Structure of hydrogenated silicon clusters. Medium-sized clusters

The structures of the Si_nH_m clusters containing 10 to 70 silicon atoms and different numbers of hydrogen atoms are calculated in the MINDO/3 approximation using the Monte Carlo technique. The geometry optimization of the clusters showed the existence of several structural varieties that determine the optimal geometry of the clusters differing in size and hydrogen content. Small clusters (n < 20) with various geometrical configurations often have a hollow structure if the number of silicon atoms in the cluster is more than 12. For 20 ≤ n < 60 and the hydrogen content m ≤ n, hollow spheroidal geometry is most favorable. Staring from n ≈ 56−60, diamond structures are more favorable. The ratio c = m/n < 1, at which the spheroidal structure remains optimal, decreases with further increase in n.