The electron-momentum-density distributions (Compton profiles) in decagonal Al72Ni12Co16 have been measured along the  and  directions with a momentum resolution of 0.14 au. The experimental valence-electron profile is decomposed into two partial profiles: an inverted parabola-like profile and a broad Gaussian-like profile. By fitting the broad partial profile with a profile made up of atomic 3d wavefunctions, it is found that a considerable portion of the 3d-electron holes of Ni and Co are filled with electrons. The directionally averaged radius of the Fermi sphere is estimated from the number of electrons under the parabolic partial profile. The radius is larger than that of the inscribed sphere of the quasi-Brillouin zone. This leads to the Hume-Rothery mechanism does not play an important role in explaining the stability of the decagonal quasicrystal. Slight anisotropies of the Compton profile are observed.
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