Theoretical estimation of the electric-field gradient in amorphous alloys

In order to explain recent experimental data, electric-field-gradient (EFG) calculations are performed for amorphous ${\mathrm{Zr}}_{70}$ ${\mathrm{Cu}}_{30}$ and its closer crystalline compound c-${\mathrm{Zr}}_2$Cu. To be able to calculate the EFG of non-axially-symmetric systems, we set up a scheme of approximations within a linear combination of atomic orbitals description of d electrons. The model used for the atomic structure of the amorphous alloy is a distribution of hard spheres relaxed by molecular dynamics, constructed in a previous work. The information on electron distributions required for the EFG calculation is obtained by direct diagonalization of a parametrized tight-binding Hamiltonian of the cluster with periodic boundary conditions. We found that, for the EFG at the Zr sites, local and lattice contributions are of the same order. An excellent agreement with experimental results is obtained, which supports both the structural model for the amorphous phase and the proposed set of approximations used in present calculations.