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Rhenium diboride
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<h2 id="synthesis">Synthesis</h2> <p>ReB2 can be synthesized by at least three different methods at standard atmospheric pressure: solid-state <a href="/facts/Salt_metathesis_reaction/YpadY3lq">metathesis</a>, melting in an <a href="/facts/Electric_arc/1hUPPAG0">electric arc</a>, and direct heating of the elements.<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a> </p><p>In the metathesis reaction, <a href="/facts/Rhenium_trichloride/8VW5YuPy">rhenium trichloride</a> and <a href="/facts/Magnesium_diboride/fsgNsNPP">magnesium diboride</a> are mixed and heated in an inert atmosphere and the <a href="/facts/Magnesium_chloride/uk1f69LG">magnesium chloride</a> byproduct is washed away. Excess boron is needed to prevent the formation of other phases such as Re7B3 and Re3B. </p><p>In the arc-melting method, rhenium and boron powders are mixed and a large electric current is passed through the mixture, also in an inert atmosphere. </p><p>In the direct reaction method, the rhenium-boron mixture is sealed in a vacuum and held at a high temperature over a longer period (1,000 °C for five days). </p><p>At least the last two methods are capable of producing pure ReB2 without any other phases, as confirmed by <a href="/facts/X-ray_crystallography/WtI0F5DN">X-ray crystallography</a>. </p> <h2 id="hardness">Hardness</h2> <p>Rhenium diboride is occasionally, and controversially,<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a><a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a> cited as a "<a href="/facts/Superhard_material/0Gb67FEw">superhard material</a>" due to its high hardness level. However, tested in the asymptotic-hardness region, as recommended for hard and superhard materials,<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a> rhenium diboride demonstrates a <a href="/facts/Vickers_hardness_test/xz7kYRsS">Vickers hardness</a> of only 30.1 ± 1.3 GPa at 4.9 N, well below the generally-accepted threshold of 40 GPa or more needed to classify it as "superhard".<a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a> Another research has estimated the Hv of full-dense ReB2 at about 22 GPa under an applied load of 2.94 N,<a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a> comparable to that of <a href="/facts/Tungsten_carbide/2w0zcaWJ">tungsten carbide</a>, <a href="/facts/Silicon_carbide/kQLVAYbr">silicon carbide</a>, <a href="/facts/Titanium_diboride/2pXEpx3v">titanium diboride</a> or <a href="/facts/Zirconium_diboride/PRQSefK0">zirconium diboride</a>.<a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a> </p><p>Values greater than 40 GPa have been observed only in tests with very low loads, which is not a suitable testing method for this type of solids.<a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a> In one test, the lowest tested load of 0.49 N yielded the <i>average</i> hardness of 48 ± 5.6 GPa and a <i>maximum</i> hardness of 55.5 GPa, which is comparable to the hardness of <a href="/facts/Boron_nitride/kumlPQRg">cubic boron nitride</a> (c-BN) under an equivalent load.<a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a> Such phenomenon of inverse relationship between the applied load and hardness is known as the <a href="/facts/Indentation_size_effect/0DoBe7IJ">indentation size effect</a>.