ELECTRONIC-STRUCTURES OF 3 SEMICONDUCTING INTERMETALLICS - RUAL2, RUGA2, AND OSAL2

Results of first-principles, density-functional, LMTO-ASA calculations on the closely related C11, C40, and C54 structures of the three titl e compounds are reported. Only for OsAl2 in the C11 structure were the structural degrees of freedom optimized; this gave results in goad ag reement with experimental values. On the other hand, the calculations were not capable of reproducing the correct relative stability of the different structures; this was ascribed to the atomic-sphere approxima tion. All of the compounds were found to be small-gap semiconductors w ith, however, a slightly larger gap for OsAl2. The occurrence of a gap at the Fermi level is mainly due to hybridization between d functions of Os or Ru and p functions of Al or Ga, but, in particular for OsAl2 , charge-transfer effects can also be considered to be responsible for the occurrence of a gap. Several hat bands just above the Fermi level lead to a corresponding high density of states there, but also to low carrier velocities. As a by-product we propose a scheme for studying current densities using parameter-free methods, and by applying this a pproach to the title compounds we predict that they will prove to have transport properties similar to those of Si, and that in particular t he n-doped systems should be interesting from a technological point of view.