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Experimental and Computational Studies on Superhard Material Rhenium Diboride Under Ultrahigh Pressures

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Experimental and Computational Studies on Superhard Material Rhenium Diboride Under Ultrahigh Pressures materials Article Experimental and Computational Studies on Superhard Material Rhenium Diboride under Ultrahigh Pressures Kaleb C. Burrage, Chia-Min Lin, Wei-Chih Chen, Cheng-Chien Chen * and Yogesh K. Vohra * Department of Physics, University of Alabama at Birmingham (UAB), Birmingham, AL 35294, USA; [email protected] (K.C.B.); [email protected] (C.-M.L.); [email protected] (W.-C.C.) * Correspondence: [email protected] (C.-C.C.); [email protected] (Y.K.V.) Received: 3 March 2020; Accepted: 31 March 2020; Published: 3 April 2020 Abstract: An emerging class of superhard materials for extreme environment applications are compounds formed by heavy transition metals with light elements. In this work, ultrahigh pressure experiments on transition metal rhenium diboride (ReB2) were carried out in a diamond anvil cell under isothermal and non-hydrostatic compression. Two independent high-pressure experiments were carried out on ReB2 for the first time up to a pressure of 241 GPa (volume compression V/V = 0.731 0.004), with platinum as an internal pressure standard in X-ray diffraction studies. 0 ± The hexagonal phase of ReB2 was stable under highest pressure, and the anisotropy between the a-axis and c-axis compression increases with pressure to 241 GPa. The measured equation of state (EOS) above the yield stress of ReB2 is well represented by the bulk modulus K0 = 364 GPa and its first pressure derivative K0´ = 3.53. Corresponding density-functional-theory (DFT) simulations of the EOS and elastic constants agreed well with the experimental data. DFT results indicated that ReB2 becomes more ductile with enhanced tendency towards metallic bonding under compression. The DFT results also showed strong crystal anisotropy up to the maximum pressure under study. The pressure-enhanced electron density distribution along the Re and B bond direction renders the material highly incompressible along the c-axis. Our study helps to establish the fundamental basis for anisotropic compression of ReB2 under ultrahigh pressures. Keywords: transition metal borides; superhard materials; high pressure studies; diamond anvil cell; ab initio calculations; elastic constants; crystal anisotropy 1. Introduction Transition metal borides have shown intriguing mechanical and structural properties combining the attractive features of metallic bonding with rigid covalent boron-boron bonding [1–3]. In moving across the periodic table from a group IV transition metal boride like TiB2 to a group VI transition metal boride like ReB2, the boron layer transitions from a planar hexagonal net to a more puckered structure. In particular, rhenium diboride (ReB2) has shown desirable mechanical properties with a high average hardness of 30–60 GPa [4–7] and bulk modulus of 334–360 GPa [4,5], comparable to that of diamonds (442 GPa) [8]. Such materials are useful for their applicability under extreme conditions requiring a combination of high-temperature chemical stability and resistance to plastic deformation. Many superhard materials (hardness above 40 GPa) such as diamonds are prone to oxidation in high-temperature environments and have a propensity for chemical reactivity with transition metals. ReB2 shows promise as an alternative to diamonds for mechanical uses due to strong covalent bonding between B-B and Re-B atoms and high electron density [4], the compound’s stability up to 2000 K, and the ease of machining by electric discharge [9]. In this study, we investigate hexagonal ReB2 Materials 2020, 13, 1657; doi:10.3390/ma13071657 www.mdpi.com/journal/materials Materials 2020, 13, 1657 2 of 11 compressed under non-hydrostatic conditions at ultrahigh pressure. Axial compression of the lattice parameters is investigated for the first time up to 241 GPa, and the equation of state is determined from the measured volume compression. ReB2 shows strong crystal anisotropy and high incompressibility along the c-axis up to the maximum pressure. The experimental data are directly compared with first-principles simulations, showing good theory-experiment agreements. The scientific novelty of ourMaterials work 2020 lies, 13 in, x FOR combining PEER REVIEW ultrahigh-pressure X-ray diffraction experiments with density functional3 of 11 theory to gain fundamental understanding of anisotropic behavior. B:2s22p1 configurations were treated as valence electrons, and the valence wave functions were 2.expanded Materials in anda plane Methods wave basis up to a kinetic energy of 420 eV. The Monkhorst–Pack k-point samplingUltrahigh of thepressure Brillouin waszone achieved [21] was bychosen utilizing by a Γ a-centered diamond k-point anvilcell mesh (DAC) with consistinga fine resolution of two = diamonds0.01 × 2π/Å facing (33 × each33 × other13). The in anconvergence opposed configuration criteria for self-consistent (Figure1a). The field strong and structure structural relaxation integrity −6 −3 ofwere