Chemists Make a Bendable Form of Boron That Conducts Electricity 10 Million Times Better
The new allotrope, Imma-B60, was predicted on paper for more than a decade. Researchers finally built it by baking the sodium out of a boron crystal.
Boron is usually a stubborn material. Its common forms are superhard, shatter under stress and barely conduct electricity. Now chemists have made a version that does the opposite on all three counts, a pure boron crystal that bends and conducts electricity about 10 million times better than ordinary boron.
The new form, called Imma-B60, is described in a study published in Nature Chemistry by a team led by Feng Chen. Chemists call different structural forms of the same element allotropes, the way carbon can be diamond or graphite. Boron has many, but none had combined high conductivity with the ability to deform without breaking. Theorists had predicted such a form for more than a decade using computer calculations, yet no one could make it.
The obstacle was the usual recipe. Boron allotropes are normally made in one step under high pressure and high temperature, which crushes the atoms into dense, tightly packed crystals. An open, porous structure requires a different approach: build the boron framework around temporary guest metal atoms, then remove the guests. That second step kept failing because boron, which is short of electrons, clings fiercely to metals.
The team found a way through. They started with sodium boride, Na4B60, and added zinc layers during crystal growth so the crystals came out large and clean, with the sodium atoms sitting inside open channels running between cages of boron. They then baked the crystals in a vacuum furnace at 900 degrees Celsius for two days. The vacuum drew the sodium out through the channels and left behind an intact framework made only of boron.
That framework is built from 12-atom boron cages linked by three-atom triangles. It behaves as a narrow-bandgap semiconductor, with a bandgap under 0.2 electron volts compared with more than 1.5 for conventional boron. The bandgap is the energy an electron needs to start carrying current, so a smaller one means much easier conduction. At room temperature the material reached roughly 900 siemens per meter, about seven orders of magnitude above standard rhombohedral boron.
The mechanical results surprised the researchers too. Compression tests on tiny pillars of the material showed it could take about 32% strain without fracturing. High-resolution imaging showed why. Under stress, planes of atoms slide smoothly past one another, a process known as dislocation-mediated slip that is common in metals but rare in brittle materials like boron.
Boron already matters in semiconductors, and its unusually strong interaction with neutrons makes it essential in nuclear technology and neutron research. A form that is tough, flexible and conductive could widen that list. The authors say the two-step scaffolding method could serve as a template for designing other mechanically resilient inorganic materials, and that Imma-B60 opens possibilities for boron well beyond its traditional semiconducting forms.
