CommunicationThermal expansion of α-boron and some boron-rich pnictides
Introduction
During last few decades elemental boron and boron-rich compounds have been attracting a considerable attention due to their unusual crystal structure and properties. The common features of these compounds are B12 icosahedra and other closo-clusters with polycentric metal-like bonding system [1]. At the same time, boron atoms within closo-clusters form strong covalent bonds with neighboring atoms of the adjacent clusters and various interstitial atoms (e.g. C, N, O, P, Zr, Y), giving a big verity of boron-rich compounds: B4C, B12O2, B13N2, ZrB12, YB12, etc. A combination of the metal-like intra-cluster bonds and strong covalent inter-cluster bonding makes boron-rich compounds extremely stable, eventually leading to high melting temperatures, chemical inertness, outstanding mechanical properties and interesting electronic properties (e.g. self-healing resistance to radiation damage, superconductivity, etc.) [[1], [2], [3], [4], [5], [6], [7], [8], [9]]. It should be noted that properties of boron-rich solids are significantly influenced by the interstitial atoms [3,7]. Thus, study of the impact of interstitial atoms on various properties (hardness, compressibility, thermal expansion, band gap, etc.) is of great interest from both fundamental and application points of view.
Very recently thermal expansion of two boron-rich nitrides synthesized under extreme conditions has been studied by synchrotron X-ray diffraction [10]. In the present paper we report the results on thermal expansion of α-rhombohedral boron (α-B12) and two boron-rich pnictides, B12P2 and B12As2. The retrieved thermal expansion coefficients compared with those reported earlier [10,11] allowed us to analyze the thermal expansion variation in the family of boron-rich pnictides.
Section snippets
Experimental
Highly crystalline α-B12 (99.98%) was provided by Dr. Igor N. Goncharenko, Laboratoire Léon Brillouin (CEA-Saclay, France). Boron subphosphide B12P2 was produced by self-propagating high-temperature reaction in the BPO4–MgB2–Mg system according to the method described elsewhere [12]. Boron subarsenide B12As2 was synthesized by reaction of amorphous boron with arsenic melt at 5 GPa and 2100 K in a toroid-type apparatus (for details, see Ref. [13]). According to X-ray diffraction study (TEXT 3000
Results and discussion
α-rhombohedral boron is the low-temperature allotrope, and its thermodynamic stability domain in the equilibrium phase diagram of boron [18] is still not precisely defined. It has rhombohedral crystal structure (R-3m) containing B12 icosahedra located at the vertices of the rhombohedral cell. Due to the lack of the valence electrons necessary for the formation of inter-icosahedral covalent bonds, the equatorial boron atoms of the neighboring icosahedra lying in one layer forming relatively weak
Conclusions
Thermal expansion of α-rhombohedral boron and isostructural boron-rich pnictides (B12P2, B12As2) was studied in situ by synchrotron X-ray diffraction up to 1280 K. The precise measurements of lattice parameters at different temperatures allowed us to retrieve the corresponding linear (αl) and volume (αv) thermal expansion coefficients of α-B12, B12P2 and B12As2. The obtained values were compared with the literature data for boron subnitride B13N2, and variation of αl and αv of three boron-rich
Acknowledgements
The authors thank Dr. Vladimir Mukhanov for the samples synthesis and Drs. Jasper Rikkert Plaisier, Lara Gigli and Giulio Zerauschek for assistance in measurements at Elettra. Synchrotron X-ray diffraction experiments were carried out during beam time allocated for Proposal 20160086 at Elettra Sincrotrone Trieste and Proposal I-20090172 EC at HASYLAB-DESY. This work was financially supported by the European Union's Horizon 2020 Research and Innovation Programme under Flintstone 2020 project
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