Fig. 1. Powder X-ray (Cu K_α radiation) diffraction spectra of MgB₂ (with h,k,l) for three different qualities (a) 90% pure natural boron; (b) 99.99% pure natural boron and (c) 99.95% pure isotopic enriched ¹¹B. Samples (a) and (b) were synthesized for 3 h/950 °C, and sample (c) for 4 h/950 °C from [11]. The data gaps are due to the removal of the Si peaks.

Fig. 2. Variation of the zero-field resistivity in the 5–300 K range for MgB₂ pellets with different boron purities. Inset: expanded scale near T_c.

Fig. 3. Resistivity curves normalized by their temperature derivative at room temperature, for different boron purities. Inset: expanded scale near T_c.

Fig. 4. X-ray spectra for three different nominal compositions of Mg_x¹¹B₂ for x=0.8,1.0,1.2.

Fig. 5. Temperature dependence of the normalized resistivity for representative samples with nominal composition Mg_x¹¹B₂ (0.8<x<1.2). Inset: expanded scale near T_c.

Fig. 6. Residual resistance ratio of Mg_x¹¹B₂ (0.8<x<1.2). The open symbols represent different pieces selected from the same batch. The solid symbols are the average. The dotted box delimits the smaller variation (x=1±0.1).

Fig. 7. Resistivity curves normalized by temperature derivative at room temperature, for Mg_x¹¹B₂ (x=0.8,0.9,1.0,1.1 and 1.2). x=1.2 data shown as dashed curve as discussed in the text. Inset: expanded scale near T_c.

Fig. 8. Resistivity curves of MgB₂ filaments with four boron fiber initial diameters (100, 140, 190 and 300 μm): (a) for as-grown wires and (b) after etched in ethyl alcohol with 5% of HCl. Inset: expanded scale near T_c.

Fig. 9. Resistivity curves before and after distillation process in MgB₂ wire with 300 μm boron fiber initial diameter.

Table 1. Boron form and purity (as provided by the seller)

Table 2. Main properties of MgB₂ wires and resistivity at 300 K and RRR for as-grown, etched and distilled MgB₂ filaments with different diameters