Cent. Eur. J. Chem. • 10(5) • 2012 • 1676-1680
DOI: 10.2478/s11532-012-0085-3
Central European Journal of Chemistry
C12: The building block of hexagonal diamond
Research Article
Gaspar Banfalvi
Department of Microbial Biotechnology and Cell Biology
University of Debrecen, Debrecen 4010, Hungary
Received 2 February 2012; Accepted 13 June 2012
Abstract: The crystalline structure of diamond may consist of C8, C10 or C12 building units. C8 was regarded as the building block of the
hexagonal diamond also known as Lonsdaleite. Adamantane (C10H16) alicyclic hydrocarbon has the same arrangement of carbon atoms
as the basic C10 unit of the cubic diamond lattice. C12 has five rings of mixed type, three of them in boat, two in chair conformation.
Model building revealed that the C8 unit containing exclusively three rings in boat conformation does not exist. Further addition of
carbon atoms to C8: a) results in C12 unit, b) allows the multiplication of C12 units, and c) by reducing the boat-to-chair ratio explains
the hardness of the hexagonal diamond Lonsdaleite. The Lonsdaleite nucleus can be grown to special diamond grains with the outer
atoms of the C12 building units replaced by different elements. This recognition can be utilized in the production of synthetic diamonds
under high-pressure high-temperature conditions or in the chemical vapor deposition growth technique.
Keywords: Unit structure • Cubic • Londaleite • Model building • Stability
© Versita Sp. z o.o.
1. Introduction
The sp3 hybride orbitals of carbon atoms allow the
formation of two isomers of crystalline diamond the
classic diamond which has a cubic structure and its
hexagonal modifications [1,2]. Adamantane is viewed as
the unsubstituted central unit of cubic diamond containing
ten carbon atoms. [3,4] Lonsdaleite, the hexagonal
diamond [5-10] is assumed to be a C8 modification [11].
Other hydrogenated and amorphous mixed diamond
structures have been reported [12-15]. Taking the
extreme conditions of allotrope formations of carbon
atoms one can assume that under high-pressure and
high-temperature conditions different crystal structures
of diamond could have formed.
Simple models made of plastic, macramé, wire
allowed the explanation of seemingly perplexing
subjects such as sugar puckering and the mechanism
of DNA supercoiling [16-18]. Backbone molecular
models (Blackwell Molecular Models) [19] built in three
dimension were used for the visualization of pentoses
and their basic conformational changes in the furanose
ring. It also contributed to the understanding why ribose
was selected as the exclusive sugar component of
nucleic acids [20]. Sugar puckering in double-stranded
DNA is one of the major factors that will determine
whether DNA is in B-form (C2’ endo conformation) or
in A-form (C3’ endo conformation). It was expected that
similarly to pentoses in hexoses and in the cyclohexane
polymer diamond different conformations of cyclohexane
may exist, and the smallest units of these conformations
are multiplied to form a lattice structure. The possibility
to predict the crystal structures from the molecular
conformations of building units has been coveted, but
not completely understood. This recognition has drawn
attention from conformational changes of pentoses and
hexoses, to the conformers of cyclohexane rings and
to the potential building blocks of diamond. While the
cubic arrangement of atoms in diamond crystals is well
established less attention has been paid to the basic unit
structure of hexagonal diamond.
This paper describes the building blocks of diamond
that were constructed and compared to determine which
one can be multiplicated to form a crytalline structure.
2. Computational details
Using molecular model or computer simulation the
adamantane-type (C10) arrangement of all carbon
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G. Banfalvi
Figure 1.
Potential building units of diamond. The adamantane analog C10 unit of cubic diamond contains three cyclohexane rings all in chair
conformation; C12 unit: two chairs surrounded by three boats; C8 unit: three rings all in boat conformation. a,d,g: ball and stick models;
b,e,h: space filling models; c,f,j: computer generated models (ChemDraw Ultra 3D).
bonds results in three six-membered rings that are in
the staggered chair conformation (Figs. 1a-1c). The
C12 building unit contains two chairs (C: 1, 2, 3, 4, 5,
6; C: 7, 8, 9, 10, 11, 12) and three boats (C: 1, 6, 7,
8, 9, 2; C: 4, 5, 6, 7, 12, 11 and C: 2, 9, 10, 11, 4, 3)
(Figs. 1d-1f). In the C8 modification (Lonsdaleite) the
bonds between the carbon atoms are also tetrahedrally
linked, (21) but in eclipsed conformation which defines
the axis of hexagonal symmetry with three boats turned
inward (C:1, 4, 5, 8, 7, 3; C:1, 2, 6, 8, 7, 3; C:1, 2, 6, 8,
5, 4) (Figs. 1g-1j).
The crystalline network which cannot be built from
C8 units is demonstrated in Fig. 2. The explanation
is provided by the structural differences between C8
and C12. Fig. 2a shows three C8 units referred to as
Lonsdaleite (structural analog of wurtzit, zinzite), built
around the C4 – C5 axis. One C8 unit is shown separately
(Fig. 2b). This structure cannot be multiplicated,
addition of further carbon atoms indicated by the arrows
(Fig. 2c) will change the full boat conformation to boatchair conformers and the C8 to C12 units (Fig. 2d).
