Structural transformation and vibrational properties of BaC2 at high pressure

We show that the ambient-pressure tetragonal phase of BaC2 (CaC2-type, space group I4/mmm), with sixfold coordination of Ba atoms and C-2 dumbbells, transforms reversibly to a new structure type at 4 GPa; it is an eight-coordinated rhombohedral modification (space group R (3) over barm), which can be viewed as a distorted variant of the CsCl-type structure. X-ray diffraction experiments further reveal an irreversible amorphization of BaC2 above 30 GPa. The possibility of a pressure-induced polymerization of isolated C-2 dumbbells into a network is considered. We also study lattice dynamics of both crystalline phases by Raman measurements, and compare the experimental observations on structures, phonons, and the first phase transition with the results of ab initio calculations. The latter also supply additional data on atomic positions, interatomic distances, and the volume-dependent equation of state (total energy and pressure).