How to Identify Lone Pairs, Van der Waals Gaps, and Metavalent Bonding Using Charge and Pair Density Methods: From Elemental Chalcogens to Lead Chalcogenides and Phase‐Change Materials

Abstract

Lone pairs explain the structure of many molecular solids, as well as the chain-like or layered structures encountered in many chalcogenide crystals. Such chalcogenides have enabled a plethora of applications, including phase change memories, thermoelectrics, topological insulators or photoconductors. In many of these solids, lone pairs also have been invoked to explain the unconventional material properties. The presence of so-called van der Waals gaps in several layered chalcogenides, as well as their low thermal conductivity have also been linked to lone pairs. However, for some of these systems, a second, presumably competing view of bonding has been proposed, where atoms are held together across the interlayer spacing by shared electrons. To clarify this situation, we reinvestigate several systems theoretically, in which the role of lone pairs has been frequently emphasized. By comparing the charge and electron localization analysis in terms of a Hartree-Fock like pair density obtained from Kohn Sham DFT, we verify that the structure of several chalcogenides is governed by the presence of lone pairs, while others are not. As an example, crystalline Se is demonstrated to form a structure, where two covalent bonds and a lone pair are present, whereas three metavalent bonds and no lone pairs are the essential characteristics of crystalline Sb, crystalline Te being an intermediate case.

The Orbital Origins of Chemical Bonding in Ge−Sb−Te Phase‐Change Materials**

“…This is likely connected to the break in the multicenter atomic sequence happening at the end of the layer, with two subsequent Te atoms instead of the usual alternation, resulting in an electronically different four‐center interaction. There is significant electronic localization at the gap with evidence for the presence of lone‐pairs, [41e] making the presence of the strong force constants and ICOBI even more unusual.…”

Mind the Mines: Lapieite Minerals with Ultralow Lattice Thermal Conductivity and High Power Factor for Thermoelectricity

“…51 The partial density of states (PDOS) in Figure 3a shows that mainly Cu (d)/Q (p) and then Bi (p) states dominate the VB and CB, respectively, highlighting their importance in the chemistry of lapieites. Previous studies 67,69 reveal that the so-called metavalent bonding plays a vital role in the lattice anharmonicity, which was also reported for BiCuQO (Q = Se and Te) as the importance of the soft Cu−Q bond. Weak Cu−Q (also Bi−Q) bonds in lapieites link the segments (see Figure 1) along the c-axis where the ultralow κ L value is obtained.…”

“…The LEP, however, is not the only source of anharmonicity in solids; conversely, sometimes its role in the anharmonicity of solids becomes minor compared with that of chemical bonding. − As already discussed, the soft Cu–Q bondssee the preceding discussion on chemical bondinglead to an anharmonic rattling vibration and low κ L in Cu-based compounds , such as BiCuQO (Q = Se and Te) and CuBr . The partial density of states (PDOS) in Figure a shows that mainly Cu (d)/Q (p) and then Bi (p) states dominate the VB and CB, respectively, highlighting their importance in the chemistry of lapieites.…”

Local Symmetry Breaking Suppresses Thermal Conductivity in Crystalline Solids

“…It is also important to note that there is a role of the possible metavalent nature of bonding in AgSbSe 2 as predicted by quantum mechanical descriptors like electron‐shared to electron‐transferred ratio [35] . However, detailed investigation is required to understand up to what extent and how the metavalent bonding [30c, 35, 36] affects the thermal transport properties in AgSbSe 2 and could be the way forward.…”