Because of its low mass, high strength, and response to neutron irradiation, boron has important applications in technology, including nuclear engineering and in extreme environments. The recent work builds on the discovery of superconductivity of boron in 2001 by researchers at the Carnegie Institution’s Geophysical Laboratory. That study revealed superconductivity with a relatively high transition temperature for an element, but the underlying structure and mechanism have remained puzzles. The current study is an important step toward understanding the transition to superconductivity in boron under pressure.
The research team included theorists and experimentalists using a broad range of high-pressure methods, subjecting materials to pressures above 120,000 atmospheres and temperatures above 1,400 degrees Celsius (approximately 2550 degrees Fahrenheit).
The complex structures involved and the few electrons in the boron atom pose difficulties for experimentalists and the structural complexity of the compound required the application of new theoretical structure prediction techniques, pioneered by lead author Artem Oganov of Stony Brook University and Moscow State University. Furthermore, together with Carlo Gatti from the University of Milano, Oganov found that chemical bonding has a significant ionic component, and this creates unusual physical properties.
Crucial to the characterization of the novel compound was infrared spectroscopy, which was carried out at the high-pressure synchrotron infrared beamline at the National Synchrotron Light Source run by the Geophysical Laboratory of the Carnegie Institution since 1992. This part of the work was led by Carnegie beamline scientist Zhenxian Liu, working with Jiuhua Chen, at Florida International University. To demonstrate the reproducibility of the results, samples of the new phase were synthesized at Stony Brook University by Jiuhua Chen, the University of Paris XIII by Vladimir Solozhenko, and the Geophysical Laboratory by former Carnegie post-doctoral fellow, Yanzhang Ma of Texas Tech University.
High pressure causes otherwise identical boron atoms to become chemically distinct by spontaneously ionizing and either losing or gaining electrons. The resulting ions can form different structures which bind together as a single element ‘compound’.
“The ionicity affects many properties of the new structure,” says Liu. “It creates splittings of bands and strong infrared absorption typical of ionic materials. Using intense synchrotron infrared radiation at our beamline, we measured transmission over a very broad wavelength range. The spectrum revealed the characteristic features of the predicted structure.”
“This yet another example of the surprises we are finding in materials under extreme conditions,” says Russell Hemley, director of the Carnegie Institution’s Geophysical Laboratory. “The behavior of what should be simple elements is far more complex than previously thought just a few years ago.”
The results are published online in Nature, 28 January 2009.
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Image caption: The complex structure of the new form of boron consists of clusters of 12 boron atoms plus pairs of two atoms. The study marks an important step toward understanding the transition to superconductivity in boron under pressure.