Reverse Monte Carlo study of structural relaxation in vitreous selenium

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Abstract
Vitreous selenium is used extensively for light detection. Its photoconducting properties are altered by structural relaxation near room temperature. X-ray diffraction, densiometry, and calorimetry are used to determine the change of the glass state during relaxation. Annealing for 24 h at 300 K increases the density by 0.14% and lowers the enthalpy by 105 J/mol. The structure factor of cast samples was measured before and after annealing, and reverse Monte Carlo models of the structure of selenium glass were generated to match the data for the quenched and annealed states. Atoms in amorphous selenium are arranged in randomly oriented chains. In the model clusters annealing effects are found by analyzing inter- and intrachain distances, bond angle distributions, and dihedral angle distributions. The average bond length remains unchanged upon annealing, while the distribution of bond lengths becomes narrower by 1.2%. Distances between atoms in adjacent chains decrease by about 0.03%, i.e., the selenium chains move closer together, and the distribution of interchain distances becomes narrower. Bond angles within the chains are affected slightly. The mean bond angle decreases by 0.016°, which indicates a greater folding of the selenium chains and represents an evolution towards the angles found in crystalline selenium phases. The bond angle distribution for the annealed state is 0.006° narrower.