Structural behaviour of CaC2 at high pressures
J. Nylén, S. Konar, and U. Häussermann
Department of Materials and Environmental Chemistry, Stockholm University, Arrhenius Laboratory, Svante
Arrheniusväg 16, 106 91 Stockholm, Sweden
Carbides of alkali and alkaline earth metals occur predominantly as salt-like acetylides which
consist of the C22- dumbbell anion, isoelectronic to dinitrogen. Achieving optimum mutual
coordination of dumbbells and cations in a crystal structure seems to be delicate, as most acetylide
carbides exhibit polymorphism [1]. Polymorphism is especially pronounced within CaC2 where
three structurally ordered modifications are known at room temperature, in addition to a disordered
high temperature form. When synthesizing CaC2 it is almost always obtained as a mixture of
tetragonal CaC2-I and monoclinic CaC2-II. Interestingly, from theory it has been predicted that
already at very modest pressures (below 20 GPa) CaC2 should transform into novel modifications
where carbon atoms are arranged into polymeric moieties (chains or strands) [2] (Figure 1).
Figure 1: Structures of monoclinic CaC2-II and tetragonal CaC2-I
Experiments, Results and Remaining Questions
The effects of high pressure (up to 30 GPa) on the structural properties of calcium carbide, CaC2,
was studied at room temperature in a diamond anvil cell. The experiments were performed at
beamline P02.2 using a wavelength of 0.48245 Å and 0.28995 Å with the energies of 25.6 keV and
42.7 keV respectively. Detector distance was around 600 mm, determined by refinement of a CeO2
standard and the images were obtained by 10 s exposures then integrated using the software Fit2D
[3]. Pressure was increased stepwise, using the raman fluorescent shift of ruby as an internal
pressure indicator. Monoclinic CaC2-II is not stable at pressures above 2 GPa and seems to
transform into the tetragonal CaC2-I phase (Figure 2). At higher pressures, tetragonal CaC2-I
possibly undergoes a minor structural change between 10 – 12 GPa (Figure 3). At pressures above
20 GPa CaC2 irreversibly amorphizes. This amorphization contrasts the expectation of crystalline
phases with polymeric carbon substructures, as predicted by theory. Yet, the amorphous carbide
phase is puzzling: Does it still constitute of dumbbell ions, or rather of fragments of extended
networks? And how are the Ca atoms coordinated or bonded?
Figure 2: Transformation of monoclinic CaC2-II (red arrows) to tetragonal CaC2-I (black arrows).
Figure 3: Compression of CaC2 up to 24 GPa.
References
1 U.
Ruschewitz, Coord, Chem. Rev. 244, 115 (2003).
et al, submitted for publication.
3 A. P. Hammersley, S. O. Svensson, M. Hanfland, A. N. Fitch, D. Haüsermann, J. High Pressure
Res. 14, 235 (1996).
2 D.Benson