Lattice Dynamics of the Rhombohedral Polymorphs of CaSi2.

The structures of two trigonal-rhombohedral CaSi2 polymorphs (space group R3̅m) were studied by X-ray diffraction and polarized Raman scattering spectroscopy. Raman-active even-parity vibrational modes of A1g and Eg are unambiguously identified and assigned to the specific lattice eigenmodes. Experimental data are found to be in very good agreement with those predicted by density functional theory lattice dynamics calculations. The transformation of 6R structural modification of CaSi2 into its 3R polymorph, by high-temperature annealing in vacuum is also reported.