the set diamond to 10 eV/unit anvils allowscell and for 10 sample eV/Å, compressions respectively. For to reach each given environments external similarpressure to point, those we of deepfirst performed planetary interiorsa structure and optimization study material calculation properties in the not hexagonal seen at ambient phase conditions.with fully relaxed In this lattice study, twoparameters separate and DACs atomic were positions. employed The in antheoretica opposedl lattice anvil configuration parameters at with ambient a 30-micron conditions – 8-degree are a0 = – 350-micron2.913 Å and bevel c0 = for7.504 pressures Å, which to 241 are GPa, within and a 100-micron 0.5% error – 7-degreemargin compared – 300-micron to anvilsthe corresponding for pressures toexperimental 105 GPa. Tovalues. minimize After lateral the structure flow of therelaxation sample, we material then performed during compression, calculations a steelwith gasketlattice wasdistortion indented to toobtain 25-micron the crystal’s thickness elastic and a holetensor, was which laser drilled provided for sample information placement on onmechanical the culet. Theproperties sample such hole sizesas bulk were and made shear 8 microns moduli, for theas 30-micronwell as crystal culet andanisotropy. 25 microns The for thebulk 100-micron modulus computed by DFT with the Voigt–Reuss–Hill approximation [22] is K0 = 357 GPa at ambient culet. The ReB2 sample from American Elements had a purity of 99.9% (metals basis) with major conditions, which agrees within a 2% error margin with the value K0 = 364 GPa obtained by fitting impurities of elemental Fe, Al, and Si in the 10 parts per million (ppm) range. The ReB2 sample was mixedthe experimental with Alfa-Aesar P-V curve platinum to the powder 3rd order (99.97% Birch–Murnagha purity) for pressuren equation. calibration. The theoretical structural visualization and charge distribution were plotted by the VESTA software (version 3.4.8) [23]. Figure 1.1. (a(a)) Microscope Microscope image image of twoof two diamond diamond anvils anvils with anwith opposed an opposed configuration configuration within a diamondwithin a b anvildiamond cell (DAC).anvil cell Sample (DAC). placement Sample isplac centeredement onis centered the culet, on or flatthe tip,culet, of oneor flat of thetip,anvils. of one (of) the Schematic anvils. of the DAC within experimental settings. Incident X-rays are propagated along the axis of compression (b) Schematic of the DAC within experimental settings. Incident X-rays are propagated along the axis and collected on a Pilatus 1M detector after sample scattering. of compression and collected on a Pilatus 1M detector after sample scattering. X-ray diffraction (XRD) experiments (λ = 0.4133 Å) were carried out on the High-Pressure Collaborative3. Results Access Team (HPCAT) Beamline 16-BM-D at the Advanced Photon Source in Argonne NationalFigure Laboratory. 2 shows the As integrated shown inXRD Figure powder1b, the data X-ray taken beam at the was maximum incident pressure along the of 241 axis GPa of compression,with pressure anddetermined scattered using X-rays the o ffplatinumthe sample EOS were [12]. capturedThe difference on a Pilatus curve 1Mshown detector below with the X-raypowder beam pattern size in3.7 Figureµm (vertical 2 resulted) 3.8 fromµm a(horizontal) fit to the hexagonal FWHM (full structure width to at ReB half2. maximum) The hexagonal and × sample-to-detectorphase of ReB2 was distancefound to of be 344.63 stable mm to the calibrated maximum using pr theessure CeO of2 di 241ffraction GPa. The profile measured in the Dioptas lattice software.parameters For at 241 more GPa information were a = 2.586 on the ± 0.004 optical Å and components c = 6.882 ± of 0.007 the BeamlineÅ. Platinum 16-BM-D, peaks in refer Figure to Park2 are etlabeled al. [10 with]. Structure asterisks refinements (*) and indexed of lattice to a face-cen parameterstered were cubic carried lattice. out The using platinum the GSAS-II lattice parameter software packageat maximum [11]. pressure The measured of 241 GPa pressure-volume was measured to data be a for = 3.490 the sample± 0.009 Å. were fitted to the 3rd order Birch–Murnaghan equation of state (EOS): 3 7 5 3 2 P(V) = K x 3 x 3 1 + K0 4 x 3 1 (1) 2 0 − 4 0 − − Materials 2020, 13, 1657 3 of 11 Here, V is the measured volume at high pressure and V0 is the ambient pressure volume with x = V0/V; K0 and K0’ are the bulk modulus and its first pressure derivative, respectively. To determine the initial volume V0 of the ReB2 sample, ambient pressure XRD measurements were separately recorded of the starting material, and the lattice parameters were determined to be a0 = 2.901 Å and c0 = 7.482 Å. The platinum EOS used was calibrated up to 550 GPa from Yokoo et al. [12] using the 3rd order Birch–Murnaghan EOS and employed as a pressure marker using K0 = 276.4 GPa and K0’ = 5.12 with the platinum lattice parameter a = 3.924 Å at ambient pressure. First-principles calculations are based on density functional theory (DFT) [13], which dictates that the ground state energy (or potential) of interacting electrons is a functional of charge density.
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