Baking sodium out of a boride leaves a porous boron 10 million times more conductive than common boron

Chemists had tried for more than a decade to make a plastic, highly conductive form of elemental boron, which until now existed only in theory and first-principles calculations, according to the phys.org account of the work ^[6]. The standard synthesis is a single high-pressure, high-temperature step, and it forces boron atoms into dense, tightly packed crystals ^[7]. The way to an open framework is to build it around temporary guest metal atoms and then drive the guests out. That extraction often failed. Boron is electron-deficient and bonds tightly to metals, so the guests stayed put ^[8].

The new route fixed the stage before extraction. Earlier work had produced only tiny, low-quality sodium boride crystals, and degassing them was difficult ^[9]. Adding zinc interlayers during growth gave large crystals in which the sodium sat inside open channels running between interconnected boron cages ^[9]. Under vacuum, the sodium left through those channels and the framework stayed intact ^[10].

The conductivity figures in the account are internally consistent. It gives roughly 900 siemens per metre at room temperature, about seven orders of magnitude above standard rhombohedral boron ^[11]. Ten million is seven orders of magnitude, so the headline ratio and the measurement agree ^[4]. The ratio is large partly because the baseline is so low. Dividing 900 by ten million puts common boron at about 0.00009 siemens per metre ^[15], the conductivity of a poor conductor with a bandgap above 1.5 eV ^[5]. Imma-B60's gap is under 0.2 eV ^[4].

The deformation figures are less tidy. Early in the account, the framework deforms by 23% without shattering ^[3]. Later, compression tests on nanopillars reach about 32% strain without fracture ^[12]. They are the same two digits in reverse order ^[3][12]. The account does not reconcile them or describe tests on anything larger than a nanopillar. It does describe how the material gives: high-resolution imaging showed dislocation-mediated slip, with atomic planes sliding smoothly past one another under stress ^[13].

Conventional elemental boron is still superhard, brittle under stress and a poor conductor ^[5]. Imma-B60 behaves differently because its atoms sit in an open framework of 12-atom cages linked by 3-atom triangular units ^[2]. Making it confirms what calculations had proposed for years ^[6]. The researchers believe it could be a foundation for designing mechanically resilient, functional inorganic materials ^[14].