Elettra | Xpress Pressure induced electronic topological transitions in BiTe driven by metavalent bonding The search for new quantum materials has become a cornerstone of modern materials science, offering the potential to revolutionize technologies ranging from spintronics and quantum computing to energy conversion like in thermoelectric devices. Among these, dual topological insulators have garnered considerable interest. These are exotic materials that host two different types of symmetry-protected electronic states simultaneously—both on their surface states, offering unusual transport properties. BiTe, a compound composed of bismuth (Bi) and tellurium (Te), represents one such dual topological insulator. Structurally, BiTe is quite unique. It consists of naturally stacked layers— quintuple layers of Bi and Te atoms alternating with Bi₂ bilayers. This structure forms a natural van der Waalslike heterostructure, where weakly bonded layers stack together in a way that allows for directional flexibility, making the system highly responsive to external stimuli like pressure. In this work, researchers set out to explore how the chemical bonding and electronic structure of BiTe evolve under the application of pressure. The focus is on a special kind of chemical bonding known as metavalent bonding. Unlike traditional covalent bonding, metavalent bonding is more delocalized, sitting somewhere in between metallic and covalent bonding. Atoms in metavalent systems share electrons in a diffuse, collective manner that leads to large polarizabilities, low thermal conductivities, and enhanced electronic tunability. These characteristics make such materials promising for both fundamental research and real-world applications. To probe the changes in BiTe under pressure, the team combined highpressure synchrotron x-ray diffraction experiments (Fig. 1 a) with Raman spectroscopy and advanced theoretical calculations based on density functional theory (DFT). Two critical pressure points—at approximately 1 and 3 gigapascals (GPa)—were identified where BiTe undergoes electronic topological transitions (ETTs) (Fig. 1 b). These transitions signify a change in the shape and connectivity of the electronic states at the Fermi level—the threshold energy that separates filled and empty electronic states. Such changes can dramatically alter a material’s electrical and thermal properties, even without changing its overall crystal structure. Unlike structural phase transitions that are readily visible through a change in symmetry or atomic arrangement, ETTs are subtler. They are often described as “Lifshitz transitions” and are characterized by sudden changes in the Fermi surface (Fig. 1 c)—the map of electron momentum states. In this study, these changes were not only detected through synchrotron x-ray diffraction and Raman shifts, but also visualized in DFT calculations, offering a comprehensive picture. One of the central findings is the link between BiTe’s chemical bonding and its susceptibility to these topological transitions. The insertion of Bi₂ bilayers into the BiTe structure weakens the traditional Bi–Te bonds within the quintuple layers. This weakening softens the lattice, as evidenced by shifts in Raman phonon modes and reduced sound velocity, making the structure more compressible along the stacking direction. As a result, BiTe undergoes ETTs at much lower pressures than Bi2Te3, its close structural cousin that lacks Bi₂ bilayers. Further theoretical and experimental analysis revealed that with increasing pressure, the bonding in BiTe strengthens—but not in the conventional sense. Rather, the pressure enhances its metavalent character (Fig. 1 d). Measurements of charge distribution, Born effective charges, and dielectric properties showed increased electron sharing and polarizability under pressure. These changes directly correlate with the observed ETTs, suggesting that metavalent bonding is not just a passive background feature but an active driver of topological transitions in this system. Visualizations of the Fermi surface provided clear evidence: as pressure increases, certain electron pockets in the momentum space appear, disappear, or merge. At 1 GPa, a new electron pocket forms near the L point of the Brillouin zone, marking the first ETT. By 3 GPa, another rearrangement occurs in the Fermi surface, signifying the second ETT. These electronic transitions were in excellent agreement with changes observed in x-ray diffraction and Raman measurements under high pressure. Notably, the study did not detect any signature of topological quantum phase transitions (TQPTs)—ruling out more dramatic topological changes and instead highlighting the nuanced role of chemical bonding and pressure. What makes this work especially significant is finding the correlation between nature of chemical bonding and the evolution of electronic topology. In BiTe, metavalent bonding not only defines the ground-state electronic properties but also guides how those properties respond to external perturbations like pressure. By modifying how electrons are shared between atoms, the system’s electronic topology can be tuned—offering a promising route to designing next-generation quantum materials. In summary, this research shows that BiTe, a naturally layered dual topological insulator, is a model system in which subtle changes in bonding—especially under pressure—can lead to profound electronic transitions. The work highlights how metavalent bonding enhances both the softness and responsiveness of the material, enabling topological transitions at relatively low pressures. This offers a blueprint for using pressure as a tuning knob to explore and control electronic topology in complex quantum materials and opens the door to new strategies in quantum device design for energy conversion application. Elettra Highlights 2024–2025 32
RkJQdWJsaXNoZXIy MjQ4NzI=