Figure 1

(Color online) (a) Stoichiometric composition of the samples. The Bi, Sr, Cu, and La content relative to the Pb substitution level is shown together with a line fit (dotted lines) for each element as a function of the Pb content. The resulting formula confirms the vanishing influence of the Pb on the Cu and Sr content: ${\text{Pb}}_y{\text{Bi}}_{1.95-y}{\text{Sr}}_{1.49}{\text{La}}_{0.4}{\text{Cu}}_{1.15}{\text{O}}_{6+\delta }$. (b) Superconducting transition temperature $T_C$ relative to the Pb substitution level of the samples used in this study (crosses with error bars). The thin curve was calculated by averaging over many samples and represents the average $T_C$ and its standard error, indicated by bars.Reuse & Permissions

Figure 2

(Color online) Representative, (equally oriented) STM and LEED patterns of the samples with Pb substitution of 0, 0.07, 0.09, 0.21, 0.23, 0.30, 0.40, and 0.53 f.u.. The Pb substitution level is indicated in the upper left of each pattern. In the lead-free crystal in the upper left $\left(y=0\right)$, the $1\times 5$ superstructure along the $b$ direction can be seen as a striped modulation in the STM pictures and a line of additional reflexes in the LEED pattern. At $y=0.07$, the Pb substitution causes extra bright spots in the STM picture. At $y=0.09$, these bright spots begin to form bright linear objects. At intermediate Pb levels $\left(y=0.21,0.23\right)$, the existence of bright linear objects is clearly visible in the STM pictures. These bright linear objects seem to self-organize both in regions with higher density (${\beta }^{\ast }$ phase) and with lower density (${\alpha }^{\ast }$ phase). In the LEED picture $\left(y=0.23\right)$, a blurred structure indicates still modulations in the $b$ direction. At higher Pb levels ($y=0.30$, 0.40, and 0.50), the STM pictures show a clear phase separation in an $\alpha$ and an $\beta$ phase. The fraction of the $\alpha$ phase reduces with increasing Pb level. The quasi-$1\times 5$ superstructure or its remnants are absent in the LEED pictures for Pb concentrations of 0.30 and higher. When going from $y=0.30$ to $y=0.53$, the LEED spots get sharper and the incoherently scattered background between the spots is reduced.Reuse & Permissions

Figure 3

(Color online) Quantitative lengths of the superstructures and features from self-organization obtained by the average from many STM pictures. The characteristic length produced by the near $1\times 5$ superstructure is shown by red crosses. This superstructure wavelength remains nearly constant for all Pb substitution levels. A second characteristic length is shown quantitatively as blue squares. It is the average length between the bright linear objects at $y=0.09$, $y=0.21$, and $y=0.23$ Pb concentration (to be compared with Fig. 2). For higher Pb substitutions $\left(y\ge 0.3\right)$, a third characteristic length can be established as the average length between the bright linear objects. It is depicted as green triangles which are taken solely from the $\alpha$ phase since the smooth $\beta$ phase shows no modulations any more.Reuse & Permissions

Figure 4

(Color online) (a) Ideal structure of Bi2201. The structure can be seen as built from a perovskite block, containing the ${\text{CuO}}_6$ octaeder and the Sr atoms, expanded in $c$ direction by a rocksalt component which is by the BiO and SrO layer. This arrangement is repeated in $c$ direction with a shift of $a/2$. As the substitution of Bi by Pb leaves the perovskite component nearly unchanged (see text), the substitution can be seen as facilitating the adaption of the BiO plane relative to the perovskite block. (b) Pseudobinary phase diagram, dependent on temperature and Pb level. At low Pb substitutions and temperatures, there is the trianglelike-shaped $\gamma$ phase, which is the phase with the $1\times 5$ superstructure. Beginning at low temperatures toward higher levels of Pb substitutions, there is an area denoted as $\gamma +\beta$, where a mixture between the $\gamma$ phase and the $\beta$ phase occurs. The $\beta$ phase itself is assumed to occur at the specific Pb substitution level where the (Bi,Pb)O plane is perfectly adapted on the perovskite block. This Pb level is assumed to be 0.5. This optimally adapted $\beta$ phase is shown as a vertical line marked with $\beta$ at the abscissa. The stability of the $\beta$ phase ends at the point ${\text{y}}_{\beta }$, at the horizontal eutectic line. This line is situated at a temperature of about $T=830°\text{C}$ and starts from the point ${\text{y}}_{\gamma }$, where the $\gamma$ phase forms, and goes at least to the eutectic or peritectic point to the right, which is the unmarked crossing point marked with a horizontal arrow below. Above this point is the liquid (melt). Left from this point at temperatures higher than $830°\text{C}$ is another region of phase mixture, where the liquid phase coexists with a solid phase. (For simplification, the notation $\gamma +\text{L}$ for this phase is omitted here.) (c) Crystallization process upon cooling within the phase diagram to specify and explain the certain observed morphologies. There are three points, marked as 1, 2, and 3, which represent preeminent concentrations. As an example, the red arrows and circles depict the cooling/crystallization process for composition 1. For details on this and the other points 2 and 3 see the text, where also the blue point, marked with $3^{\prime }$, is discussed.Reuse & Permissions