The nature of chemical bonding in telluride-based 'phase-change' memory (PCM) materials (e.g. Ge-Sb-Te), that form the basis of new electronic and optical non-volatile memory (NVM) technology, remains controversial. In these NVM devices, the material is switched ultra-rapidly (~ns) and reversibly between metastable low electrical-conductance/optical-reflectivity amorphous/glassy states {0} and high electrical-conductance/optical-reflectivity crystalline states {1} upon the application of appropriate voltage/optical pulses. The origin of this remarkable behaviour, viz. the large opto-electronic property contrast between amorphous and crystalline states and the ultra-rapid crystallization process, can ultimately be traced to the nature of the chemical bonding in the two metastable phases. The bonding is not simply and entirely two-centre/two-electron (2c/2e) covalent-like in either crystalline or amorphous phases, but precisely what is its nature remains contentious.

In this talk, I will describe results of calculations undertaken on models of PCM telluride, and non-PCM sulphide and selenide, chalcogenide materials obtained by density-functional-theory (DFT)-based, ab initio molecular-dynamics (AIMD) simulations. These provide evidence that the bonding in crystalline tellurides is mostly, and in amorphous tellurides is partially, multi-centre and electron-rich in nature [1]. One way of understanding this is in terms of 'hyperbonding', e.g. involving the three-centre/four-electron (3c/4e) bonding interaction between p-like lone pairs on Te atoms and antibonding states associated with neighbouring Ge-Te, Sb-Te or Ge-Sb bonds [2-4]. Hyperbonding is prevalent in tellurides but very much less so in other chalcogenides, viz. selenides or particularly sulfides. Hyperbonded configurations have characteristic near-linear triatomic geometries, as in defective-octahedral 4-coordinated 'seesaw' entities. The high degree of electronic polarizability of hyperbonds is responsible for the opto-electronic property contrast between predominantly hyberbonded atoms in the crystalline state and predominantly covalently-bonded atoms in the amorphous state. 

[1] The myth of “metavalency” in phase-change materials. R.O. Jones, S.R. Elliott and R. Dronskowski, Adv. Mat. Perspective 35, 2300836 (2023)

[2] Chemical bonding in chalcogenides: the concept of multi-centre hyperbonding. T-H. Lee and S.R. Elliott, Adv. Mat. 32, 2000340 (2020)

[3] Multi-centre hyperbonding in phase-change materials. T.H. Lee and S.R. Elliott, Phys. Stat. Sol. – Rapid Res. Lett. 15, 2000516 (2021)

[4] Hypervalency in amorphous chalcogenides. T.H. Lee and S.R. Elliott, Nat. Comm. 13, 1458 (2022)