The Orbital Origins of Chemical Bonding in Phase-Change Materials

Layered phase-change materials in the Ge–Sb–Te-system are widely used in data storage and are the subject of intense research to understand the elusive quantum-chemical origin of their unique properties. To uncover the nature of the underlying periodic wavefunction, we study the interacting atomic orbitals including their phase information as revealed by crystal orbital bond index (COBI) and fragment crystal orbital (FCO) analysis. In full accord with previous and also new findings based on projected force constants (pFC), we demonstrate the decisive role of multicenter bonding along straight atomic connectivities such as Te–Ge–Te and Te–Sb–Te. While the here found multicenter bonding resembles well-established three-center four-electron bonding in molecules, its solid-state manifestation beyond a molecular motif leads to distinct longe-range consequences, thus serving to contextualize the aforementioned material properties usually termed “metavalent”. For example, we suggest multicenter bonding to be the origin of their astonishing bond-breaking and also phase-change behavior. As a hole-in-one, multicenter bonding immediately explains the too small “van der Waals” gaps between individual layers since multicenter bonding forces these gaps to shrink below the nonbonding Te–Te distances.

Comparison of DAFH and FALDI-like approaches

Two complementary methodologies for extracting useful insights into electronic structure and bonding from contemporary wavefunctions are compared. The first of these, known as the analysis of domain-averaged Fermi holes (DAFH), mostly provides visually appealing descriptions of the role and the extent of electron sharing in chemical bonding. The second one, known as the fragment, atom, localized, delocalized and interatomic (FALDI) charge density decomposition scheme, uses the partitioning of certain localization and delocalization indices to focus on highly visual contributions associated with individual domains and with pairs of domains, respectively. Four variants of a FALDI-like approach are investigated here in some detail, mostly to establish which of them are the most reliable and the most informative. In addition to ‘full’ calculations that use the correlated pair density, the consequences for the DAFH and FALDI-like procedures of using instead a popular one-electron approximation are explored. Additionally, the geometry dependence of the degree of acceptability of the errors that this introduces for delocalization indices is assessed for different formal bond multiplicities. The familiar molecular test systems employed for these various linked investigations are the breaking of the bonds in H2 and in N2, as well as the nature of the bonding in B2H6, as a simple example of multicenter bonding. One of the key outcomes of this study is a clear understanding of how DAFH analysis and a particular variant of FALDI-like analysis could be most profitably deployed to extract complementary insights into more complex and/or controversial bonding situations.

Stabilizing a metalloid {Zn12} unit within a polymetallide environment in [K2Zn20Bi16]6−

The access to molecules comprising direct Zn–Zn bonds has become very topical in recent years for various reasons. Low-valent organozinc compounds show remarkable reactivities, and larger Zn–Zn-bonded gas-phase species exhibit a very unusual coexistence of insulating and metallic properties. However, as Zn atoms do not show a high tendency to form clusters in condensed phases, synthetic approaches for generating purely inorganic metalloid Znx units under ambient conditions have been lacking so far. Here we show that the reaction of a highly reductive solid with the nominal composition K5Ga2Bi4 with ZnPh2 at room temperature yields the heterometallic cluster anion [K2Zn20Bi16]6–. A 24-atom polymetallide ring embeds a metalloid {Zn12} unit. Density functional theory calculations reveal multicenter bonding, an essentially zero-valent situation in the cluster center, and weak aromaticity. The heterometallic character, the notable electron-delocalization, and the uncommon nano-architecture points at a high potential for nano-heterocatalysis.

Influences of Multicenter Bonding and Interstitial Elements on Twinned γ-TiAl Crystal

The bonding properties of the twin boundary in polysynthetic twinned γ-TiAl crystal and the effect of interstitial alloy elements on it are investigated by first principles. Among the three different kinds of interface relationships in the γ/γ interface, the proportion of true twin boundaries is the highest because it has the lowest interfacial energy, the reason for which is discussed by local energy and three-center bond. The presence of the interstitial atoms C, N, H, and O induces the competition for domination between their affinity to host atoms and three-center bonds, which eventually influences the values of unstable stacking fault energy (USFE) and intrinsic stacking fault energy (ISFE). The relative importance of different bonding with different alloy elements is clarified based on the analysis of local energy combined with Electron Localization Function (ELF) and Quantum Theory of Atoms in Molecules (QTAIM) schemes.

Effects of structural variations on π-dimer formation: long-distance multicenter bonding of cation-radicals of tetrathiafulvalene analogues

Replacement of sulfur with selenium or insertion of ethylenedithio-substituents into tetrathiafulvalene cation-radicals increases the stability of the π-bonded dimers.