One of the six C12 units in Fig. 3 is circled at the upper
right corner. The C8 nucleus in Fig. 3 is indicated with
white letters inside the carbon atoms. Black numbers in
Fig. 3. show that further additions of carbon atoms to C8
results in chair conformers of cyclohexans contradicting
the principal of full boat character of C8 building blocks.
3. Results and discussion
The conventional diamond cell is cubic with a crystal
structure equivalent to a face-centered (FCC) lattice
(Fig. 4a). The C10 adamantane unit is multiplicated to
become cubic diamond (Fig. 4b). The multiplication of
C12 hexagonal units (Fig. 4c) results in the hexagonal
diamond Lonsdaleite (Fig. 4d).
The extreme polymorphism of carbon allotropes
suggests that the conformation of cyclic rings could
have changed during the formation of diamonds and
some of the hexagonal chairs could have assumed boat
conformation. The production of synthetic diamonds
containg various amorphous metastable and crystalline
phases [12,22] confirms this notion. The demonstration
that the cubic diamond can be transformed to hexagonal
diamond under extreme conditions [8] indicates that
the hexagonal boat-to-chair ratio could modify the
hardness and crystalline forms of diamond. The chair
conformation containing exclusively staggered carbon
atoms has a lower energy level and is more stable than
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C12: The building block of hexagonal diamond
Figure 2.
Turning C8 to C12 unit. a) Three C8 structures surround the central C4 and C5 carbon atoms. b) One C8 structure is numbered and
shown separately. c) Addition of carbon atoms indicated by the arrows is turning C8 to C12. d) One C12 unit is numbered and shown
separately
Figure 3.
Relationship of C8 and C12 units in hexagonal diamond. White numbers inside carbon atoms indicate a C8 unit. Additions of carbon
atoms to C8 generate cyclohexane rings (black numbers outside carbon atoms). The upper right C12 unit is circled.
the boat conformation with ecclipsed carbon atoms.
Consequently, the full chair cubic diamond is more
stable than isomers of diamond with their carbon atoms
being exclusively in boat conformation. The mixture of
boat-chair conformations could explain the unexpected
hardness of Lonsdaleite. These observations have
practical implications for the industrial production of
different forms of diamonds.
The most common crystal habit is the eight-sided
octahedron or diamond shape, but diamond crystals
can also form cubes, dodecahedra, and combinations of
these shapes. These structures are manifestations of the
cubic crystal system. The crystal structure of hexagonal
diamonds is not well known. It escaped attention that
the arteficial growth of Londaleite grains was based
on the formation of C12 units the real building blocks
of hexagonal diamond. The transformation of the C8
structure to C12 unit provides a reasonable explanation
for the debated structure of hexagonal diamond.
Lonsdaleite nuclei sourronded by hydrogen atoms
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G. Banfalvi
Figure 4.
Multiplication of building blocks of diamond. a) Unit cell of cubic diamond with two interpenetrating face-centered cubic lattices.
b) Multiplication of C10 (adamantane) units forming cubic diamond. c) Unit cell structure of hexagonal lattice. d) C12 building blocks
multiplicated to form hexagonal diamond.
can be grown further to diamond grains by replacing
the outer hydrogens with carbon atoms [23]. The C8C12 conversion also helps to understand how diamond
growth is terminated. Diamond grains or the lattice
structure cannot grow indefinitely and the growth has
to come to an end either by coupling hydrogen atoms
as the most simple dopants or other elements to the
surface carbon atoms. Natural diamonds embrace about
50 trace elements that have been found on the sub-ppm
scale reflecting the environment in which the final phase
of crystallization took place [23]. The discovery of the
building block C12 has implications in artificial diamond
synthesis especially at the end of the synthetic process
when the outer atoms are being replaced by different
elements that give the diamond or monolayer special
character. Synthetic hexagonal diamonds are produced
by high-pressure high-temperature synthesis during
which graphite is converted to diamond. Although, the
resulting arteficial diamonds are usually small (few
millimeters in diameter) with uneven surfaces and sharp
edges, not acceptable as gemstones, but their edges
serve as cutting tools and drill-bits. Proper conditions
and dopants chosen for synthesis are expected to widen
the selection of arteficial diamonds. The construction
of semiconductors with new replacement elements in
thin electronic monolayers offers further applications of
synthestic diamonds. The clarification of the structure of
hexagonal diamond is expected to contribute to the better
understanding and industrial production of synthetic
diamonds especially at the end of the synthesis when
the outer carbon atoms of the nucleus will be replaced
by different heavy metals rather than by hydrogens
providing special color, lustre and hardness. Additions
of replacement metals at the final stage of synthesis
determines the application of hexagonal diamonds.
4. Conclusion
It was found that there are only two diamond structures:
the cubic diamond consisting of C10 (full chair
adamantane) building block and the hexagonal diamond
containing the so far unknown C12 units (mixed boat
and chair conformers). The C8 (full boat) structure
known earlier as Lonsdaleite turned out not to be the
unit structure of hexagonal diamond as further additions
of carbon atoms to C8 results in mixed boat and chair
conformations that are characteristic to the C12 unit.
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C12: The building block of hexagonal diamond
Acknowledgement
This work was supported by an OTKA grant T42762.
The technical contribution of Dr. Gabor Nagy and Dr.
Andras Fodor are gratefully acknowledged.
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