<a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a> </p><p>In recent times, there has been a significant amount of research into improving the hardness and other properties of the ReB2. In one study, the hardness for the ReB2(R-3m) <a href="/facts/Polymorphism_(materials_science)/bu2tqsB9">polymorph</a> was estimated at 41.7 GPa, while for the ReB2(P63/mmc) it was placed at c.a. 40.6 GPa.<a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a> In another study, a fully dense <a href="/facts/Boron_carbide/J15UuFPa">B4C</a>-27<a href="/facts/Mass_fraction_(chemistry)/YMIH9egn">wt.%</a>ReB2 ceramic composite nanopowder was fabricated by <a href="/facts/Spark_plasma_sintering/vEjzhbel">spark plasma sintering</a>. It has exhibited a <a href="/facts/Indentation_hardness/sbYCdPar">microhardness</a> of 50 ± 3 GPa under a 49 N load in the asymptotic-hardness region and had a 3.2 g/cm3 density, comparable with the hardness and density of the c-BN.<a class="footnote-ref" id="fnref:17" href="#fn:17"><sup>17</sup></a> </p><p>The hardness of ReB2 exhibits considerable <a href="/facts/Anisotropy/YMqTQWVB">anisotropy</a> because of its hexagonal layered <a href="/facts/Structural_formula/XRTmvINB">structure</a>, being greatest along the <i>c</i> axis. Two factors contribute to the high hardness of ReB2: a high density of <a href="/facts/Valence_electron/mXPA15VP">valence electrons</a>, and an abundance of short <a href="/facts/Covalent_bond/EzMLf3Pz">covalent bonds</a>.<a class="footnote-ref" id="fnref:18" href="#fn:18"><sup>18</sup></a><a class="footnote-ref" id="fnref:19" href="#fn:19"><sup>19</sup></a> Rhenium has one of the highest valence electron densities of any transition metal (476 electrons/nm3, compare to 572 electrons/nm3 for <a href="/facts/Osmium/1TjXD69n">osmium</a> and 705 electrons/nm3 for diamond<a class="footnote-ref" id="fnref:20" href="#fn:20"><sup>20</sup></a>). The addition of boron requires only a 5% expansion of the rhenium lattice because the small boron atoms fill the existing spaces between the rhenium atoms. Furthermore, the <a href="/facts/Electronegativity/H7Vq7Uah">electronegativities</a> of rhenium and boron are close enough (1.9 and 2.04 on the <a href="/facts/Electronegativity/H7Vq7Uah">Pauling scale</a>) that they form covalent bonds in which the electrons are shared almost equally. </p> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Ruthenium_boride/JHasH94V">Ruthenium boride</a></li> <li><a href="/facts/Network_covalent_bonding/7ykpwQqV">Network covalent bonding</a></li> <li><a href="/facts/Superhard_material/0Gb67FEw">Superhard material</a></li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>La Placa, S. J.; Post, B. (1962). "The crystal structure of rhenium diboride". Acta Crystallographica. 15 (2): 97. Bibcode:1962AcCry..15...97L. doi:10.1107/S0365110X62000298. <a href="https://doi.org/10.1107%2FS0365110X62000298" target="_blank">https://doi.org/10.1107%2FS0365110X62000298</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>Dubrovinskaia, Natalia; Dubrovinsky, Leonid; Solozhenko, Vladimir L. (2007-12-07). "Comment on "Synthesis of Ultra-Incompressible Superhard Rhenium Diboride at Ambient Pressure"". Science. 318 (5856): 1550. Bibcode:2007Sci...318.1550D. doi:10.1126/science.1147650. ISSN 0036-8075. OCLC 264200699. PMID 18063772. S2CID 1640087. Retrieved 2024-01-27. <a href="https://www.science.org/doi/10.1126/science.1147650" target="_blank">https://www.science.org/doi/10.1126/science.1147650</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Chung, Hsiu-Ying; et al. (April 20, 2007). "Synthesis of Ultra-Incompressible Superhard Rhenium Diboride at Ambient Pressure". Science. 