A 2212 BISMUTH CUPRATE WITH A NONMODULATED STRUCTURE BI2-XLAXBA2.5LA0.5CU2O8.25

A new solid solution with the 2212 type structure, Bi2-xLaxBa2.5La0.5C u2O8.25, has been isolated for 0.2 less-than-or-equal-to x < 0.70. X-r ay and electron diffraction studies and EDAX analysis show that lantha num replaces bismuth in the [BiO]infinity layers. The originality of t his structure compared to other bismuth cuprates has to do with the co mplete absence of modulation as soon as x > 0.20. Taking into consider ation the fact that bismuth and lanthanum are isovalent and exhibit th e same size, and that the oxygen content does not vary owing to the ex perimental conditions, this study demonstrates that the electronic 6s2 lone pair of Bi(III) should be at the origin of the modulation of the structure in bismuth cuprates. The absence of superconductivity in th is phase is discussed by comparing with the 2212 Sr based cuprates sub stituted with lanthanides.