NA-4(3+) CLUSTERS IN SODIUM SODALITE

We have measured and calculated the absorption spectrum of the Na-4(3+) clusters in Na3[AlSiO4]3 sodalite prepared by high-vacuum deposition of sodium atoms. The samples with a Na-4(3+):Na-3(3+) cluster ratio up to 1:10 show a single-absorption feature with lambda(max) = 628 nm (1.99 eV). The absorption originates from the individual sodalite cages containing the Na-4(3+) cluster. For the Na-4(3+):Na-3(3+) cluster ratio larger than 1:10, when some of the Na-4(3+) clusters are likely to interact, the changes in the absorption spectra indicate the onset of insulator-metal transition. Time-dependent quantum mechanical calculations of the photon absorption cross section of Na-4(3+) clusters in sodalite at infinite dilution were carried out. The dependence of the calculated spectra on sodalite framework charges and cluster geometry was used to determine which of the proposed charge distribution models are consistent with the absorption spectra. The best agreement between measurements and calculations is obtained for Na = +1, Si = +1.9, Al = +0.9, and O = -0.95.