week ending PRL 102, 015506 (2009) PHYSICAL REVIEW LETTERS 9 JANUARY 2009 Ultimate Metastable Solubility of Boron in Diamond: Synthesis of Superhard Diamondlike BC5 Vladimir L. Solozhenko* and Oleksandr O. Kurakevych LPMTM-CNRS, Universite´ Paris Nord, F-93430 Villetaneuse, France Denis Andrault LMV, Universite´ Blaise Pascal, F-63000 Clermont-Ferrand, France Yann Le Godec IMPMC, Universite´ P&M Curie, F-75015 Paris, France Mohamed Mezouar European Synchrotron Radiation Facility, F-38043 Grenoble, France (Received 13 October 2008; published 9 January 2009) Here, we report the synthesis of cubic BC5 (c-BC5), the diamondlike B-C phase with the highest boron content ever achieved, at 24 GPa and about 2200 K, using both a laser-heated diamond anvil cell and large-volume multianvil apparatus. The synthesized phase is low compressible (bulk modulus of 335 GPa), conductive, and exhibits extreme Vickers hardness (71 GPa), unusually high for superhard materials fracture toughness (9:5 MPa m0:5), and high thermal stability (up to 1900 K); this makes it an exceptional superabrasive and promising material for high-temperature electronics. DOI: 10.1103/PhysRevLett.102.015506 PACS numbers: 61.66.Fn, 61.05.cp, 62.50.Àp Diamond has found a wide variety of applications in A number of graphitelike B-C phases of different stoi- modern science and technology due to its unique proper- chiometry (so called boron-substituted graphites with ties, such as extreme hardness, high thermal conductivity, boron content up to 50 at%) have been synthesized by wide band gap, high electron and hole mobility [1]. At the thermal chemical vapor deposition [8]. These materials same time, it is nonresistant to oxidation and reactive with should be perfect precursors for diamondlike B-C phases; ferrous metals. The growing demand for advanced super- however, their high-pressure, high-temperature behavior hard materials in cutting and shaping hard metals and has not been well understood so far. Recent experiments ceramics [2], as well as in electronic [3] and electrochem- on graphitelike BC3 showed that at 20 GPa and 2200 K in a ical [4] applications, has stimulated the search for novel multianvil press, the phase decomposes into the mixture of diamondlike phases that are more thermally and chemi- boron-doped diamond (1:8 at% of boron) and boron cally stable than pure diamond. carbide [9]. Very recently, phase segregation of graphite- The traces of boron impurities change the electrical like BC1:6 into a mixture of diamond, boron carbide, and properties of diamond from an insulator into a semicon- elemental boron was observed in a diamond anvil cell at ductor [3,4]. Moreover, boron-doped diamond was re- 45 GPa and 2230 K [10]. Hence, all previous attempts to ported to be a type-II superconductor with transition synthesize diamondlike B-C phases with boron concentra- temperature Tc 5K [5], while heavily boron-doped tion higher than 2 at% have failed. diamond (20 at% B) is predicted to be a superconduc- In the present study, we have performed a systematic tor with very high, up to 55 K, Tc [6]. However, only very investigation of phase transformations of turbostratic limited amount of boron (2 at%) can be introduced into graphitelike BCx phases (t-BCx) at pressures up to diamond lattice by common methods of thermal chemical 25 GPa and temperatures up to 2500 K with the aim to vapor deposition and high-pressure synthesis. The limiting establish the ultimate boron solubility in diamond and concentration of boron that can be incorporated into dia- synthesize diamondlike B-C phase(s) with a high boron mond structure has not been established yet, although content. under extreme pressure-temperature conditions, this con- Our in situ studies of t-BC and t-BC3 in the 3–7 GPa tent could be significantly increased. Since all B-C mate- pressure range using multianvil system MAX80 and rials show higher resistance to oxygen and ferrous metals energy-dispersive x-ray diffraction with synchrotron radia- than similar carbon materials [7], the diamondlike B-C tion at HASYLAB-DESY have shown that neither phase phases with high boron content are expected to combine undergoes any transformation up to 2000 K. the best properties of the elements including advanced In situ experiments at higher pressures and temperatures electrical and optical properties, very high hardness, and have been conducted at ID30 and ID27 beam lines of the high thermal and chemical stability. European Synchrotron Radiation Facility using laser- 0031-9007=09=102(1)=015506(4) 015506-1 Ó 2009 The American Physical Society week ending PRL 102, 015506 (2009) PHYSICAL REVIEW LETTERS 9 JANUARY 2009 heated diamond anvil cell (DAC) and angle-dispersive previously reported for other graphitic phases of the B-C-N x-ray diffraction. At pressures above 20 GPa in the system [12,14]. 