2.4.3. Zn

Zn substitution is known to induce a T*c* depression in both Bi-2212 [15] and (Bi,Pb)-2212 [8]. Such depression has been explored in [8] for Zn content up to *x* = 0.04 and results to be in the order of Δ T*c*∼ 10 K. We have extended the explored range up to *x* = 0.32, panel (*d*) of Figure 4, finding that the T*c* depression maintains approximately the same magnitude even at such grea<sup>t</sup> doping content. A close inspection highlights the occurrence of diamagnetic signal even in the range above T*c* of the Bi-2212 phase: this suggests the possible formation of Bi-2223 phase induced by a high level of Zn doping.

## 2.4.4. Ti

Ti doping in the BSCCO system yields the formation of Bi-2212/2223 multiphase samples with an improving of the superconducting properties for small Ti content *x*, and successive worsening for further *x* increasing up to the highest essayed value *x* = 0.15 [9,16]. The ionic radius of Ti4<sup>+</sup>(68 pm) is similar to Cu2<sup>+</sup>(72 pm), so it has been suggested that the former can substitute the latter in the crystal lattice. In a similar way, our Ti-doped samples feature the emerging of the high-T*c* Bi-2223 phase at the expense of the Bi-2212 phase, panel (*c*) of Figure 4. According to literature, the nominal doping values of samples Ti01 and Ti05 should induce a sizable decrease on T*c* in both the Bi-2223 and Bi-2212 components [16]. On the contrary, samples Ti01 and Ti05 have the same T*c* values, while in sample Ti10, the variation of T*c* can be considered negligible if one takes into account the very high *x* value.

#### **3. Discussion and Conclusions**

In the Nd- and Y-doped samples, our results confirm the worsening of the superconducting properties with *x* exceeding the values usually reported. For these samples, SEM micrographs show a reduced grain size, with deterioration of connectivity. On the contrary, Zn- and Ti-doped samples feature a substantial independence in the high *x* range. This could be at least partially related to the different grain sizes that appear to be greater than what can be found in literature. This is true for both Zn doping [8] and even more for Ti doping, where T*c* appears to be independent on *x*. In principle, this could sugges<sup>t</sup> that the majority of the Ti ions were segregated as impurities, rather than entering the BSCCO crystal lattice. However, SEM and EDX show a uniform compositional distribution, with no evidence of substituents segregation or inhomogeneities in platelet borders and surfaces. Moreover, by comparison with the images reported in Reference [9], one can notice the greater size of platelets, even from the low doping values. The latter, likely due to the different conditions of synthesis and sintering, can probably be the true origin of the different behaviors.

In short, the main conclusions are the following:


Owing to the structural and chemical complexity of the BSCCO HTSs and in spite of the long experimental activity, this work confirms the need of detailed studies to take into account the multiple structural changes induced by different chemical substitutions of (Bi,Pb)-2223 phase, and to clarify the controversial issue of the correlation with T*c*.

#### **4. Materials and Methods**

Polycrystalline samples have been prepared as reported in Reference [17], starting from stoichiometric mixture of the high-purity binary oxides which were calcined in air at 800 ◦C for 24 h. The powder pressed into disk-shaped pellets using a manual hydraulic press type (SPECAC) under different pressures around 0.6 GPa. The pellets were sintered in air at 835 ∼850 ◦C for 140 h. From the obtained pellets, fragments of irregular shape, and linear dimensions of ∼2 mm have been obtained and addressed to the various characterization techniques.

The phase identification was carried out by PXRD, using the Siemens D500 diffractometer, emitting CuK*<sup>α</sup>*1 and CuK*<sup>α</sup>*2 wavelengths (average *λ* = 1.54178 Å), with no filter for the CuK*β*.

The surface morphology of the samples was studied using SEM (Philips 515), operating at 25 kV, and equipped with an EDX "Phoenix" detector for compositional analysis. ZFC magnetization curves M(T) have been measured in applied magnetic field H = 100 Oe using a commercial SQUID magnetometer MPMS-5T (Quantum Design Co., San Diego, CA, USA).

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4352/10/6/462/s1, Figure S1. EDX patterns for the analysed samples, Figure S2. PXRD patterns for the analysed samples, Figure S3. Expanded PXRD patterns for Y05, Nd05 and reference sample X000.

**Author Contributions:** Conceptualization, R.C., D.D. and E.G.; methodology, R.C., D.D. and E.G.; validation, R.C., D.D. and E.G.; formal analysis, R.C., D.D. and E.G.; resources, R.C., D.D., M.M.A., A.R.A. and E.G.; investigation, R.C., D.D., M.M.A., A.R.A. and E.G.; writing—original draft preparation, R.C.; writing—review and editing, R.C., D.D. and E.G.; visualization, R.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors thank P. Ferro (IMEM-CNR) for PXRD, F. Pattini (IMEM-CNR) for SEM data collection and S. Rampino (IMEM-CNR) for fruitful discussion.

**Conflicts of Interest:** The authors declare no conflict of interest.