316 (5823): 436–9. Bibcode:2007Sci...316..436C. doi:10.1126/science.1139322. PMID 17446399. S2CID 10395879. <a href="http://www.sciencemag.org/cgi/content/abstract/316/5823/436" target="_blank">http://www.sciencemag.org/cgi/content/abstract/316/5823/436</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>Dubrovinskaia, Natalia; Dubrovinsky, Leonid; Solozhenko, Vladimir L. (2007-12-07). "Comment on "Synthesis of Ultra-Incompressible Superhard Rhenium Diboride at Ambient Pressure"". Science. 318 (5856): 1550. Bibcode:2007Sci...318.1550D. doi:10.1126/science.1147650. ISSN 0036-8075. OCLC 264200699. PMID 18063772. S2CID 1640087. Retrieved 2024-01-27. <a href="https://www.science.org/doi/10.1126/science.1147650" target="_blank">https://www.science.org/doi/10.1126/science.1147650</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>Dubrovinskaia, Natalia; Dubrovinsky, Leonid; Solozhenko, Vladimir L. (2007-12-07). "Comment on "Synthesis of Ultra-Incompressible Superhard Rhenium Diboride at Ambient Pressure"". Science. 318 (5856): 1550. Bibcode:2007Sci...318.1550D. doi:10.1126/science.1147650. ISSN 0036-8075. OCLC 264200699. PMID 18063772. S2CID 1640087. Retrieved 2024-01-27. <a href="https://www.science.org/doi/10.1126/science.1147650" target="_blank">https://www.science.org/doi/10.1126/science.1147650</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>Chung, Hsiu-Ying; et al. (April 20, 2007). "Synthesis of Ultra-Incompressible Superhard Rhenium Diboride at Ambient Pressure". Science. 316 (5823): 436–9. Bibcode:2007Sci...316..436C. doi:10.1126/science.1139322. PMID 17446399. S2CID 10395879. <a href="http://www.sciencemag.org/cgi/content/abstract/316/5823/436" target="_blank">http://www.sciencemag.org/cgi/content/abstract/316/5823/436</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>Dubrovinskaia, Natalia; Dubrovinsky, Leonid; Solozhenko, Vladimir L. (2007-12-07). "Comment on "Synthesis of Ultra-Incompressible Superhard Rhenium Diboride at Ambient Pressure"". Science. 318 (5856): 1550. Bibcode:2007Sci...318.1550D. doi:10.1126/science.1147650. ISSN 0036-8075. OCLC 264200699. PMID 18063772. S2CID 1640087. Retrieved 2024-01-27. <a href="https://www.science.org/doi/10.1126/science.1147650" target="_blank">https://www.science.org/doi/10.1126/science.1147650</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>Qin, Jiaqian; He, Duanwei; Wang, Jianghua; Fang, Leiming; Lei, Li; Li, Yongjun; Hu, Juan; Kou, Zili; Bi, Yan (2008). "Is Rhenium Diboride a Superhard Material?". Advanced Materials. 20 (24): 4780. Bibcode:2008AdM....20.4780Q. doi:10.1002/adma.200801471. S2CID 98327405. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>Dubrovinskaia, Natalia; Dubrovinsky, Leonid; Solozhenko, Vladimir L. (2007-12-07). "Comment on "Synthesis of Ultra-Incompressible Superhard Rhenium Diboride at Ambient Pressure"". Science. 318 (5856): 1550. Bibcode:2007Sci...318.1550D. doi:10.1126/science.1147650. ISSN 0036-8075. OCLC 264200699. PMID 18063772. S2CID 1640087. Retrieved 2024-01-27. <a href="https://www.science.org/doi/10.1126/science.1147650" target="_blank">https://www.science.org/doi/10.1126/science.1147650</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li> <li id="fn:10"><p>Dubrovinskaia, Natalia; Dubrovinsky, Leonid; Solozhenko, Vladimir L. (2007-12-07). "Comment on "Synthesis of Ultra-Incompressible Superhard Rhenium Diboride at Ambient Pressure"". Science. 