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Abstract
The temperature dependence of the phonon frequencies and bandwidths has been studied using Raman spectroscopy between 293 and 350 K. It can be seen that in the region of the phase transition at≈347 K there is a change in the slope of both curves. We have tried to explain these features by means of reorientation of molecules. Calculations have revealed an additional minimum of the lattice energy where the molecules are rotated by an angle of 180° around their axes relatively to their room temperature position. The stability of this proposed structure has been further tested by the calculation of the dispersion curves. No imaginary frequencies have been detected for the wave vectors chosen. Also, there are no great differences in the phonon frequencies of the two structures for k = 0, as has also been observed in the spectra recorded.
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