FURTHER EVIDENCE FOR TL3-BASED SUPERCONDUCTORS FROM IMPROVED BOND STRENGTH PARAMETERS INVOLVING NEW STRUCTURAL DATA OF CUBIC TL2O3( IN TL)

We report on refined structural parameters for cubic Tl2O3 (avicennite , space group Ia3, a=10.5344(3) angstrom) using Guinier powder diffrac tion data. The final reliability factor of the structure analysis was R(F)=0.015. Empirical bond valence sums of the two different Tl3+ ions in the structure were calculated from bond lengths. These values devi ate negatively from the actual 3+ valence by the same amount that has been frequently observed for Tl-based superconductors. Therefore, a le ast squares fit of the Tl3+ bond strengths s on the bond lengths d acc ording to s=(d/d0)-N has been performed giving improved parameters d0= 2.04(1) angstrom, N=6.4(2) and as a result persuasive evidence for the 3+ valence of Tl in high-T(c) superconductors, even though the Tl sit e is occupied to some extent by Cu1+.