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.

Probing the Validity of the Zintl−Klemm Concept for Alkaline-Metal Copper Tellurides by Means of Quantum-Chemical Techniques

Understanding the nature of bonding in solid-state materials is of great interest for their designs, because the bonding nature influences the structural preferences and chemical as well as physical properties of solids. In the cases of tellurides, the distributions of valence-electrons are typically described by applying the Zintl−Klemm concept. Yet, do these Zintl−Klemm treatments provide adequate pictures that help us understanding the bonding nature in tellurides? To answer this question, we followed up with quantum-chemical examinations on the electronic structures and the bonding nature of three alkaline-metal copper tellurides, i.e., NaCu3Te2, K2Cu2Te5, and K2Cu5Te5. In doing so, we accordingly probed the validity of the Zintl−Klemm concept for these ternary tellurides, based on analyses of the respective projected crystal orbital Hamilton populations (−pCOHP) and Mulliken as well as Löwdin charges. Since all of the inspected tellurides are expected to comprise Cu−Cu interactions, we also paid particular attention to the possible presence of closed-shell interactions.