Loss of superconductivity and structural transition in Mg1-xAlxB2

The basic magnetic and electronic properties of most binary compounds have been well known for decades. Therefore the recent announcement of superconductivity at 39 K in the simple binary ceramic compound MgB2 [1] is surprising. This compound, available from common chemical suppliers, and used as a starting material for chemical metathesis reactions [2], has been known and structurally characterized since the mid 1950’s [3]. 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 10% Al content. This indicates that superconducting MgB2 is poised very near a structural instability at slightly higher electron concentrations. Electronic structure calculations on MgB2 have shown that the states at the Fermi energy (EF) are primarily derived from boron orbitals [4-6]. Doping on the Mg site is therefore expected to introduce electrons into the bands at EF with relatively little disruption of the electronic network. Solid solutions of the type Mg1-xAlxB2 were therefore synthesized by direct reaction of the elements to test the effect of electron concentration on Tc. AlB2 is the prototype compound for this structure type, and the Mg1xAlxB2 series has been previously reported [7] but not physically characterized. Starting materials were bright Mg flakes (Aldrich chemical), fine Al powder (Alfa Inorganics), and sub-micron amorphous B powder (Callery Chemical). Due to the poor reactivity of crystalline boron at low temperatures, and MgO contamination of fine Mg powders, the selection of starting materials is important. Starting materials were lightly mixed in half-gram batches, and pressed into pellets. The pellets were placed on Ta foil, which was in turn placed on Al2O3 boats, and fired in a tube furnace under a mixed gas of 95% Ar 5% H2. The samples were heated for one hour at 600 C, one hour at 800 C, and one hour at 900 C. After cooling, they were pressed into pellets and fired for an additional 2 hours at 900 C and quickly cooled to room temperature. The resulting pellets have very low densities, making them inappropriate for resistivity measurements. Identical results to those reported here were obtained for materials synthesized by heating in sealed evacuated Ta tubes at 925 C for 4