title: A quenchable superhard carbon phase synthesized by cold compression of carbon nanotubes.
abstract: A quenchable superhard high-pressure carbon phase was synthesized by cold compression of carbon nanotubes. Carbon nanotubes were placed in a diamond anvil cell, and x-ray diffraction measurements were conducted to pressures of approximately 100 GPa. A hexagonal carbon phase was formed at approximately 75 GPa and preserved at room conditions. X-ray and transmission electron microscopy electron diffraction, as well as Raman spectroscopy at ambient conditions, explicitly indicate that this phase is a sp(3)-rich hexagonal carbon polymorph, rather than hexagonal diamond. The cell parameters were refined to a(0) = 2.496(4) A, c(0) = 4.123(8) A, and V(0) = 22.24(7) A (3). There is a significant ratio of defects in this nonhomogeneous sample that contains regions with different stacking faults. In addition to the possibly existing amorphous carbon, an average density was estimated to be 3.6 +/- 0.2 g/cm(3), which is at least compatible to that of diamond (3.52 g/cm(3)). The bulk modulus was determined to be 447 GPa at fixed K' identical with 4, slightly greater than the reported value for diamond of approximately 440-442 GPa. An indented mark, along with radial cracks on the diamond anvils, demonstrates that this hexagonal carbon is a superhard material, at least comparable in hardness to cubic diamond.
authors.affiliation: Los Alamos National Laboratory, Los Alamos, NM 87545, USA. z_wang@lanl.gov
authors.forename: Zhongwu
authors.initials: Z
authors.lastname: Wang
authors.forename: Yusheng
authors.initials: Y
authors.lastname: Zhao
authors.forename: Kimberly
authors.initials: K
authors.lastname: Tait
authors.forename: Xiaozhou
authors.initials: X
authors.lastname: Liao
authors.forename: David
authors.initials: D
authors.lastname: Schiferl
authors.forename: Changsheng
authors.initials: C
authors.lastname: Zha
authors.forename: Robert T
authors.initials: RT
authors.lastname: Downs
authors.forename: Jiang
authors.initials: J
authors.lastname: Qian
authors.forename: Yuntian
authors.initials: Y
authors.lastname: Zhu
authors.forename: Tongde
authors.initials: T
authors.lastname: Shen
mesh_terms: D002244:Carbon; D003080:Cold Temperature; D003460:Crystallization / Q000379:methods; D018130:Diamond; D014961:X-Ray Diffraction
chemical_list: D002244:Carbon; D018130:Diamond