Electron Requirements of Bonds in Metal Borides

Abstract

Electrical measurements have been made on CaB6, SrB6, BaB6, YB2, YB4, YB6, and YB12 for the purpose of testing models of the electronic structure of the boron atoms in the four boride types represented. The compounds CaB6, SrB6, and BaB6 were found to be semiconductors. The Hall coefficients of YB2, YB6, and YB12 were measured and found to correspond to one free electron per yttrium atom in each compound. The Hall coefficient of YB4 was also measured, but could not be interpreted because the free-electron theory does not provide a theoretical value for comparison with experiment. The results are in agreement with the models which have been proposed.

Fabrication of semiconducting SrB6− thin films on ultrasmooth sapphire substrates by laser molecular beam epitaxy

“…Interestingly, electrical properties of hexaborides vary widely from semiconducting to superconducting [3] on doping divalent or trivalent metallic ions into the crystal structure [4]. The electrical conduction of SrB 6 bulk ceramics was reported to show both semiconducting [5][6][7] and semimetallic [8,9] behavior. Energy band calculations and experimental investigations between 0.05 and 300 K of a very high quality SrB 6 sample have indicated semiconducting behavior with a energy gap of 1.1 eV [6,7].…”

Magnetic susceptibility of Ce and Pr in SrB6 and YB6

“…The observed decay times are considerably faster than those previously measured by an indirect temperature-variation method and shorter than those predicted by phonon processes [2]. The decay times are independent of temperature in the range 2.2 to 4.2OK but decrease with increasing Cr concentration, indicating that Cr-Cr interactions are important in photon-echo decay.…”

“…Theory [1] and experiment [2] agree that divalent metals form semiconducting hexaborides, trivalent metals electrically conducting hexaborides with one conduction electron per unit cell. Fig.…”

Hole superconductivity in MgB2: a high Tc cuprate without Cu

“…The conduction in AlB 2 is mainly electron-like, as suggested by Fig. 2 and by the measured negative Hall coefficient of the similar compound Y B 2 [6]. Hence AlB 2 is not a superconductor.…”