Crystal structures and electronic properties of BaC₂ isomers by theoretical study based on DFT

Band structures and electronic properties of two BaC₂ isomers were calculated by using density functional theory (DFT) properly. The ionic bond features are all typical between cation (Ba) and anion clusters (C ₂) in both structures of the isomers. However, a much stronger covalent bond exists in anion clusters which can be seen by inspecting the electron distribution contour that has a dull bell like shape between two carbon atoms. The shortest distance between Ba²⁺ and C ₂ ^2- and the bond length in anion clusters are different in these isomers of BaC₂, which are 0.2945 nm and 0.1185 nm for the structure with the I4/mmm space group and 0.2744 and 0.1136 nm with the C2/c type, respectively. Band structures were clarified by combining the DOS to indicate the ionic bonding features more clearly. Population analysis provided further evidence on these ideas. Thermodynamical calculation results reveal that the transition temperature of these two polymorphs of BaC₂ locates near 132 K, which is consistent with the recent experimental results.