The synthesis of polymeric carbon dioxide has long been of interest to many chemists and materials scientists. Very recently, the polymeric phase of carbon dioxide (called CO2-V) at high pressures and temperatures was discovered [1]. This new material can be quenched to ambient temperature above 1 GPa. The vibration spectrum of CO2-V is similar to that of SiO2 polymorphs, indicating that it is an extended covalent solid with carbon-oxygen single bonds. It also exhibits a very interesting optical behaviour, generating the second harmonic of Nd:YLF laser at a wavelength of 527 nm with a conversion efficiency near 0.1%.

The determination of crystal structure of CO2-V has, however, been a challenge due to (i) the incomplete transformation resulting in a mixture phase of CO2-III and V, (ii) relatively low-Z materials, (iii) complex, low symmetry crystal structure, and (iv) a highly strained lattice with strongly preferred orientation. Despite these difficulties, the characterisation of the crystal structures of various CO2 phases has been possible, including the polymeric phase by using an intense 10 µm-size focused monochromatic synchrotron X-ray beam and a fast scanning image-plate detector at ID30 at the ESRF [2].

Carbon dioxide crystallises to an optically isotropic cubic Pa3 structure, CO2-I at 1.5 GPa and ambient temperature (Figure 39a). Above 11 GPa, CO2-I transforms to the orthorhombic Cmca phase, CO2-III, which exists in a wide range of pressures above 70 GPa. The large quadruple moment of linear CO2 molecule is considered to stabilise both CO2-I and III at least at relatively low pressures. However, above 30 GPa, CO2-III develops a very characteristic texture indicative of a highly strained lattice (Figure 39b). It also shows an abnormally large pressure gradient exceeding 20% of the maximum pressure of the sample within 100 µm. These observations clearly indicate that CO2-III has high material strength at these pressures, which is rather unusual for a molecular crystal. In fact, it has been found that the bulk modulus of CO2-III is unusually high at 87 GPa, comparable to that of elemental silicon.

Above 35 GPa and 1800 K, highly strained CO2-III transforms into a new phase CO2-V that can be quenched at ambient temperature (Figure 39c). It clearly shows that the texture in the transformed area is distinct from that in a non-transformed area. In this study, the crystal structure of CO2-V has been found to be orthorhombic (P212121), analogous to SiO2 tridymite (a distorted high temperature phase of ß-quartz). Each carbon atom is bonded to four oxygen atoms at a carbon-oxygen distance of 1.36 Å at 40 GPa and an O-C-O angle of 110°. These CO4 tetrahedral units share their corner oxygen atoms to form a layered structure in the ab-plane; whereas, the apices of the tetrahedra are connected through oxygen atoms along the c-axis (Figure 40). This interconnected layer structure of tetrahedra results in a C-O-C angle of 130°, substantially smaller than those of SiO2 tridymites 174-180° or of quartz 145°. Such rigidity in the C-O-C angles may reflect the fact that oxygen atoms in CO2-V are more tightly bound than in SiO2 and result in a higher covalence and bulk modulus for CO2-V than for any of the SiO2 polymorphs. In fact, we found that the bulk modulus of CO2-V is about 362 GPa, substantially higher than SiO2-quartz (37 GPa) and even stishovite (310 GPa). It is nearly the same value with cubic-BN (369 GPa) and thus could be a good candidate for a superhard polymer.

[1].V. Iota, C. Yoo, H. Cynn, Science, 283, 1510 (1999).
[2].C.S. Yoo, H. Cynn, F. Gygi, G. Galli, M. Nicol, D. Häusermann, S. Carlson, C. Mailhiot, Phys. Rev. Lett., in print (1999).

C.S. Yoo (a), H. Cynn (a), F. Gygi (a), G. Galli (a), V. Iota (a), M. Nicol (b), S. Carlson (c), D. Häusermann (c), C. Mailhiot (a).

(a) Lawrence Livermore National Laboratory, Livermore (USA)
(b) High Pressure Science Center at the University of Nevada (USA)
(c) ESRF