Canyon Diablo lonsdaleite is a nanocomposite containing c/h stacking disordered diamond and diaphite

Abstract

In 1967, a diamond polymorph was reported from hard, diamond-like grains of the Canyon Diablo iron meteorite and named lonsdaleite. This mineral was defined and identified by powder X-ray diffraction (XRD) features that were indexed with a hexagonal unit cell. Since 1967, several natural and synthetic diamond-like materials with XRD data matching lonsdaleite have been reported and the name lonsdaleite was used interchangeably with hexagonal diamond. Its hexagonal structure was speculated to lead to physical properties superior to cubic diamond, and as such has stimulated attempts to synthesize lonsdaleite. Despite numerous reports, several recent studies have provided alternative explanations for the XRD, transmission electron microscopy and Raman data used to identify lonsdaleite. Here, we show that lonsdaleite from the Canyon Diablo diamond-like grains are a nanocomposite material dominated by subnanometre-scale cubic/hexagonal stacking disordered diamond and diaphite domains. These nanostructured elements are intimately intergrown, giving rise to structural features erroneously associated with h diamond. Our data suggest that the diffuse scattering in XRD and the hexagonal features in transmission electron microscopy images reported from various natural and laboratory-prepared samples that were previously used for lonsdaleite identification, in fact arise from cubic/hexagonal stacking disordered diamond and diaphite domains. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 2)'.

Keywords: diaphite; hexagonal diamond; lonsdaleite; nanocomposite; structural complexity.

Figure 1.

Photograph of a section of Canyon Diablo iron meteorite slice (left) containing hard carbon grains (formerly called diamonds) and a selection of grains extracted after acid dissolution and heavy density mineral separation (right). The iron was ground, polished and etched with nital and shows the characteristic mineralogy of a hard carbon-grain-bearing region. The hard carbon grains (indicated by the white arrows), which sit proud of the surface, occur within the graphite rim surrounding the crystalline troilite core. The iron meteorite slice is sample #34.102x in the Buseck Center for Meteorite Studies and is the mirror slice of the sample studied by Ksanda and Henderson in 1939 [41]. Scale bars next to the extracted hard carbon grains = 1 mm.

Figure 2.

XRD data of an approximately 0.1 mm-size hard carbon grain, reported in [24], analysed (a) as a physical mixture of c and h diamond structures and (b) with c/h stacking disorder using MCDIFFaX. (c) Example structure of c/h stacking disordered diamond projected along $⟨10\overline{1}0⟩$. Hexagonal and cubic stacking are indicated by ‘h’ and ‘c’, respectively.

Figure 3.

Structure model of type 1 and type 2 diaphite structures and their corresponding simulated XRD patterns plotted on XRD data measured from 2.0 × 2.0 µm⁻² areas of two Canyon Diablo grains referred to as grain 8 and grain 7 in [31]. (a) For type 1 diaphite graphene layers with ‘g+’ and ‘g−’ type stackings are inserted within c/h stacking disordered diamond. (b) For type 2 diaphite the ‘dd’ and ‘gg’ indicate the continuations of cubic diamond and graphene regions, respectively, whereas the grey-shaded areas show the structures required to transition from cubic diamond to graphene and vice versa. (c) The simulated patterns of type 1 (Φ_c= Φ_h= 0.4, Φ_dg= Φ_gd= 0.2, Φ_g+= Φ_g−= 0.4) and type 2 diaphite (Φ_gg = 0.9, Φ_dg = 0.0053, Φ_g = 0.1) obtained by DIFFaX analysis are shown in blue and pink, respectively, on the measured XRD patterns.

Figure 4.

Structure models and calculated diffraction patterns of h, c and c/h stacking disordered diamond as well as 2H graphite and diaphite along major projections. The arrangement of the diffraction patterns reveals the three-dimensional crystallographic relationship, reported by Garvie et al. [56] and Németh et al. [27], among the various structures. Double reflections marked by black circles for diaphite structures are expected to occur as a broad reflection.

Figure 5.

Low-magnification TEM images and corresponding SAED patterns taken from crushed grains (a,b) and FIB lamellae (c,d) of Canyon Diablo hard grains. Small inset in (c) shows the orientation of the FIB samples. The reflections spread over a large area roughly correspond to d spacings of 2.2 and 1.8 Å. The SAED patterns are indexed as $⟨01\overline{1}⟩$ (a,c) and $⟨\overline{2}11⟩$ (b,d) c diamond. White arrows mark indices for $⟨10\overline{1}0⟩$ (a,c) and $⟨0001⟩$ (b,d) c/h stacking disordered diamond. Vertical streaks on the SAED pattern (b) indicate (000l) c/h stacking disorder, i.e. the imaged area contains $⟨10\overline{1}0⟩$ and $⟨0001⟩$ projected c/h stacking disordered diamond domains. Black arrows in (c) and (d) point to graphite 000l reflections. Original BFTEM images and SAED patterns of (c) and (d) were reported in [31] (copyright, PNAS).

Figure 6.

HRTEM image of c/h stacking disordered diamond (a), with corresponding FFT indexed as c diamond and c/h stacking disordered diamond (b), and its structure model plotted on an HRTEM image (c). Original HRTEM image (a) was reported in [24], copyright Springer Nature Ltd.

Figure 7.

Complex (011) twins (a,b) with corresponding FFTs, and their structure models. The indices refer to c diamond structure. White arrows on the FFTs (a,b) mark a second set of streaking that resulted from the (011) twins. Original HRTEM images (a) were reported in [24], copyright Springer Nature Ltd.

Figure 8.

HRTEM images of type 1 and type 2 diaphite structures viewed along $⟨01\overline{1}⟩$ c diamond and their corresponding FFTs. Type 1 and type 2 diaphite intergrowth (a,b) and their corresponding structure model (c). Vertically superimposed diaphite and c/h stacking disordered diamond structures give rise to complexity for an approximately 50–60 nm thick sample (d).

Figure 9.

uHRTEM image (a) and its corresponding FFT (b) show evidence for $⟨111⟩$ c diamond and $⟨0001⟩$ c/h stacking disordered diamond intergrowth. White arrow marks in (b) split reflections with an approximately 10° rotation.

Figure 10.

Type 2 diaphite viewed along $⟨12\overline{1}⟩$ c diamond and $⟨0001⟩$ graphene. The original HRTEM image of (a) and the structure model (b) are reported in [24] (copyright Springer Nature Ltd.) and [27] (copyright ACS). White circles in (a) mark hexagonally arranged reflections and the dotted lines indicate the interface between $(13\overline{1})$ c diamond and $(0001)$ graphene. Type 2 diaphite examples with variable c diamond and graphene contents (c). Black dotted lines mark the interface between $(1\overline{1}3)$ c diamond and (0001) graphene. The $⟨12\overline{1}⟩$ c diamond contribution is evidenced by the $(1\overline{1}3)$ elongation of the domains (white dotted lines) and the streaking of reflections is marked by a white arrow (d) in a 50–70 nm thick sample.