Pressure-Induced Electron-Deficient Multicenter Bonding: Exploring Unconventional Bonds in Pnictogens and Chalcogens

Chemical bonds are like the invisible threads holding materials together, quietly shaping how they act and perform. Researchers have relied before on familiar models such as covalent, ionic, and metallic bonds to explain how atoms interact and why materials behave the way they do. These models work well for many materials, however, when it comes to more advanced materials like phase-change materials (PCMs), bismuth, and compressed elements, such as arsenic and selenium, things get complicated. These special materials display unusual behaviors that don’t fit neatly into the old bonding categories. They conduct electricity in unexpected ways, handle heat in ways that seem almost counterintuitive, and reflect light in unique patterns. Take PCMs, for example. They’re fascinating because they can flip between crystalline and amorphous states, which makes these materials used in data storage, thermal energy management, and thermoelectric devices due to their exceptional properties, such as low band gaps and high thermoelectric efficiency; however, the in-depth reason behind what makes their bonds so special is still puzzling.

Scientists have suggested various theories such as resonant bonding and metavalent bonding, where electrons move freely in a partially delocalized system, while others talk about hypervalent bonding (also known as electron-rich multicenter bonding (ERMB)), where electrons are shared among three atoms instead of the typical two. These ideas help a little but fall short of fully explaining what’s happening. Resonant and metavalent bonding do not account for all the structural and vibrational quirks, and hypervalency doesn’t apply universally to electron-rich systems as previously supposed. This lack of understanding of how these bonds work limits our ability to design new materials with specific properties. To this account, a new research paper published in Journal of Material Chemistry C  and conducted by Hussien Osman and Professor Francisco Javier Manjón from the Universitat Politècnica de València together with Dr. Placida Rodríguez-Hernández and Professor Alfonso Muñoz from the Universidad de La Laguna and with Alberto Otero-de-la-Roza from the Universidad de Oviedo in Spain investigated a particular kind of bond: the electron-deficient multicenter bond (EDMB).

Using advanced quantum simulations, they investigated how these bonds form in elements like pnictogens and chalcogens, especially under compression. In their experiments, the researchers explored in particular arsenic, antimony, bismuth, selenium, tellurium, and polonium—and how they respond to extreme pressure. For instance, arsenic provided an intriguing and paradigmatic example because under normal conditions, its atoms arrange themselves in a trigonal pattern known as the A7 phase. But when pressure rises above 25 GPa, this structure transforms into a simple cubic arrangement called the A_h phase. This phase transition is not just about how atoms are positioned; it also changes the bonds that hold them together. The typical covalent bonds that define the A7 phase begin to break down as pressure increases, making way for something entirely different, the EDMBs. These bonds are unique because electrons are shared across multiple atoms, creating an unusual, highly flexible bonding network. The team confirmed these changes by mapping electron distributions using advanced computational tools. One of the most fascinating discoveries involved lone electron pairs (LEPs), which are often thought of as passive players in bonding that result in distorted structures with small atomic coordination due to the intrinsic LEP stereochemical activity. The researchers found that as pressure increased the LEPs changed their roles. While LEPs became much more involved in bonding interactions when ERMBs are formed, they lose their stereochemical activity and barely participate in bonding when EDMBs are formed. Moreover, the authors analyzed the atomic vibrations within these crystalline structures, known as phonon modes, to gather further evidence of pressure-induced EDMB formation. In arsenic, they observed that certain vibrational frequencies in the A7 structure softened under compression, signaling that traditional bonds were weakening due to the electronic charge transfer to newly formed bonds. On the other hand, they found that all vibrational modes became stiffer when the A_h phase was formed above 25 GPa, thus reflecting the dynamic balance between localized and delocalized electrons inherent to EDMBs. They also observed similar trends in other elements, including antimony, bismuth, selenium, and tellurium. It is noteworthy to mention that polonium stood out as a particularly important reference point. Unlike the other elements studied, polonium naturally forms an octahedral structure with EDMBs at normal pressure, which makes it an ideal candidate for studying these bonds. Its behavior helped the researchers draw broader conclusions and demonstrated how these unconventional bonds consistently emerge under extreme conditions.

In conclusion, the new study by Professor Francisco Javier Manjón and his colleagues advances our understanding of unconventional chemical bonding, especially in PCMs, which are important materials in data storage, thermal energy management, and thermoelectric devices. The ability to understand and predict EDMBs allows scientists to fine-tune these properties which opens the door to design materials that perform better and meet specific needs. This could lead to significant advancements in technologies where precise control over electronic and structural properties is essential. The study also highlights the role of pressure as a powerful tool for transforming materials. Under high-pressure conditions, bonds can shift, and entirely new phases of matter can emerge which offers exciting possibilities for innovation. Moreover, this ability to manipulate bonding interactions opens new opportunities for developing high-performance systems in quantum computing and optoelectronics, where unconventional bonding patterns might provide distinct advantages. Perhaps most importantly, the research work showcases the role of unconventional bonding in creating sustainable technologies because materials with EDMBs have moderate electrical conductivity, low thermal conductivity, and high energy efficiency which make them ideal for green energy applications.