Symmetry Coordinates and Lattice Vibrations of LaCl3

Symmetry coordinates for the 24k=0 modes of LaCl3 are derived. These symmetry coordinates form a basis in terms of which the normal modes are expanded. From a group theoretical analysis of the C6h2(p63/m) space group, it is determined that there are three infrared-active (1A1u and 2E1u), six Raman-active (2A1g, 1E1g, and 3E2g), and five nonactive (2Bg, 2Bu, and 1E2u) vibrational modes. Three infrared absorption bands were observed in evaporated LaCl3 films at 138, 165, and 210 cm—1. Three force parameters were determined using the wavenumbers of the infrared-active modes. The wavenumbers of the other modes were expressed as functions of the fourth force parameter. For one value of this parameter, agreement with the observed Raman lines was obtained with an average deviation of 6.6 cm—1.