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The dependence of the infrared-absorption properties on size variation and surface relaxation of rectangular MgO microcrystals (clusters) has been studied in the context of the rigid-ion model. The frequency ωₘ of maximum absorption (for a given polarization direction) increases slightly with increase in cluster size from 180 to 900 ions, and also when surface relaxation of the cluster is taken into account. In all cases studied, ωₘ is extremely close to the Fr\"ohlich frequency ωF for spherical particles treated in continuum theory. The total absorption is independent of crystal orientation and relaxation, and proportional to the number of particles in the cluster.
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Chen et al. (1978) studied this question.
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