2000–2500 K range, t-BCx phases (1 x 4) decompose At 24.0 GPa, no change in the diffraction patterns of into boron-doped diamond (1–2 at% of boron) and boron t-BC5 has been observed below 1550 K [Fig. 1(b)]. At carbides (B4C and B50C2) that are accompanied by the higher temperatures, the profile of the only broad line of formation of cubic B-C phases (c-BCz) with lattice pa- the pattern that is located in the region of 111 reflection of rameters up to 2% higher than that of diamond [Fig. 1(a)]. diamond shows the appearance of a fine structure. Finally, The decrease of the maximal temperature in an experiment at 2200 K, the formation of the cubic BCz phase has been from 2500 down to 2000 K leads to a higher amount of observed, and the diffraction pattern exhibits only 111, forming c-BCz. At the same time, the higher is the pres- 220, and 311 lines of the cubic (or, to be precise, pseudo- sure, the lower is the starting temperature of t-BCx decom- cubic) lattice, which indicates that the sample contains a position, especially in the case of the phases with high single crystalline phase. Since no amorphous phase has boron content. Thus, at 41 GPa, phase segregation of been observed in the quenched samples by both transmis- t-BC and t-BC1:5 was observed already at 1750 K. Based sion electron microscopy and Raman spectroscopy, one on these results, one can conclude that synthesis of a pure can assume that the composition of the cubic phase is the cubic B-C phase corresponding to the ultimate solubility same as that of the precursor, namely, BC5. The 1:5 boron- of boron in diamond may be expected only in the case to-carbon ratio has been additionally confirmed by electron t BC of - x precursors with relatively low boron content energy loss spectroscopy (GIF2000, Gatan) and by elec- x 5 ( ). tron microprobe analysis (Cameca SX-50, Camebax). BC At room temperature, compression of turbostratic 5 is Being instantly heated up to 2440 K at the same pres- accompanied by a pronounced decrease of intensities of sure, the as-synthesized cubic phase undergoes the decom- 00l the lines similar to that observed for turbostratic CBN position into boron-doped diamond (2 at% of boron solid solutions [11,12]. Upon compression to 24 GPa, these according to the lattice parameter) and boron carbide lines almost disappear [Fig. 1(b)]. Also, with increasing B4C [Fig. 1(b)]. The relatively narrow temperature range pressure, the profile of the two-dimensional 10 reflection (200 K) of the c-BC5 formation clearly indicates the becomes more symmetric, and the intensity of this line metastable character of this phase; e.g., its slight over- increases. These features, observed independently on the heating leads to the phase segregation into more thermo- diffraction geometry in relation to the compression axis dynamically stable boron-doped diamond and boron [12,13], point to the reconstruction of the turbostratic 2 3 carbides. Thus, the value of 16 at% of boron may be graphitelike sp -layers into the disordered sp -structure, considered as a concentration limit of existence of meta- stable diamondlike B-C phases. 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 do- 2440 K BDD + B4C main of their formation is determined by the boron con- 111 centration dependencies of the activation barriers for t-BCx and c-BCx decomposition, and t-BCx into c-BCx 2200 K 220 c-BCz transformation. BC 1940 K According to the transmission electron microscopy (JEM 2010HR, JEOL), cubic BC5 quenched down to am- 1550 K bient conditions occurs as nanocrystalline aggregates with 300 K clearly visible but very small (average size: 10–15 nm) ambient grains. The three strongest diffraction rings of the selected BC BC3 5 area electron diffraction (SAED) patterns, i.e., 111, 220, 10 11 003 001 002 and 311, allowed us to evaluate the lattice parameter of 6 8 10 12 14 16 18 20 6 8 10 12 14 16 18 20 BC a ¼ 0:359ð7Þ nm 2θ (λ = 0.03738 nm) 2θ (λ = 0.03738 nm) cubic 5 as . The more precise value of (a) (b) lattice parameter was established by angle-dispersive x-ray diffraction with synchrotron radiation and was found to be FIG. 1 (color online). (a) Representative diffraction patterns of a ¼ 0:3635 Æ 0:0006 nm, which is larger than those of BC the 3 and BC samples quenched from 26 GPa and 2400 K. both diamond (0.35667 nm for single crystal diamond 1:8 Both phases decompose into boron-doped diamond with at according to JCPDS No. 6-0675, and 0.35688 nm for % of B (BDD), cubic BCz phase (according to lattice parameters, nanodiamond [15]) and cubic boron nitride (0.36158 nm, BC9 and BC5 for starting t-BC3 and t-BC, respectively), and JCPDS No.