated. The former was stable for a wid e Ba-rich range (1.8 less-than-or-equal-to x less-than-or-equal-to 3.2 ), and the latter was observed for a smaller Ba content range (1.2 les s-than-or-equal-to x less-than-or-equal-to 1.6). The phase stability w as found to depend on the synthesis condition. The maximum T(c) of the as-synthesized specimen in this series occurred at about 70 K around x = 2.0. The Meissner volume fraction of the as-synthesized specimen ( x = 2.4) was 30% at 5 K.