Subscriber access provided by UNIVERSITY OF TOLEDO LIBRARIES
Communication
A Narrow Gap Semiconducting Germanium Allotrope from the Oxidation of a layered Zintl Phase in Ionic Liquids Zhongjia Tang, Alexander P. Litvinchuk, Melissa Gooch, and Arnold M. Guloy J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b03503 • Publication Date (Web): 21 May 2018 Downloaded from http://pubs.acs.org on May 21, 2018
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Journal of the American Chemical Society
A Narrow Gap Semiconducting Germanium Allotrope from the Oxidation of a Layered Zintl Phase in Ionic Liquids Zhongjia Tang1, Alexander P. Litvinchuk2,3, Melissa Gooch2,3 and Arnold M. Guloy1,3* 1
Department of Chemistry, University of Houston, Houston, TX 77204-5003, USA
2
Department of Physics, University of Houston, Houston, TX 77204-5005, USA
3
Texas Center for Superconductivity, University of Houston, Houston, TX 77204-5002, USA
Supporting Information Placeholder ABSTRACT: A metastable germanium allotrope, Ge(oP32), was synthesized as polycrystalline powders and single crystals from the mild-oxidation/delithiation of Li7Ge12 in ionic liquids. Its crystal structure, from single crystal X-ray diffraction (Pbcm, a = 8.1527(4) Å, b = 11.7572(5) Å, c = 7.7617(4) Å), features a complex covalent network of 4-bonded Ge, resulting from a well-ordered topotactic oxidative condensation of 7[Ge12] layers. It is a diamagnetic semiconductor (Eg = 0.33 eV), and transforms exothermically and irreversibly to α-Ge at 363 °C. This demonstrates the potential of ionic liquids as reactive media in the mild oxidation of Zintl phases to new highly crystallized modifications of elements and simple compounds.
The concept that an element forms structural modifications having different chemical and physical properties illustrates the most fundamental aspects of an element’s chemical nature. Due to the limited chemistry involved, synthetic allotropes of non-molecular elemental structures are rare. Most solid allotropes are successfully made by subjecting an element under extreme conditions, often under high pressures 1-4 and temperatures. Many solid elements undergo structural transitions from simple crystal structures at ambient conditions to more complex structures at very high pressures. The high-pressure allotropes of germanium, an important semi5-14 conductor, effectively exemplifies this. Allotropes synthesized through chemical reactions at ambient conditions, from reactants more complex than the element are more uncommon. Although development of ‘soft’ chemical routes to synthetic allotropes is a challenge, its elegant use significantly expands the phase field of the search for ‘kinetically’ stable elemental allotropes well beyond pressure and temperature. Applications of ‘soft chemistry’ to Zintl phases and related compounds are still limited. Nonetheless, Zintl anions of Si and Ge present viable precursors to a chemical ‘bottom-up’ approach to new elemental crystal15-19 line framework structures. Controlled oxidation of highly reducing Ge anions to its elemental structures is thermodynamically favoured over the endothermic reduction of weakly oxidizing Ge cations. This was successfully demonstrated
