Anharmonic calculation of bandwidths and frequency shifts in crystalline CO 2

Abstract

The bandwidths and the anharmonic frequency shifts of the optical lattice phonons of crystalline CO 2 are calculated as a function of temperature using three different intermolecular potentials. A previously available potential (MOSMD) which reproduces correctly the structure, energy and harmonic frequencies of the CO 2 crystal provides anharmonic, calculated bandwidths about 10 times larger than the experimental values. An improved potential (PRC-1) is obtained by moving inside the OC bonds the oxygen interaction center and by increasing the molecular quadrupole. This potential predicts bandwidths and shifts of the right order of magnitude but still too large. The third potential (PRC-2) is obtained from the previous one by using a non-collinear distribution of negative charges. The negative charges are localized on exagons around the OC bond at a distance d = 0.3 Å. This potential reproduces correctly the experimental bandwidths and yields small anharmonic shifts. The temperature dependence of the bandwidths and shifts is correctly reproduced in terms of three-phonon decay processes for the T + g and for the T − g phonons. In the case of the E g phonon four-phonon processes are required in order to fit the experimental data.