In the light of the tremendous progress that has been made in raising the transition temperature of the copper oxide superconductors (for a review, see ref. 1), it is natural to wonder how high the transition temperature, Tc, can be pushed in other classes of materials. At present, the highest reported values of Tc for non-copper-oxide bulk superconductivity are 33 K in electron-doped CsxRbyC60 (ref. 2), and 30 K in Ba1-xKxBiO3 (ref. 3). (Hole-doped C60 was recently found4 to be superconducting with a Tc as high as 52 K, although the nature of the experiment meant that the supercurrents were confined to the surface of the C60 crystal, rather than probing the bulk.) Here we report the discovery of bulk superconductivity in magnesium diboride, MgB2. Magnetization and resistivity measurements establish a transition temperature of 39 K, which we believe to be the highest yet determined for a non-copper-oxide bulk superconductor.

Superconductivity at 39 K in magnesium diboride

Many of the fundamental questions regarding the solid-state chemistry of boron are still unsolved, more than 200 years after its discovery. Recently, theoretical work on the existence and stability of known and new modifications of the element combined with high-pressure and high-temperature experiments have revealed new aspects. A lot has also happened over the last few years in the field of reactions between boron and main group elements. Binary compounds such as B(6)O, MgB(2), LiB(1-x), Na(3)B(20), and CaB(6) have caused much excitement, but the electron-precise, colorless boride carbides Li(2)B(12)C(2), LiB(13)C(2), and MgB(12)C(2) as well as the graphite analogue BeB(2)C(2) also deserve special attention. Physical properties such as hardness, superconductivity, neutron scattering length, and thermoelectricity have also made boron-rich compounds attractive to materials research and for applications. The greatest challenges to boron chemistry, however, are still the synthesis of monophasic products in macroscopic quantities and in the form of single crystals, the unequivocal identification and determination of crystal structures, and a thorough understanding of their electronic situation. Linked polyhedra are the dominating structural elements of the boron-rich compounds of the main group elements. In many cases, their structures can be derived from those that have been assigned to modifications of the element. Again, even these require a critical revision and discussion.

Boron: elementary challenge for experimenters and theoreticians.

The basic magnetic and electronic properties of most binary compounds have been well known for decades. The recent discovery of superconductivity at 39 K in the simple binary ceramic compound magnesium diboride, MgB2, was therefore surprising. Indeed, this material has been known and structurally characterized since the mid 1950s (ref. 2), and is readily available from chemical suppliers (it is commonly used as a starting material for chemical metathesis reactions). Here we show that the addition of electrons to MgB2, through partial substitution of Al for Mg, results in the loss of superconductivity. Associated with the Al substitution is a subtle but distinct structural transition, reflected in the partial collapse of the spacing between boron layers near an Al content of 10 per cent. This indicates that superconducting MgB2 is poised very near a structural instability at slightly higher electron concentrations.

Loss of superconductivity with the addition of Al to MgB2 and a structural transition in Mg1-x AlxB2.

We report on the preparation of dense monofilamentary MgB2/Ni and MgB2/Fe tapes with high critical current densities. In annealed MgB2/Ni tapes, we obtained transport critical current densities as high as 2.3×105 A/cm2 at 4.2 K and 1.5 T, and for MgB2/Fe tapes 104 A/cm2 at 4.2 K and 6.5 T. An extrapolation to zero field of the MgB2/Fe data gives a critical current value of ∼1 MA/cm2, corresponding to a critical current value well above 1000 A. The high jc values obtained after annealing are a consequence of sintering densification and grain reconnection. Fe does not react with MgB2 and is thus an excellent sheath material candidate for tapes with self-field jc values at 4.2 K in excess of 1 MA/cm2.

Large transport critical currents in dense Fe- and Ni-clad MgB2 superconducting tapes

https://science.sciencemag.org/content/sci/153/3734/373.full.pdf

Framework Rearrangement in Boranes and Carboranes

The Preparation and Structure of Magnesium Boride, MgB2

The structure of HCOF has been investigated by electron diffraction in the gas phase. The parameters were refined by a least squares technique based upon the observations and calculated intensity curves. The parameter values and limits of error (taken as σ) are C-F = 1.351 ± 0.013 A., C=O = 1.192 ± 0.011 A7., O•••F = 2.225 ± 0.019 A., C-F/C=O = 1.134 ± 0.005 and ∠O-C-F = 121.9 ± 0.9°.

The Molecular Structure of Formyl Fluoride

On the structure of tetraborane

Stable pentaborane, B5H9, is one of the boron hydrides, a short, comparatively little studied series of extraordinary compounds for which a satisfactory elementary valence theory is lacking. In 1947 we decided to undertake new electron diffraction studies of the molecular structures. The early diffraction work and most of the theoretical discussion had been too much influenced (it now seems) by unfortunate analogies to ordinary valence compounds, and it had become reasonably clear that at least the old, ethane-like structure for diborane was incorrect and that a bridge structure (I) was more likely.

The Structure of Stable Pentaborane.

/pdf/on-the-structure-of-stable-pentaborane-4u8k6uz9yy.pdf

Morton E. Jones

Papers

The Preparation and Structure of Magnesium Boride, MgB2

The Molecular Structure of Formyl Fluoride

On the structure of tetraborane

The Structure of Stable Pentaborane.

On the structure of stable pentaborane