The atoms of the 5th element of the periodic table often find themselves in the company of each other, forming allotropes with a rich variety of structural motifs, each carrying a unique set of chemical and physical properties. Despite the long catalog, most boron allotropes do not simultaneously possess high electrical conductivity and plasticity, but a recent study has expanded the portfolio.
Scientists have now designed a new allotrope called Imma-B60 by washing out the sodium from the sodium boride compound Na4B60. Their findings are published in Nature Chemistry.
Unlike the dense, tightly packed atomic arrangements found in standard forms of elemental boron, Imma-B60 forms a porous open framework built from 12-atom boron cages connected by 3-atom triangular boron units. This unique structure shifts internally under stress, allowing the allotrope to be flexible and deform by 23% without shattering. Imma-B60 also conducts electricity 10 million times better than the common form of boron, thanks to its very narrow bandgap of under 0.2 eV.
Going beyond brittle boron
Boron is known to wear multiple hats. Sometimes it plays a key role in semiconductor technology; other times, because of its unusually strong neutron-scattering ability, it has also become indispensable in neutron-scattering research and nuclear applications.
The structural properties that make it useful also bring certain drawbacks. For instance, conventional forms of elemental boron are naturally superhard, brittle under mechanical stress, and are poor electrical conductors with wide bandgaps above 1.5 eV.
For years, a plastic and highly conductive form of elemental boron existed only on paper, a promising idea confined to theories and first-principles calculations. Actually making it has stumped chemists for over a decade. The usual route for synthesizing boron allotropes relies on one-step high-pressure, high-temperature methods, which naturally push boron atoms into dense, tightly packed crystals.
For open frameworks, scientists theorized constructing a precursor scaffold around temporary guest metal atoms, then baking the metal away. Even that often failed because boron, being electron-deficient, fiercely bonds with metal atoms, making that guest extraction step stubbornly difficult.
A new kind of boron
In this study, researchers found a way to overcome the obstacles. They started with sodium boride (Na4B60) as a scaffold for constructing the boron allotrope.
Earlier studies could only produce tiny, low-quality sodium boride crystals, which made degassing—an essential step for allotrope formation—rather difficult, so the researchers introduced zinc interlayers during the formation of sodium boride. The zinc enabled the growth of large crystals in which sodium atoms sat inside open channels formed by interconnected boron cages.
They then placed the sodium boride crystals in a vacuum furnace and baked them at 900°C for two days. The vacuum pulled the sodium atoms out through the open structural channels, leaving behind the intact framework of Imma-B60, a new, pure form of elemental boron.
Imma-B60 behaved like a narrow-bandgap semiconductor, conducting electricity at room temperature about seven orders of magnitude higher than standard rhombohedral boron, reaching roughly 9×102 S m-1. It also turned out to be more plastic than expected, as compression tests on Imma-B60 nanopillars showed the material can achieve about 32% strain without fracturing.
High-resolution imaging revealed that its bendiness is possible due to a dislocation-mediated slip mechanism in which atomic planes slide smoothly past each other under stress.
The new two-step scaffolding route gives a new material made entirely of boron, yet with properties unlike those of its allotropic counterparts. The researchers believe that Imma-B60 could provide a foundation for designing mechanically resilient, functional inorganic materials, opening up possibilities for boron well beyond its conventional semiconducting phases. https://phys.org/news/2026-09-flexible-boron-million-electrically.html
Recent Comments