Self-consistent augmented-plane-wave electronic-structure calculations for the A15 compounds V3X and Nb3X, X = Al, Ga, Si, Ge, and Sn

We have performed self-consistent (SC) band structure calculations for the A15 compounds V3X and Nb3X, X = Al, Ga, Si, Ge, and Sn, using the augmented-plane-wave (APW) method. Relativistic effects (except the spin-orbit interaction) have been included in each SC cycle, along with corrections to the usual muffin-tin approximation. The latter apply the APW wave functions outside of the muffin-tin spheres to compute the interstitial charge densities and potentials. The resulting interstitial potential has full cubic symmetry (no spherical averaging), although a spherically averaged muffin-tin form is retained inside the spheres. The final SC potentials were used to generate energies and wave functions on a cubic mesh of 35 k points in 1/48th of the Brillouin zone. These results were interpolated onto a finer mesh of 969 k points using a symmetrized Fourier method; the densities of states (DOS), N (E), were determined using tetrahedral integration. These accurate interpolation methods allow us to determine the DOS on a fine energy scale ( +- 3 mRy) around the Fermi level E/sub F/, where we find large variations for the compounds V3Ga, V3Si, and Nb3Sn. This correlates well with the fact that these compounds have high superconducting transition temperatures (T/sub c/) and anomalous electronically derived properties. The energy bands of the A15 materials exhibit significant variations even amongst the isoelectronic compounds, both as the band shapes and positions of E/sub F/. All compounds possess very flat bands that evolve from the GAMMA12 state near E/sub F/ and give rise to the sharp peaks in N (E). For V3Ga, V3Si, and Nb3Sn, E/sub F/ falls within several mRy of GAMMA12 so that these flat bands are responsible for the sharp structure in N (E) at E/sub F/. We find that Nb3Ge and Nb3Si have relatively low values of N (E/sub F/), which suggests that the high T/sub c/'s observed and predicted are due to unusual mechanisms