Unusual Electronic and Bonding Properties of the Zintl Phase Ca_5Ge_3 and Related Compounds. A Theoretical Analysis

摘要

Theoretical reasons for metallic behavior among diverse Zintl phases have generally not been pursued at an advanced level. Here, the electronic structure of Ca_5Ge_3 (Cr_5B_3 type), which can be formulated (Ca~(+2))_5(Ge_2~(-6))Ge~(-4) in oxidation states, has been explored comparatively by means of semiempirical and first-principles density functional methods. The FP-APW calculations show that alkaline-earth-metal and germanium orbitals, particularly the d orbitals on the cations and the p-π~* orbitals of the halogen-like dimeric Ge_2~(-6), mix considerably to form a conduction band. This covalency perfectly explains the unusual metallic properties of the nominally electron-precise Zintl phase Ca_5Ge_3 and its numerous relatives. Similar calculational results are obtained for Sr_5Ge_3, Ba_5Ge_3, and Ca_5Sn_3. Cation d orbitals appear to be a common theme among Zintl phases that are also metallic.