NEW PHASES AND CHEMICAL REACTIONS IN SOLID CO UNDER PRESSURE

The Raman scattering of solid carbon monoxide was studied in a diamond cell from 15 to 297 K at pressures from 1.0 to 5.8 GPa. At low temperature a transition occurs near 3.4 GPa from the known ..cap alpha..-phase (space group P2/sub 1/3) to a new element of-phase (structure unknown), rather than to the predicted ..gamma..-phase (P4/sub 2//mnm). A transformation from ..beta..-CO (space group P6/sub 3//mmc) into a new sigma-phase (Pm3n.) was found near 5.2 GPa at room temperature. Above about 4.6 GPa and 80 K, CO reacts photochemically when irradiated with visible laser light. The photoreactivity may be associated with the formation of a yellow polymer, which can be recovered at zero pressure. 31 references, 3 figures.