First-principles study of crystalline and amorphous AlMgB14-based materials

We report first-principles investigations of crystalline and amorphous boron and M1xM2yXzB14−z (M1, M2 = Al, Mg, Li, Na, Y; X = Ti, C, Si) phases (so-called “BAM” materials). Phase stability is analyzed in terms of formation energy and dynamical stability. The atomic configurations as well as the electronic and phonon density states of these phases are compared. Amorphous boron consists of distorted icosahedra, icosahedron fragments, and dioctahedra, connected by an amorphous network. The presence of metal atoms in amorphous BAM materials precludes the formation of icosahedra. For all the amorphous structures considered here, the Fermi level is located in the mobility gap independent of the number of valence electrons. The intra-icosahedral vibrations are localized in the range of 800 cm−1, whereas the inter-icosahedral vibrations appear at higher wavenumbers. The amorphization leads to an enhancement of the vibrations in the range of 1100–1250 cm−1. The mechanical properties of BAM materials are investig...