Structure and properties of solid solutions in the Mg-Al-B system

Abstract

Ê Ú ¿½ Å Ý ¾¼½¼ ÔØ ¾  ÒÙ ÖÝ ¾¼½½ ×ØÖ ØCompounds with the general formula Mg 1− Al B 2 were obtained by two-step ceramic synthesis. All compounds were characterized by X-ray diffraction, NMR spectroscopy, and by four point probe resistivity measurements in various magnetic fields method. The diborides unit cell parameters were determined as a function of the Al mole fraction. With the vaues of up to 0.40 (where is the composition of the stock prepared for sintering), the unit cell parameters of Mg 1− Al B 2 are similar to those of pure MgB 2 and the superconducting transition temperature was lowered. For stock compositions of 0 25 ≤ ≤ 0 60, the products contain a superstructure, also superconducting phase, which becomes the only product at = 0 50, and at > 0 60 this phase is replaced by AlB 2 -based solid solutions.

Synthesis and characterization of aluminum diboride products using 27Al, 11B NMR and ab initio studies

“…The peak around 0 ppm could rise from aluminum oxide, presumably from the surface of the aluminum metal particles and from aluminum metal reacting with boron trioxide. This resonance around 0 ppm differs somewhat in shift from the lone report [18] of 100 ppm as the assignment for the 27 Al resonance for AlB 2 . However, there is a possible explanation for the observation of a 27 Al shift for AlB 2 differing from the numerous reports of a resonance around 880 ppm.…”

“…1. It should also be noted that 27 Al NMR spectra [14,17,18] previously in the literature for AlB 2 indicate the presence of significant aluminum metal in the samples. Due to the presence of aluminum metal in all of these samples (including the literature reports), there was interest in comparing the spectra of the products from different preparations.…”

“…Figure 3 shows the 27 Al MAS spectrum of the commercial AlB 2 sample from Sigma-Aldrich that contains three resonances. As mentioned in the ''Introduction,'' the literature for AlB 2 reports three different 27 Al shifts at 1640 ppm [12], 880 ppm [13,15,17], and 100 ppm [18]. Despite its assignment as the resonance for AlB 2 in Ref.…”

“…Several NMR investigations of AlB 2 have been previously reported [11][12][13][14][15][16][17][18]. However, while there is but a single aluminum site [10] in AlB 2 , the literature for AlB 2 shows spectra with a single 27 Al NMR resonance, but at three different reported shifts of 1640 ppm [12], 880 ppm [13,15,17], and 100 ppm [18].…”

Al-doped MgB2 materials studied using electron paramagnetic resonance and Raman spectroscopy

“…19,20 Many researchers have reported structural changes and superconductivity of Al-doped MgB 2 materials (Mg 1Àx Al x B 2 , x ¼ 0-1) in four phase regions: (i) the region with Al doping up to x ¼ 0.1 shows almost the same structure and a T c similar to pure MgB 2 ; 4 (ii) for 0.1 x 0.25, the coexistence of Al-rich and Mg-rich phases was reported; 8,21 (iii) for 0.25 x 0.6, a super-structured phase (Mg 0.5 Al 0.5 B 2 ) occurs, in which hexagonal Mg layers are alternatively replaced by Al layers. This phase was reported to be superconductive but at very low T c ; 10,22 and (iv) for x > 0.6, the material is no longer superconductive and it exhibits an AlB 2 -type structure. 22,23 To date, a number of experimental and theoretical studies dealt with the superconductive properties of both undoped and Al-doped MgB 2 materials.…”