Structure and Glass Transition Behavior of As-Rich As-Se Glasses: Modeling and Experiment

Abstract

Chalcogenide glasses have been widely used in various technologies due to their wide IR transparency and good glass forming ability. Arsenic selenide glasses (AsxSe100-x), in particular, have been readily adopted for IR applications and widely studied to provide insights into fundamental structure-property relationships. Arsenic rich AsxSe100-x glasses (? > 40), however, are marked by the presence of unusual structural phenomena which influence the properties of these glasses and are not yet fully understood, including features such as molecular units (As4Se3) and a second calorimetric signal during the glass transition. Therefore, the structure of these glasses has been reexamined in As-rich compositions using Raman spectroscopy and atomic simulations in an effort to clarify the structure and elucidate the origin of the second calorimetric signal. High-resolution Raman spectra of the As-Se glass binary reveal a complex evolution of the structure with increasing As content. The evolution of the structure in Se-rich compositions is consistent with the chain-crossing model, as previously established in the literature. However, analysis of the As-rich spectra with multivariant curve resolution (MCR) techniques reveals mixed As-(Se3-x, Asx) units and As-(As3) units as well as molecular As4Se3 and As4 butterfly units. A model of the spectrum made by fitting Gaussian functions to the MCR components was then used for structural analysis. The population of As4Se3 units was derived, which shows a maximum at x = 60% and a decoupling from the population of As-rich units, consistent with a local minimum in 𝑇𝑔. Ab initio molecular dynamics modelling of the structure of As60Se40 glass was also performed. Comparison of the structure derived from characterization of the model to reported experimental measurements of the glass show generally good agreement. Analysis of the structural units found in the model support the existence of As4Se3 molecules in the glass while casting further doubt on the presence of As4Se4. Furthermore, the individual molecular As4 units previously attributed to the 203 cm-1 peak in the Raman spectrum are found to be absent, with As4 butterfly units incorporated into the glass network present instead. First-principles Raman spectrum calculations performed on the As4 butterfly units find that the Raman spectrum of these units vary significantly depending on the configuration. Using various chain configurations and modifying the relaxation of these chains through tension suggests a convergence of the asymmetric stretching mode of these units toward the spectral feature near 203 cm-1 found in the glass, supporting the assignment of this feature to As4 butterfly units. Finally, the dynamics of the glass transition for As-rich As-Se glasses were characterized through modulated differential scanning calorimetry (MDSC) and reveal a bimodal glass transition exhibiting different relaxation behaviors for the two observed components. The prominence of this bimodal signal is maximized at the As60Se40 composition, coinciding with the local minimum in 𝑇𝑔 and the maximum population of As4Se3 molecules. In situ Raman spectroscopy of the As60Se40 composition heated through the 𝑇𝑔 show that the first peak in MDSC measurements correlates with the relaxation of the amorphous backbone while the second peak is associated with the decomposition of As4 butterfly units. Features associated with As4Se3 molecules appear to alter at the onset of 𝑇𝑔, indicative of decoupling of these units from the glass network in accordance with reported behavior of molecular glasses.