Natural Carbynes,Including Chaoite, on Earth, in Meteorites, Comets, Circumstellar and Interstellar Dust - Polyynes: Synthesis, Properties, and Applications - page 349

crystal grains. A synthesis of available data [20,61,77] suggests carbyne
nanocrystals, specifically chaoite, could exist in meteorites after thermal
annealing of a pure carbon, amorphous precursor with disseminated
sp 1 -hybridized entities.

16.9 LABORATORY CARBON CONDENSATION EXPERIMENTS

Chaoite, possibly carbon VI, a -carbyne, b -carbyne and carbon VIII were
reported among carbynes that condensed in carbon vapors produced by
resistive heating and high-energy laser irradiation of a carbon starting
material [92]. Except chaoite and carbon VI, carbynes in the experiments
were identified by the a 0 value [92]. In general, low-number (<22) elemental
carbon crystals are defined by default as being a carbyne when the hexa-
gonal unit cell dimensions are larger than those of hexagonal graphite
(a 0 ¼ 0.670 nm). This is fair practice but whether distinctions

within a broad ''carbyne group'' are possible remains questionable.
Are carbynes produced in laboratory carbon and C-H 2 -Ar vapors
designed to simulate the properties carbon dust in astrophysical environ-
ments? Field-emission scanning electron microscope (FESEM) and HRTEM
studies of vapor-condensed carbon smokes showed a wide range of carbon
morphologies that were free of metal impurities (at EDS detection limits),
viz.1.C 60 and higher fullerenes that agglomerated into amorphous soot
grains, 2. fullerenic nanotubes and onions, 3. amorphous carbon sheets,
4. individual poorly- graphitized carbon loops or clusters on the sheets, and
5. pre-graphitic carbon ribbons [4,76]. A thermally annealed (1073K; 3 h)
condensed carbon sample showed clusters of very thin, hexagonal plates

0.246 nm; c 0 ¼

( Figure 16.4 ) with a maximum d-value of 0.53 nm in hexagonal single-crystal

c-axis electron diffraction patterns. This d-value was probably not a 0 but the
(110) hexagonal plane. These plates with a 0 ¼ 1.06 nm are indeed very thin.

While C 60 fullerene was confirmed by high-performance liquid chromato-
graphy and mass spectrometry [76], no independent data are available to
confirm a carbyne nature of the plates. Assuming the plates are carbyne they
might have formed by a solid-state transformation of condensed amorphous
carbon sheets, which should then have contained sp 1 -hybrizied linear carbon
domains. But, there could be an alternative.
A laser ablation experiment using a target of amorphous carbon with
carbyne ''nanodomains'' found that the presence of sp 1 -hybridized carbon
inhibited C 60 fullerene formation but promoted the formation of higher
fullerenes, C 74 to C 160 , by the step-wise addition of discrete [C

C] units [93].
The target used in the carbon-vapor condensation experiments [4] was
apparently carbyne-free since abundant C 60 did form and agglomerated into
amorphous soot grains [76]. Higher fullerenes in the soot grains could have
formed according to the fullerene growth reaction that introduces C 2
molecules into the gas phase [94]. When, in that case, the gas molecules were