Medium-range order in amorphous selenium: Molecular dynamics simulations

In this paper we present a theoretical study on a glassy structure and infrared (IR)/Raman spectra of amorphous selenium (a-Se). The vibrational spectra are calculated for two-hundred independent amorphous samples obtained by performing molecular-dynamics (MD) simulations for one linear chain of 216 Se atoms. The interaction potential used in the MD simulation is constructed on the basis of ab initio molecular-orbital calculations (with the Hartree-Fock and second-order Moller-Plesset methods) for various Se clusters. The parameters in the bond-current and bond-polarizability models needed for the spectral calculations are also determined by the same ab initio approach. The calculated static structure factor, vibrational density of states, and IR/Raman spectra well reproduce experimental results. It is found that there exists a remarkable medium-range order in the disordered Se chain; i.e., a chain segment having four consecutive dihedral angles with alternate signs [i.e., (+, -, +, -) or (-, +, -, +)] tends to be excluded from the chain structure, which is attributed to the steric hindrance effect existing in those configurations. By comparing the polarized Raman spectra for two different random chains with and without the above steric hindrance effect, we find that only the former spectrum exhibits a distinct peak at 80 cm(-1) which is observed also in the experiment. Therefore the appearance of this peak provides useful information on the medium-range order existing in the disordered Se chain.