Ab initiostudy of icosahedral Al-Pd-Mn quasicrystals: Structural model, electric field gradients, and negative valence of Mn

The atomic and electronic structures of the Quandt-Elser model of i-Al70.8Pd21.5Mn7.7 [A. Quandt and V. Elser, Phys. Rev. B 61, 9336 (2000)] are studied by pseudopotential and full-potential linear augmented plane-wave methods. A model with a lower total energy than the original Quandt-Elser model (ΔE=-0.030.3em0exeV/atom) is obtained by moving one of the Al atoms on a cube in the Mackay void away from its high symmetry position and by performing a subsequent structural relaxation. The electronic density of states of this modified Quandt-Elser model is found to be similar to that of the model of Katz and Gratias, but with a more pronounced pseudogap near the Fermi energy. Good agreement is obtained with an available photoemission spectrum, but calculated electric field gradients do not reproduce the angular dependence of a ²⁷Al nuclear magnetic resonance spectrum. By introducing vacancies at Mn sites and by comparing the filling of bands near the Fermi energy for the models with and without a vacancy, it is found that the Mn atoms have an apparent negative number of -3.2 valence electrons independent of the Mn site. It is argued that this apparent negative number of valence electrons is due to the filling of Mn-d states below the Fermi energy.