Atomic and electronic structure of crystalline and amorphous alloys. II. Strong electronic bonding effects in Ca-Al compounds

Abstract

Calculations of the atomic and electronic structure of ${\mathrm{Ca}}_{\mathrm{x}}$ ${\mathrm{Al}}_{1\mathrm{-}\mathrm{x}}$ glasses (with x=0.70, 0.60, 0.50, and 0.33) and of the crystalline intermetallic compound ${\mathrm{CaAl}}_2$ are presented. For the amorphous alloys the calculations are based on realistic models for the atomic structure constructed by a molecular-dynamics simulation linked to a steepest-descent potential-energy mapping. The effective interatomic forces are calculated using pseudopotential theory. We find that both the atomic and the electronic structures are dominated by strong electronic bonding effects: (a) The strong (s,p,d) hybridization of the free-electron-like conduction band of pure Al is broken up on alloying with Ca, and we find nearly separate Al 3s and Al 3p bands which are much narrower than the s and p bands in pure Al; (b) only the Al 3p states interact substantially with the Ca states. The interatomic electron transfer is small, but we find a substantial intra-atomic d-to-s transfer on the Al sites and an s-to-d transfer on the Ca sites. In the atomic structure the s-d promotion leads to a strong contraction of the Ca-Ca bonds in both the crystalline and the amorphous alloys compared to pure Ca, and a preferential Ca-Al bonding in the Al-rich but not in the Ca-rich alloys. The calculated electronic structure is well confirmed in all its details by heat-capacity, photoemission, and soft-x-ray emission spectra. The x-ray-diffraction data for the atomic structure corroborate the predicted compression of the Ca-Ca distances and the overall form of the correlation functions (which points to a local topology best described as a disordered tetrahedral close packing) but show distinctly lower and broader peaks. We argue that this is due to the short mean free path of the electrons, which will lead to a damping of the oscillations in the interatomic interactions.