Structural behavior of Pb y Bi 1.95-y Sr 1.49 La 0.4 Cu 1.15 O 6+δ for 0

In the Bi cuprates, the presence of a near 1x5 superstructure is well known. Usually, this superstructure is suppressed by the substitution of lead, but there have been reports of a phase separation in so called alpha and beta phases. This paper shows in high detail for the first time how and why the phase separation develops and what happens to the quasi 1x5 superstructure upon lead substitution. For this purpose, the lanthanum and lead substituted single-layered superconductor Bi2201 has been investigated by scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). The La content was kept constant at slightly under-doped concentration while the Pb content was changed systematically. Thermodynamic considerations show that a phase mixture of alpha and beta phases is inevitable.