Lithium-richest phase of lithium tetrelides Li₁₇TT₄ (TT = Si, Ge, Sn, and Pb) as an electride

The lithium-richest phase in the binary Li-Tt system (Tt = Si, Ge, Sn, and Pb) has a stoichiometry of Li₁₇Tt₄. In the beginning of this paper, the structural complexity of Li₁₇Tt₄ is gradually stripped away using the concept of the M26 cluster found in γ-brass structures and a Tt-centered polyhedral representation. By means of the first-principles electronic structure calculations, which are followed by the analyses of the electron localization function (ELF), Bader charges, and spin density, we observe non-nuclear maxima of the ELF, electron density, and spin density. Since the electron densities off the atoms are confined in crystalline voids, separated from each other, and behaving as an anion, Li₁₇Tt₄ can be identified as a potential zero-dimensional electride. This finding agrees with a simple Zintl picture, which suggests a valence electron count of [(Li⁺)₁₇(Tt⁴¹)₄¢e¹]. Detailed analyses on the band structures, the projected density of states, and crystal orbitals at the ¥ point in the reciprocal space hint at the potential of forming a bond between the non-nuclear electron density and the neighboring atoms. Signatures of bonding and anti-bonding orbital interactions can be witnessed.