by the bulk synthesis of the clathrate-II germanium allo20 trope, Ge(cF136). The low-density allotrope was obtained as bulk polycrystalline powders by reacting the cluster-based (alkali-metal) ‘Na4Ge9’ with a low-temperature ionic liquid. Improved optical properties, with direct and tunable band gaps, have been proposed in low-density germanium allotropes which are generally inaccessible to high-pressure syn21,22 thesis. Oxidation of Zintl anions in ionic liquids, based on long chain quaternary alkyl ammonium salts, take advantage of 23 the inherent reactivity of ionic phases. The reactions of alkali-metal germanides and silicides at moderate temperatures (100-300°C) in ionic liquids follow the Hofmann elimi24 nation reaction. The low acidity of the β-H's (C-H) in the quaternary alkyl ammonium and the inherent thermal stability of the ionic liquids allow the mild oxidation of the anions at low to moderately high temperatures where metastable allotropes can be obtained. Polycrystalline powders and single crystals of Ge(oP32) were obtained from the reaction of Li7Ge12 in ionic liquids (IL): dodecyltrimethyl ammonium aluminum tetrachloride (DTAC) or hexyltrimethyl ammonium aluminum tetrabro25 mide (HTMAB), at 135-145 ºC for 3-7 days. Reactions at T ≥ 150 ºC leads to the significant formation of α-Ge, and reactions at T > 180 ºC yield only α-Ge. The synthesis produces bulk quantities of the crystalline allotrope, yields >95% of the crystalline products by XRPD (amorphous Ge is present). Moreover, use of a lower melting and less viscous ionic liquid (HTMAB) with shorter alkyl ‘tails’ leads to improved rates of reaction, better crystallinity and larger crystal sizes of the products. Single crystals of Ge(oP32) (0.05-0.25 mm in size) were obtained from reactions of coarsely ground powders of Li7Ge12 with HTMAB, heated at 140 ºC for 7 days. Proper temperature control and keeping the reactions undisturbed were critical in the growth of single crystals. The dark gray polycrystalline powders and dark silver rhombic-shaped single crystals of Ge(oP32) are stable to air and moisture. All preparative manipulations were performed within an Aratmosphere glove-box, with total H2O and O2 levels 2σ(I)], R1 = 0.0370, wR2 = 0.0946, R indices (all data): R1 = 0.0436, wR2 = 0.0967. (28) Sheldrick, G.M. Acta Cryst. A 2008, 64, 112-122. (29) Kiefer, F.; Karttunen, A. J.; Doeblinger, M.; Faessler, T. F. Chem. Mater. 2011, 23(20), 4578-4586. (30) Kiefer, F.; Hlukhyy, V.; Karttunen, A. J.; Faessler, T. F.; Gold, C.; Scheidt, E.-W.; Scherer, W.; Nylen, J.; Häussermann, U. J. Mater. Chem. 2010, 20(9), 1780-1786. (31) Zaikina, J. V.; Muthuswamy, E.; Lilova, K. I.; Gibbs, Z. M.; Zeilinger, M.; Snyder, G. J.; Fassler, T. F.; Navrotsky, A.; Kauzlarich, S. M. Chem. Mater. 2014, 26(10), 3263-3271. (32) Conesa, J. C. J. Phys. Chem. B 2002, 106, 3402–3409. (33) Chadi, D. J. Phys. Rev. B 1985, 32, 6485–6489. (34) Kiefer, F.; Faessler, T. F. Solid State Sci. 2011, 13(3), 636-640. (35) Ceperley, D. M.; Alder, B. J. Phys. Rev. Lett. 1980 45, 566-569. (36) Perdew, J.P.; Zunger, A. Phys. Rev. B, 1981 23, 5048-5079. (37) Monkhorst, H. J.; Pack, J. D. Phys. Rev. B 1976 13, 5188-5192. (38) Clark, S. J.; Segall, M. D.; Pickard, C. J.; Hasnip, P. J.; Probert, M. J.; Refson, K.Z.; Payne, M. C. Z. Kristallogr. 2005, 220, 567-570. (39) Lopez-Cruz, E.; Cardona, M. Solid St. Commun.1983, 45(9), 787-789. (40) Vogg, G.; Brandt, M. S.; Stutzmann, M. Adv. Mater. 2000, 12(17), 1278-1281. (41) Bianco, E.; Butler, S.; Jiang, S.; Restrepo, O. D.; Windl, W.; Goldberger, J. E. ACS Nano 2013, 7(5), 4414-4421.
ACS Paragon Plus Environment
Page 4 of 9
Page 5 of 9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Journal of the American Chemical Society SYNOPSIS TOC:
ACS Paragon Plus Environment
Journal of the American Chemical Society 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35
ACS Paragon Plus Environment
Page 6 of 9
Page 7 of 9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
Journal of the American Chemical Society
ACS Paragon Plus Environment
Journal of the American Chemical Society 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
ACS Paragon Plus Environment
Page 8 of 9
Page Journal 9 of 9 of the American Chemical Society 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
ACS Paragon Plus Environment
