Structural behavior of the acetylide carbides Li2C2 and CaC2 at high pressure.

The effects of high pressure (up to 30 GPa) on the structural properties of lithium and calcium carbide, Li(2)C(2) and CaC(2), were studied at room temperature by Raman spectroscopy in a diamond anvil cell. Both carbides consist of C(2) dumbbells which are coordinated by metal atoms. At standard pressure and temperature two forms of CaC(2) co-exist. Monoclinic CaC(2)-II is not stable at pressures above 2 GPa and tetragonal CaC(2)-I possibly undergoes a minor structural change between 10 and 12 GPa. Orthorhombic Li(2)C(2) transforms to a new structure type at around 15 GPa. At pressures above 18 GPa (CaC(2)) and 25 GPa (Li(2)C(2)) Raman spectra become featureless, and remain featureless upon decompression which suggests an irreversible amorphization of the acetylide carbides. First principles calculations were used to analyze the pressure dependence of Raman mode frequencies and structural stability of Li(2)C(2) and CaC(2). A structure model for the high pressure phase of Li(2)C(2) was searched by applying an evolutionary algorithm.