Abstract
Using group theoretical principles, a complete set of symmetry coordinates can be determined for a complex crystal. The symmetry coordinates for k = 0 form a basis in terms of which the normal modes can be expanded. A model based on central forces and involving only four force parameters has been used to calculate the normal mode frequencies of LaCl3. From an analysis of the C26h(P63/m) space group, one can determine that there are three infrared active (1A1u and 2E1u), six Raman active (2A1g, 1E1g, and 3E2g) and five non-active (2Bg, 2Bu, and 1E2u) vibrational modes. Three force parameters were determined by using the infrared active fundamentals which were observed(1) at 138, 165 and 210 cm−1. The frequencies of the other modes were expressed as functions of the fourth force parameter. For one value of this parameter, agreement with the observed(2) Raman lines was obtained with an average deviation of 6·6 cm−1.
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