Vibrational properties of nanometricAB2 ionic clusters

Abstract

A broad survey of harmonic dynamics inAB2 clusterswith up to N = 3000 atoms is performed using a simple rigid ion model, with ionic radii selected togive rutile as the ground state structure for the corresponding extended crystal.The vibrational density of states is already close to its bulk counterpart forN∼500, with characteristic differences due to surfaces, edges and vertices. Two methods areproposed and tested to map the cluster vibrational states onto the rutile crystal phonons.The net distinction between infrared (IR) active and Raman active modes that exists forbulk rutile becomes more and more blurred as the cluster size is reduced. It isfound that, in general, the higher the IR activity of the mode, the more this isaffected by the system size. IR active modes are found to spread over a widefrequency range for the finite clusters. Simple models based on either a crudeconfinement constraint or surface pressure arguments fail to reproduce the results ofthe calculations. The effects of the stoichiometry and dielectric properties of thesurrounding medium on the vibrational properties of the clusters are also investigated.