Site-different structures from dilithium hexaboride (Li2b6) to dimagnesium hexaboride (Mg2B6) by first-principles

The structural, mechanical, electronic and bonding properties of dilithium hexaboride (Li2B6) and isostructural hypothetic compounds obtained by replacing Li atoms in different sites to magnesium atoms have been investigated by first-principles density functional pseudopotential plane-wave calculations. It is shown that calculated lattice parameters of Li2B6 agree with the experimental results. All of designed hypothetical structures have negative formation enthalpies, thus all of them are thermodynamically stable and the most stable structure is Mg2B6. At the same time, from calculated single crystal elastic constants, it is shown that all structures are mechanically stable and related mechanical properties such as bulk, shear and Young moduli are calculated. It is shown that adding magnesium to the structure of Li2B6 is decreasing values of the moduli. Further, hardnesses of the structures are determined theoretically and it is obtained that hardness exhibits same trend with the moduli. From electronic structure calculations including band structure and site-dependent density of states, all structures are metallic, and fully magnesium substituted structure (Mg2B6) has the highest metallicity among the structures. Additionally, bonding nature of the structures are analyzed by using electron density maps, Mulliken atomic charges and bond overlap populations.