A superlattice made of a topological insulator (Sb₂Te₃) and a normal insulator (ferroelectric GeTe), also called an interfacial phase‐change material, are switched between the Dirac semimetal and gapped phases by applying an external electric field, which provides a platform to study topological insulators and opens new possibilities for designing phase‐change memory devices.

For the case example of GeTe, we demonstrate that the use of techniques that probe the average structure may lead to misleading conclusions regarding the nature of phase change. GeTe is a narrow band-gap semiconductor and a ferroelectric with the simplest conceivable structure that\char22{}according to previous studies\char22{}undergoes a ferroelectric-to-paraelectric displacive phase transition at $\sim${}705$\hskip0.3em${}K. In this work, we provide direct experimental evidence that, contrary to the existing paradigm, the local distortion remains essentially unchanged with temperature and argue that the previous conclusion about the displacive nature of the ferroelectric-to-paraelectric transition was due to misinterpretation of Bragg diffraction, a technique that is only sensitive to the average structure and does not ``see'' random local distortions. The reported results have far-reaching implications for other materials exhibiting displacive phase transitions where conclusions have been reached based on results obtained using averaging techniques.

Through the use of first-principles Ge $K$-edge XANES simulations we demonstrate that the structure of melt-quenched amorphous Ge-Sb-Te is intrinsically complex and is a mixture of Ge(3):Te(3) and Ge(4):Te(2) configurations in comparable concentrations, in contrast to the as-deposited amorphous phase that is dominated by the Ge(4):Te(2) configurations. The reasons for Ge-Te polyvalency are discussed and it is argued that both configurations are compatible with the Mott 8–N rule and the definition of an ideal amorphous solid. The near-perfect Te-Te distance match between the two major configurations accounts for the high cyclability of phase-change materials. Stable compositions in the Ge-Sb-Te system are suggested.

Although GeTe-${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$ (GST) alloys are widely used in data storage, many fundamental issues are still under debate. Here, we demonstrate that the presence of vacancies in the crystalline phase has far-reaching consequences, namely, a triad of twofold coordinated Te atoms with lone-pair electrons generated around the vacancy enables the formation of soft three-center four-electron bonds, whose properties provide an explanation for the unusual characteristics of GST, in particular, the increase in local disorder upon crystallization, the co-existence of a very fast switching rate with a large property contrast, the possibility of a solid-solid amorphization process that excludes conventional melting, and the drastic difference in crystallization behavior between GST and the ideal binary GeTe. Anisotropy of the three-center bonds may serve as an additional degree of freedom for information recording and provide a unified explanation for a variety of unique effects observed in lone-pair semiconductors.

Nonvolatile memory, of which phase-change memory (PCM) is a leading technology, is currently a key element of various electronics and portable systems. An important step in the development of conceptually new devices is the class of van der Waals (vdW)-bonded GeTe/Sb₂Te₃ superlattices (SLs). With their order of magnitude faster switching rates and lower energy consumption compared to those of alloy-based devices, they are widely regarded as the next step in the implementation of PCM. In contrast to conventional PCM, where the SET and RESET states arise from the crystalline and amorphous phases, in SLs, both the SET and RESET states remain crystalline. In an earlier work, the superior performance of SLs was attributed to the reduction of entropic losses associated with the one-dimensional motion of interfacial Ge atoms located in the vicinity of Sb₂Te₃ quintuple layers. Subsequent experimental studies using transmission electron microscopy revealed that GeTe and Sb₂Te₃ blocks strongly intermix during the growth of the GeTe phase, challenging the original proposal but at the same time raising new fundamental issues. In this work, we propose a new approach to switching in SLs associated with the reconfiguration of vdW gaps accompanied by local deviation of stoichiometry from the GeTe/Sb₂Te₃ quasibinary alloys. The model resolves in a natural way the existing controversies, explains the large conductivity contrast between the SET and RESET crystalline states, is not compromised by Ge/Sb intermixing, and provides a new perspective for the industrial development of memory devices based on such SLs. The proposed concept of vdW gap reconfiguration may also be applicable to designing a broad variety of engineered two-dimensional vdW solids.