Phase transformations in boron under shockwave compression

Boron is the only pure element whose phase diagram remained elusive until the last ~decade, due, in part, to challenges associated with isolating pure boron and the influence of impurities on its phase diagram. Boron is found in several important applications including as a building block in superhard materials, in dielectrics, corrosion-resistant alloys, and nuclear reactor materials, and it has recently-discovered superconducting properties. Boron exhibits a rich phase diagram with at least 6 reported phases below 150 GPa and 4000 K.^1-4 Boron polymorphs are composed of arrangements of B₁₂-icosahedra and B₂-“dumbbell” structures. At modest pressures and temperatures, a superhard phase gamma-B₂₈ was recently discovered which is a NaCl-lattice structure of B₁₂-icosahedra bridged by B₂ “dumbbells”. Above 74 GPa, theoretical calculations have also predicted a alpha-Ga metallic phase.⁴ Very little is known experimentally about boron’s phase transformations under dynamic loading. Only a single literature report of its principal Hugoniot exists from the LASL compendium of Marsh.⁴ A large degree of scatter in the experimental shock data is likely indicative of multiple solid-solid phase transformations. Here, we will report the results of recent gas gun-driven plate impact experiments on boron using a 50 mm-bore two stage light gas gun at LANL, and impact facilities at the Dynamic Compression Sector. These experiments, performed in a transmission geometry on ~ 1 mm thick samples, aimed to repeat the historical shockwave experiments with modern velocimetry diagnostics, and confirm the gamma-B₂₈ and alpha-Ga structures by time-resolved x-ray diffraction. Data on related boron-containing compounds, such as B₄C will also be presented for context.

¹A. R. Oganov and V. L. Solozhenko, J. Superhard Mat. 31, 285 (2009). ²S. Zhang, H. D. Whitley, T. Ogitsu, Solid State Sci. 108, 106376 (2020). ³K. Shirai, Jpn. J. Appl. Phys. 56, 05FA06-1 (2017). ⁴A. R. Oganov et al. Nature 457, 863, (2009). ⁵LASL Shock Compendium, S. P. Marsh (Ed.), University of California Press (1980).