To introduce the crystal growth of high-quality single crystals of the Bi2201-phase, it is appro-priate to start with the equilibria ternary phase diagram. The superconducting Bi2201-phase exists only in a very narrow composition range. This can be seen in Fig. 3.5 where as an exam-ple, a part of the pseudo-ternary phase diagram for Bi2+zSr2−zCuO6+δ at T≈900◦C is shown.
This is shown relative to the composition of the elements SrO-1/2 Bi2O3-CuO. One competition in the growth is the existence of the above-mentioned non-superconducting stoichiometric phase next to the Bi2201-phase [130] (phase 2:2:1 in Fig. 3.5). The Bi2−yPbySr2−xLaxCuO6+δ pseudo-ternary phase diagram is in principle the same as the one shown. The difference is that one has to replace 1/2 Bi2O3 by 1/2 Bi2O3 + LaO. Because the La has a higher melting temperature compared to Bi, the shown diagram may reassemble the one for Bi2−yPbySr2−xLaxCuO6+δ at slightly higher temperatures. The effect of Pb substituting Bi is very weak in this diagram.
An important feature of the Bi2201-phase is that it can only be reached from the melt via a peritectic point. This situation is depicted in Fig. 3.6 by an experiment-based pseudo-binary phase diagram [131]. In this diagram, x=0 represents Bi2(Sr,Ca)O4 and x=∞is (Sr,Ca)O2. As illustrated, the phase can be solely reached via the peritectic point at x≈0.85 and T≈835◦C.
Among the methods existing for growing the Bi2201-phase as single crystals, the floating zone technique (e.g., [132, 133]), and the flux method (e.g., [134, 135]) are often applied. Here, the growth was achieved using a flux method. In my opinion, the advantage of this method is that the crystals grow without any extra strain because of a small temperature gradient within the crucible. Also, the risk of producing intercalations between the BiO-planes might be reduced because possible pollution will be concentrated on a certain crystallization region. The flux method can be applied with or without solution (then also called ’stoichiometric growth’). The solution can be an additional transport material such as KCl [136, 137, 138], or possibly also Li and Sb (done for the Bi2212-phase by [139]), or an element of the final composition weighted in a higher amount. With the latter type of solution, the method is then called ’self flux’. For this growth of Bi2201, theoretically Cu, Pb and Bi can be used. Because Cu does not seem appropriate as solution [140], and Pb is not wanted in La free crystals, here, Bi is used in typical additional amounts of 10% of the total Bi formula unit. The use of Bi as a solvent has the additional advantage of lowering the melting temperature. In the following I would like to describe the processing steps of growing the Bi2Sr2−xLaxCu1+wO6+δ and Bi2−yPbySr2−xLax -Cu1+wO6+δ single crystals used here. The process can be roughly summarized by the following:
The total mass must be more than a critical mass (>50g). The dependence of the La starting composition and the resulting average La content for the crystals used here is shown in Fig. 3.7.1.
The starting composition is also often called the ’nominal composition’ whereas the resulting
3.2 Crystal Growth
La content is the ’actual composition’. Interestingly, the curve is quite similar to that published by Yang et al. [135]. The first process step is divided between crystals with Lead and for those without:
Figure 3.5: From [124]: En-larged portion of the subsolidus pseudo ternary phase diagram in equilibria of the system SrO-1/2 Bi2O3-CuO at T≈ 900
◦C. The two phases Bi2Sr2 -CuO6(2:2:1) and Bi2+zSr2−z -CuO6+δ(RSS) can be seen. The bunches of thin lines represent the tie lines.
Figure 3.6: By [131] pro-posed pseudo-binary phase di-agram: x=0 is Bi2(Sr,Ca)O4
and x=∞ is (Sr,Ca)O2. The Bi2201-phase is here denoted as
’221’ and at x=1. Please note the peritectic point at x≈0.85 and T≈835◦C.
(i) For crystals without Lead, the oxides Bi2O3, CuO, LaO and the carbonate SrCO3 are composed. To get a homogeneous mixture, the composition is dissolved in ethanol and then grounded. It follows a calcination at T≈800◦C to decompose the Strontium-Carbonate: SrCO3
→SrO + ↑ CO2, afterwards the composition is again dissolved in ethanol and grounded.
(ii) For crystals containing Lead, first the oxides Bi2O3, CuO and the carbonate SrCO3 are
3.7.1: Dependence of the La starting composition and the resulting La content measured by EDX.
3.7.2: Typical temperature-time dependence of the growing steps in the oven.
composed, dissolved in ethanol and then grounded. This composition is compared to the proce-dure without Lead calcinated at higher temperatures of T≈930◦C. After the calcination, LaO and PbO are added and the composition is dissolved in ethanol and grounded.
The composition is placed in an oven and melted beyond the liquidus temperature Tm in a zirco-nium oxide crucible. Tmis approximately in the range from 950◦C to 1050◦C. This temperature is maintained for 1-2 hours to thermally homogenize the melt. The typical temperature-time dependence of the growing steps in the oven are shown in Fig. 3.7.2. For crystals with Pb concentration y>0.3 formula units, additional air is supplied. After the homogenization, the system is cooled down to the upper crystallization temperature T2. For Bi2201 with x=0.4 and y=0 formula units, T2 ≈850◦C. In the crystallization temperature range [T1,T2] the system is cooled down very slowly at a rate of 1-2 K/h to allow the system only to form a few crystalliza-tion nuclei. For Bi2201 with x=0.4 and y=0, T1 ≈800◦C. As already mentioned, the system is peritectic and therefore must be cooled down very slowly in the crystallization process. With the help of additional Oxygen, it is then possible to reach the solely Bi2201-phase (please com-pare Fig. 3.6). After the system is stable and depending on the number of crystallization seeds desired, it is rapidly quenched to room temperature.
Photographs of the crystals in the zirconium crucible after the growth process are shown in Fig.
3.7. It is then necessary to prepare some crystals from the mixture of slightly different phases and La concentrations in the crucible to get a true one-phase crystal of Bi2201. As a rule, the crystal properties vary from the center of the crucible to the surface. The cleave requires some manual skill but can be handled quite well because the crystals cleave easily between the BiO-layers due to their weak van der Waals bonds. Typically, the samples are flat with a thickness of about 300 µm and a typical size of about 2 x 1 mm2. The longer side is in most cases the growth direction, which is the a-axis.