Theoretical study of electronic and structural properties of amorphous arsenic chalcogenides

We have carried out electronic structure calculations on arsenic chalcogenide molecules and atomic clusters. The molecules and clusters have been studied as possible models for the structure of the amorphous arsenic chalcogenides, As$\sb2$S$\sb3$ and As$\sb2$Se$\sb3$. The molecules studied are the As$\sb4$S$\sb4$, As$\sb4$S$\sb5$, As$\sb4$S$\sb6$, and As$\sb4$Se$\sb4$ molecules, while the clusters include the eclipsed and staggered AsS$\sb3$ pyramids, the As$\sb2$S$\sb3$ helical chains, the As$\sb2$S$\sb3$ outrigger raft, and the As$\sb2$S$\sb3$ and As$\sb2$Se$\sb3$ hexagonal rings. Our results show that it is possible to calculate bond charge distributions, cohesive properties, and densities of states using small clusters, and obtain good agreement with experiments performed on the corresponding amorphous solids. We have also demonstrated the advantage of the ab initio norm-conserving pseudopotential method in calculating cohesive and bonding properties of clusters and molecules over all electron approaches. Based on the results of these calculations, we have demonstrated the transferability of the arsenic-sulfur bond among different clusters and molecules. The arsenic-sulfur bond energy is also transferable among majority of clusters. For the clusters that do not show the transferability of the arsenic-sulfur bond energy, we can explain the discrepancies by other factors. We have developed a model for the relative stabilities of the arsenic sulfide molecules. The arsenic-arsenic bonds in As$\sb4$S$\sb4$ and As$\sb4$S$\sb5$ are suggested as the main reasons for the stability of these molecules. The As$\sb4$S$\sb4$ and As$\sb4$Se$\sb4$ molecules represent well the electronic properties of the corresponding molecular solids. A randomly packed networks of either As$\sb4$S$\sb5$ and As$\sb4$S$\sb6$ molecules have been ruled out as the models for the local structure of the amorphous As$\sb2$S$\sb3$. The eclipsed and staggered pyramids have also been ruled out as possible structural models for the amorphous As$\sb2$S$\sb3$. However, flipping of the pyramidal units may provide a mechanism for amorphization of arsenic chalcogenides. We cannot make any conclusive choice among the outrigger raft, helical chain, and As$\sb2$S$\sb3$ hexagonal ring as structural models for the amorphous As$\sb2$S$\sb3$. The arsenic-sulfur bond charge distributions, bond energies, and densities of states are very similar among these clusters and all agree quite well with experimental data. More specific experimental results are therefore necessary to allow discrimination among these possible models