Cubic BC5: a diamond-like material with unique properties

A diamond-like material with high boron content could be expected to combine the best properties from its constituent elements, boron and carbon. In a recent study, a superhard diamond-like BC5 phase with extremely high boron content was synthesised under high pressure/high temperature conditions. This new material represents the ultimate solubility of boron in diamond. Like diamond, it possesses exceptional hardness and resistance to fracture. Unlike diamond, cubic BC5 is a semiconductor rather than an insulator. These properties make cubic BC5 an interesting material for electronics and electrochemistry under extreme conditions.

Diamond’s unique properties, such as extreme hardness, high thermal conductivity, wide band gap, high electron and hole mobility make it suitable for a variety of scientific and technological applications [1]. However, it remains susceptible to oxidation and is reactive with ferrous metals. The growing demand for advanced superhard materials for the cutting and shaping of metals and ceramics [2], as well as for electronic [3] and electrochemical [4] applications, have stimulated the search for novel diamond-like materials that are more thermally and chemically stable than pure diamond.

Recently we performed a systematic investigation of phase transformations of turbostratic graphite-like BCx phases of various stoichiometry (x = 1-5) at pressures up to 25 GPa and temperatures up to 2500 K in a laser-heated diamond-anvil cell using in situ angle-dispersive X-ray diffraction at beamline ID27. At pressures above 20 GPa in the 2000-2500 K range, the t-BCx phases (1 ≤ x ≤ 4) decomposed into boron-doped diamond (1-2 at% of boron) and boron carbides (B4C and B50C2) that were accompanied by the formation of cubic B-C solid solutions.

The diamond-like BC5 phase corresponds to the ultimate solubility of boron in diamond. It was synthesised under controlled conditions of pressure and temperature (24 GPa and around 2200 K) and then quenched to ambient conditions (Figure 1). The quenched samples were characterised using X-ray diffraction, Raman spectroscopy, electron probe microanalysis, transmission electron microscopy and electron energy loss spectroscopy.

Figure 1. Laser-heating sequence of diffraction patterns of a BC5 sample taken in situ at 24 GPa for various temperatures. The lowest pattern corresponds to t-BC5 at ambient conditions

The structure of cubic BC5 is closely related to diamond but with boron atoms that are randomly distributed throughout the diamond-like lattice (Figure 2). The lattice parameter of c-BC5 at ambient conditions was found to be a = 3.635 ± 0.006 Å, which is larger than those of both diamond (3.5667 Å) and cubic boron nitride (3.6158 Å) and is in good agreement with the ideal mixing (Vegard's law) between diamond and "diamond-like boron" (a = 4.04 Å corresponding to the B-B bond length of 1.75 Å). The relatively narrow (~200 K) temperature range of the c-BC5 formation clearly indicates the metastable character of the phase, e.g. its slight overheating leads to the phase segregation into more thermodynamically stable boron-doped diamond and boron carbides. Thus, the value of 16 at% of boron may be considered as a concentration limit of existence of metastable diamond-like B-C solid solutions. These metastable phases do not participate in the phase equilibria in the B-C system at high pressures and temperatures; while the p-T-x domain of their formation is determined by the boron concentration dependencies of the activation barriers for t-BCx and c-BCx decomposition, and the t-BCx to c-BCx transformation

Figure 2. Suggested crystal structure of cubic BC5. The red and black balls represent the boron and carbon atoms, respectively. The boron atoms are randomly distributed throughout the diamond-like lattice.

Well-sintered millimetre-sized bulks of nanostructured c-BC₅ have been produced in a large-volume multi-anvil press. The material exhibits extreme hardness and fracture toughness, and very high thermal stability; this makes cubic BC₅ an exceptional superabrasive capable of overcoming diamond. The beneficial combination of the electrical conductivity, a band structure that is unusual for diamond-like phases due to electron deficiency of boron atoms, and high thermal stability will eventually allow the expansion of the boundaries of high-temperature electronics and electrochemistry at extreme conditions. This discovery, the new cubic-BC₅ material and its process of fabrication, is the subject of a patent from the authors institutes

http://www.esrf.eu/news/spotlight/spotlight77

Eurix Janeth Gómez Vera
CI 18392113
ESS