Structures and Stabilities of CaO and MgO Clusters and Cluster Ions: An alternative interpretation of the experimental mass spectra

The structures and relative stabilities of doubly-charged nonstoichiometric (CaO)$_n$Ca$^{2+}$ (n=1--29) cluster ions and of neutral stoichiometric (MgO)$_n$ and (CaO)$_n$ (n=3,6,9,12,15,18) clusters are studied through {\em ab initio} Perturbed Ion plus polarization calculations. The large coordination-dependent polarizabilities of oxide anions favor the formation of surface sites, making the critical cluster size where anions with bulk coordination first appear larger than that found in the related case of alkali halides. Thus, we show that there are substantial structural differences between alkali halide and alkaline-earth oxide cluster ions, contrary to what is suggested by the similarities in the experimental mass spectra. An alternative interpretation of the magic numbers for the case of oxides is proposed, which involves an explicit consideration of isomer structures different from the ground states. A comparison with the previously studied (MgO)$_n$Mg$^{2+}$ cluster ions shows that the emergence of bulklike structural properties with size is slower for calcium oxide. Nevertheless, the structures of the doubly charged clusters are rather similar for the two materials. On the contrary, the study of the neutrals reveals interesting structural differences between MgO and CaO, similar to those found in the case of alkali halides.