12230-32-9

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Chemical Properties

refrac solid

InChI:InChI=1/12B.Mg

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• 7439-95-4 magnesium 

Relevant academic research and scientific papers

A reactivity study in the Mg-B system reaching for an improved synthesis of pure MgB2

The binary Mg-B system was studied with respect to the existing phases MgB2, MgB4, MgB7, and MgB12 and their transformations into each other. As a result of these studies, a new synthesis route is reported for MgB2 by reacting MgB4 and Mg with each other. The obtained MgB2 was characterized by Rietveld refinement of the powder XRD pattern (RBragg = 3.56%, Rwp = 10.6%), magnetic measurements, and by electron probe microanalysis (EPMA). With this new synthesis route, a better phase homogeneity is obtained when compared with MgB2 samples prepared from the elements.

Synthesis, crystal growth and structure of Mg containing β-rhombohedral boron: MgB17.4

For the first time, single crystals of Mg containing β-rhombohedral boron MgB17.4 were synthesised from the elements in a Mg/Cu melt at 1600 °C. The crystal structure determined by the refinement of single crystal data (space group R-3m, a = 10.991 (2) A, c = 24.161 (4) A, 890 reflections, 123 variables, R1 (F) = 0.049, w R2 (I) = 0.122) improves and modifies the former structure model derived from earlier investigations on powder samples. Mg is located on four different positions with partial occupation. While the occupation of the sites D (53.3%), E (91%) and F (7.2%) is already known from other boron-rich borides related to β-rhombohedral boron, the occupation of the fourth position (18h, 6.7%) is observed for the first time. Two boron positions show partial occupation. The summation reveals the composition MgB17.4 and Mg5.85B101.9, respectively, confirmed by WDX measurements. The single crystals of MgB17.4 show the highest Mg content ever found. Preliminary measurements indicate no superconductivity.

Synthesis, Crystal Structure, and Properties of MgxB 50C8 or Mgx(B12)4(CBC) 2(C2)2 (x = 2.4-4)

Single crystals of a new magnesium boride carbide MgxB 50C8 (x = 2.4-4) were synthesized from the elements in a metallic melt using tantalum ampules. Crystals were characterized by single crystal X-ray diffraction and electron microprobe analysis. The variation of the Mg content results from different reaction conditions. The composition Mg ～3B50C8 is by far the most favored. It fulfills the electron counting rules of Wade and Longuet-Higgins and thus explains the light-green to yellow transparent color. The structure of Mg ～3B50C8 (C2/m, Z = 1, a = 8.9384(12) A, b = 5.6514(9) A, c = 9.6021(13) A, β = 105.86(1)°) consists of B12 icosahedra. The icosahedra are interconnected by four exohedral B-B bonds to layers. The layers are connected to a three-dimensional covalent network by C2 and CBC units and further exohedral B-B bonds. The Mg sites are partially occupied. Different site occupation factors cause the various compositions and colors (Mg 2.4B50C8, brown; Mg4B 50C8, black). The vibrational spectra show the modes of B12 icosahedra and C2 and CBC units as well. Measurements of the microhardness according to Vickers and Knoop revealed remarkably high values of HV = 3286 (32.0 GPa) and HK = 3165 (31.5 GPa), which exceed the values of B4C. Optical spectra reveal a band gap of 2.7 eV for Mg～3B50C8, in agreement to the observed color. This justifies an ionic description, and the formula can be written as (Mg2+)3(B122-) 4(CBC+)2(C2)2.

Electron probe microanalysis of Mg-B compounds: Stoichiometry and heterogeneity of superconductors

Electron probe microanalysis (EPMA) by wavelength-dispersive X-ray spectroscopy (WDX) was applied to determine the chemical composition of oxygen-bearing magnesium boride compounds. Extremely smooth samples, and the area-mode integration of measurement for the boron Ka peak are necessary to obtain precise results. An uncorrected analytical total near 100 wt% is a good quality control parameter. A careful sample preparation is necessary to avoid an artificial enrichment of oxygen. The analyzed samples were powders, ceramics, sheathed wires and tapes. MgB2 MgB4, MgB7 and MgB12 were identified as the relevant phases, confirming phase diagram studies. No other compounds (boric oxide, ternary Mg-B-oxides) are necessary to explain the data. Deviations from stoichiometry in Mg-B phase analysis are mainly caused by sample heterogeneities on a sub-μm scale, the compounds were found to be largely stoichiometric. Bulk samples of superconducting MgB2 tapes and wires are highly heterogeneous and contain much more oxygen than our other samples. Besides MgB2 they consist of other magnesium borides and MgO with grain sizes from the nm-scale to several microns. Modeling the electron microprobe analyses provides evidence for the development of complex microstructures by oxidation reactions in addition to primary inhomogeneities within the starting material.

Synthesis and crystal structure of MgB12

Single crystals of MgB12 were synthesized from the elements in a Mg/Cu melt at 1600 °C. MgB12 crystallizes orthorhombic in space group Pnma with a = 16.632 (3) A, b = 17.803 (4) A and c = 10.396 (2) A. The crystal structure (Z = 30, 5796 reflections, 510 variables, R1 (F) = 0.049, w R2 (I) = 0.134) consists of a three dimensional net of B12 icosahedra and B21 units in a ratio 2:1. The B21 units are observed for the first time in a solid compound. Mg is on positions with partial occupation. The summation reveals the composition MgB12.35 or Mg0.97B12, respectively. This is in good agreement with the value of MgB11.25 as expected by electronic reasons to stabilize the boron polyhedra B122 - and B214 -.