In situ study of the formation of rhenium borides from the elements at high-(p, T) conditions: Extreme incompressibility of Re 7 B 3 and formation of new phases Erick A. Juarez-Arellano a, * , Björn Winkler b , Alexandra Friedrich b , Lkhamsuren Bayarjargal b , Wolfgang Morgenroth b , Martin Kunz c , Victor Milman d a Instituto de Química Aplicada, Universidad del Papaloapan, Circuito Central 200, Parque Industrial, 68301 Tuxtepec, Oaxaca, Mexico b Institut für Geowissenschaften, Goethe-Universität Frankfurt, Altenhöferallee 1, 60438 Frankfurt a.M., Germany c Advanced Light Source, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA d Accelrys, 334 Science Park, Cambridge, UK article info Article history: Received 21 June 2013 Received in revised form 9 July 2013 Accepted 27 July 2013 Available online 6 August 2013 Keywords: Rhenium borides Laser-heated diamond-anvil cell Synchrotron DFT calculations abstract Based on in situ synchrotron X-ray diffraction experiments employing laser heated diamond anvil cells to investigate the reaction of rhenium and boron from the elements at high-(p, T) conditions, Re 7 B 3 was found to be extremely incompressible, with B Re7B3 ¼ 435ð14Þ GPa, making it one of the least compressible binary compounds known to date. We also have determined the previously unknown bulk modulus of Re 3 B, B Re3B ¼ 320ð15Þ GPa, and have confirmed earlier reports of the bulk modulus of ReB 2 , B ReB2 ¼ 360(18) GPa. The experimental findings were supported by density functional theory calcula- tions, which were also employed to compute elastic stiffness coefficients and estimates for the hardness. At different high-(p, T) conditions the formation of new phases were observed. Ó 2013 Elsevier Masson SAS. All rights reserved. 1. Introduction Transition metal borides, TM-borides, are a large family of compounds many of which have outstanding physical properties, such as the high bulk modulus of OsB (453 GPa [1]), or the ultra- hardness of ReB 2 (Hv ¼ 30e48 GPa [1,2]) and WB 4 (Hv ¼ 46 GPa [1]). Hence, TM-borides have been studied extensively both theo- retically and experimentally; and most are well characterized at ambient conditions and high temperatures (e.g. Refs. [3e6]). The synthesis of most TM-borides at ambient pressure is relatively straightforward and can be achieved by heating a mixture of the elements above z1500 K in air. All transition metals from periods 4e6 with the exception of Cd and Hg have been shown to form binary borides [7]. In contrast to the numerous ambient pressure studies and very many theoretical high pressure studies, compara- tively few in situ high pressure studies have been presented [8e15]. Rhenium diboride, ReB 2 , is one of the most studied TM-borides due to its hardness and incompressibility. Investigations are favored by the fact that it can be synthesized at ambient pressure [16]. Several synthesis methods have been employed to obtain ReB 2 as powder, single crystals or as thin films, including solid state re- actions, self-propagating high-temperature synthesis, arc melting, zone melting, floating zone furnace-based techniques and pulsed laser deposition, among others [2,16e19]. In contrast, the other two binary phases that have been observed in the ReeB system, namely Re 3 B and Re 7 B 3 have attracted much less attention [20,21]. Kawano et al. [22] reported the superconductivity of Re 7 B 3 (T c ¼ 3.3 K) and Re 3 B(T c ¼ 4.8 K), while Takagiwa et al. [23] reported the magnetic properties of Re 3 B. Phase stabilities at high-(p, T) conditions are currently unknown in the ReeB system and the compressibilities of Re 3 B and Re 7 B 3 have not been determined yet. No in situ high pressure synthesis studies of rhenium borides have been reported up to now. In the present study we therefore explore the reaction of rhenium and boron by in situ experiments to understand processes occurring at high-pressures and high temperatures using laser- heated diamond anvil cell experiments. We have also determined the compressibilities of the rhenium borides and discuss our * Corresponding author. Tel.: þ52 28787 59240x220; fax: þ52 28787 59240x230. E-mail addresses: eajuarez@unpa.edu.mx, eradjuar@hotmail.com (E.A. Juarez- Arellano). Contents lists available at ScienceDirect Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie 1293-2558/$ e see front matter Ó 2013 Elsevier Masson SAS. All rights reserved. http://dx.doi.org/10.1016/j.solidstatesciences.2013.07.020 Solid State Sciences 25 (2013) 85e92