Probing the Zintl–Klemm Concept: A Combined Experimental and Theoretical Charge Density Study of the Zintl Phase CaSi

Abstract

The nature of the chemical bonds in CaSi, a textbook example of a Zintl phase, was investigated for the first time by means of a combined experimental and theoretical charge density analysis to test the validity of the Zintl-Klemm concept. The presence of covalent Si-Si interactions, which were shown by QTAIM analysis, supports this fundamental bonding concept. However, the use of an experimental charge density study and theoretical band structure analyses give clear evidence that the cation-anion interaction cannot be described as purely ionic, but also has partially covalent character. Integrated QTAIM atomic charges of the atoms contradict the original Zintl-Klemm concept and deliver a possible explanation for the unexpected metallic behavior of CaSi.

Polar‐Covalent Bonding Beyond the Zintl Picture in Intermetallic Rare‐Earth Germanides

“…The CaGe structure contains one‐dimensional Ge zig‐zag chains consistent with the formal (2b)Ge 2− picture. It is isotypic to CaSi, for which Ca−Si partially covalent bonds were identified on the basis of the electron density topology (QTAIM method) and discussed to lead to a significant deviation from the idealized Zintl picture . In this respect it is not unimportant to know, that both CaSi and CaGe display metallic character with only a pseudo‐gap found in the band structure at the Fermi level.…”

Solvate‐Induced Semiconductor to Metal Transition: Flat [Bi1−] Zigzag Chains in Metallic KBi⋅NH3 versus [Bi1−] Helices in Semiconducting KBi

“…The Zintl phase CaSi is a text book example of an electron‐precise Zintl phase with sulfur analogue two‐connected “Si 2− ” anions, which however, because of the planar conformation of the polyanion, is metallic. Experimental electron density measurements show that Ca orbital contributions play an important role for the planarity of the Si zigzag chain (in contrast to the expected helical chain) enforcing the metallic properties . However, the Zintl concept does not allow specific conformers of polyanions to be predicted since the role of the counter ions is neglected at first glance .…”

“…Experimental electron density measurements show that Ca orbital contributions play an important role for the planarity of the Si zigzag chain (in contrast to the expected helical chain) enforcing the metallic properties . However, the Zintl concept does not allow specific conformers of polyanions to be predicted since the role of the counter ions is neglected at first glance . An exemplary variation of structure–property relations already arises in the elements of the pnictogen ( Pn ) group, which embraces the whole spectrum of element types, reaching from the clear non‐metal nitrogen through phosphorous that adapts non‐metallic and metallic modifications, to the semimetals arsenic and antimony and finally to bismuth that is best described as semi‐metal with zero band gap .…”

“…The metallic CaSi is iso(valence)electronic to the semiconductor KBi, which, as mentioned before, contains helical chains. This discrepancy might be explained by an effect of a partly covalent bonding of the Ca atoms to the delocalized π system of the Si zigzag chains …”

“…[3] However, the Zintl concept does not allow specific conformers of polyanions to be predicted since the role of the counter ions is neglected at first glance. [3,4] An exemplary variation of structure-property relations already arises in the elements of the pnictogen (Pn) group, which embraces the whole spectrum of element types, reaching from the clear non-metal nitrogen through phosphorous that adapts non-metallic and metallic modifications, to the semimetals arsenic and antimony and finally to bismuth that is best described as semi-metal with zero band gap. [5] The various allotropes of the heavier elements exhibit a huge diversity in their structures, and their affinity to form localized bonds is mirrored in the different cages of polycyclic polyanions, with [Pn 4 ] 2À , [Pn 7 ] 3À , [Pn 11 ] 3À , or [Pn 14 ] 4À as typical examples (Pn = P, As, Sb, Bi).…”