318 (5856): 1550. Bibcode:2007Sci...318.1550D. doi:10.1126/science.1147650. ISSN 0036-8075. OCLC 264200699. PMID 18063772. S2CID 1640087. Retrieved 2024-01-27. <a href="https://www.science.org/doi/10.1126/science.1147650" target="_blank">https://www.science.org/doi/10.1126/science.1147650</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></p></li> <li id="fn:11"><p>Qin, Jiaqian; He, Duanwei; Wang, Jianghua; Fang, Leiming; Lei, Li; Li, Yongjun; Hu, Juan; Kou, Zili; Bi, Yan (2008). "Is Rhenium Diboride a Superhard Material?". Advanced Materials. 20 (24): 4780. Bibcode:2008AdM....20.4780Q. doi:10.1002/adma.200801471. S2CID 98327405. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></p></li> <li id="fn:12"><p>Qin, Jiaqian; He, Duanwei; Wang, Jianghua; Fang, Leiming; Lei, Li; Li, Yongjun; Hu, Juan; Kou, Zili; Bi, Yan (2008). "Is Rhenium Diboride a Superhard Material?". Advanced Materials. 20 (24): 4780. Bibcode:2008AdM....20.4780Q. doi:10.1002/adma.200801471. S2CID 98327405. <a href="/wiki/Bibcode_(identifier)" target="_blank">/wiki/Bibcode_(identifier)</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></p></li> <li id="fn:13"><p>Dubrovinskaia, Natalia; Dubrovinsky, Leonid; Solozhenko, Vladimir L. (2007-12-07). "Comment on "Synthesis of Ultra-Incompressible Superhard Rhenium Diboride at Ambient Pressure"". Science. 318 (5856): 1550. Bibcode:2007Sci...318.1550D. doi:10.1126/science.1147650. ISSN 0036-8075. OCLC 264200699. PMID 18063772. S2CID 1640087. Retrieved 2024-01-27. <a href="https://www.science.org/doi/10.1126/science.1147650" target="_blank">https://www.science.org/doi/10.1126/science.1147650</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></p></li> <li id="fn:14"><p>Chung, Hsiu-Ying; et al. (April 20, 2007). "Synthesis of Ultra-Incompressible Superhard Rhenium Diboride at Ambient Pressure". Science. 316 (5823): 436–9. Bibcode:2007Sci...316..436C. doi:10.1126/science.1139322. PMID 17446399. S2CID 10395879. <a href="http://www.sciencemag.org/cgi/content/abstract/316/5823/436" target="_blank">http://www.sciencemag.org/cgi/content/abstract/316/5823/436</a> <a href="#fnref:14" class="footnote-back-ref">↩</a></p></li> <li id="fn:15"><p>Chung, Hsiu-Ying; et al. (April 20, 2007). "Synthesis of Ultra-Incompressible Superhard Rhenium Diboride at Ambient Pressure". Science. 316 (5823): 436–9. Bibcode:2007Sci...316..436C. doi:10.1126/science.1139322. PMID 17446399. S2CID 10395879. <a href="http://www.sciencemag.org/cgi/content/abstract/316/5823/436" target="_blank">http://www.sciencemag.org/cgi/content/abstract/316/5823/436</a> <a href="#fnref:15" class="footnote-back-ref">↩</a></p></li> <li id="fn:16"><p>Maździarz, Marcin; Mościcki, Tomasz (2015-10-12). "Structural, mechanical, optical, thermodynamical and phonon properties of stable ReB2 polymorphs from density functional calculations". Journal of Alloys and Compounds. 657 (published 2016-02-05): 878–888. doi:10.1016/j.jallcom.2015.10.133. ISSN 0925-8388. OCLC 5937517569. S2CID 138931858. Retrieved 2024-01-27. <a href="https://www.sciencedirect.com/science/article/abs/pii/S092583881531389X" target="_blank">https://www.sciencedirect.com/science/article/abs/pii/S092583881531389X</a> <a href="#fnref:16" class="footnote-back-ref">↩</a></p></li> <li id="fn:17"><p>Mnatsakanyan, R.; Davtyan, D.; Minasyan, T.; Aydinyan, S.; Hussainova, I. (2021-02-15). "Superhard B4C-ReB2 composite by SPS of microwave synthesized nanopowders". Materials Letters. 285: 129163. doi:10.1016/j.matlet.2020.129163. ISSN 0167-577X. OCLC 8885940810. S2CID 229404472. <a href="https://doi.org/10.1016%2Fj.matlet.2020.129163" target="_blank">https://doi.org/10.1016%2Fj.matlet.2020.129163</a> <a href="#fnref:17" class="footnote-back-ref">↩</a></p></li> <li id="fn:18"><p>Chung, Hsiu-Ying; et al. (April 20, 2007). 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