# Johari-Goldstein relaxation in quenched and irradiated chalcogenide glasses

## TL;DR

This study shows that X-ray irradiation can increase the Johari-Goldstein relaxation in chalcogenide glasses, offering a new way to control their mechanical properties.

## Contribution

X-ray irradiation is introduced as a new method to control structural heterogeneity and relaxation dynamics in glasses.

## Key findings

- X-ray irradiation and rapid quenching both increase the strength of the Johari-Goldstein relaxation in GeSe3 glasses.
- Irradiated glasses show a stronger Johari-Goldstein relaxation than isenthalpic glasses produced by thermal quenching.
- The Johari-Goldstein relaxation is linked to defect regions in the glass network, which act as plastic units.

## Abstract

When a liquid is cooled down to temperatures close to the glass transition, the relaxation dynamics are characterized by two timescales associated with the structural relaxation and a secondary process known as the Johari-Goldstein (JG) or slow β relaxation. The JG relaxation is related to many crucial properties of glasses, such as their plastic response, and is here investigated using fast-scanning calorimetry in high-enthalpy GeSe3 glasses. High-enthalpy states are reached by two methods: (1) increasing the cooling rate used to quench the melt and (2) irradiating the glass with X-rays. Both methods make the JG relaxation visible in the calorimetric traces, where it appears as an exothermic signal at temperatures below the glass transition. The JG relaxation can be associated with mobile regions produced by quenching or defect regions produced by irradiation. These findings strongly support a general connection between the JG relaxation and local defect regions in the glass network and offer a new strategy to control via X-ray irradiation a key feature of the glass transition and, thereby, the related mechanical properties of the glass.

•High-enthalpy glasses are produced by thermal quenching and X-ray irradiation•Both methods increase the strength of the secondary Johari-Goldstein relaxation•Iso-enthalpic glasses produced by different methods are different glasses•The secondary relaxation originates from defect regions within the glass network

High-enthalpy glasses are produced by thermal quenching and X-ray irradiation

Both methods increase the strength of the secondary Johari-Goldstein relaxation

Iso-enthalpic glasses produced by different methods are different glasses

The secondary relaxation originates from defect regions within the glass network

Understanding relaxation processes in glasses is essential for linking their microscopic dynamics to macroscopic mechanical behavior. As a liquid is cooled toward the glass-transition temperature (Tg), its relaxation dynamics separate into two distinct processes: the structural (α) relaxation, involving large-scale cooperative rearrangements, and the faster Johari-Goldstein (JG) or β relaxation, a localized motion that persists below Tg. The JG relaxation governs the residual mobility of glasses and plays a central role in phenomena such as plastic flow and crystallization. Here, the JG relaxation is investigated in high-enthalpy GeSe3 glasses using fast-scanning calorimetry. High-enthalpy glassy states are reached through two independent routes: rapid thermal quenching from the melt and X-ray irradiation of the solid glass. Both methods increase structural disorder and reveal the JG relaxation as an exothermic signal below Tg in the calorimetric traces, corresponding to the release of stored enthalpy from defect regions. Although rapidly quenched and X-ray-irradiated samples can exhibit comparable enthalpy levels, their calorimetric traces differ in shape, indicating distinct microscopic pathways to high-enthalpy states. Notably, the strength of the JG relaxation is higher in irradiated glasses than in isenthalpic glasses produced by thermal procedures, although this difference is reduced for long-irradiated glasses. These results support the view that the secondary relaxation originates from defect or loosely connected regions within the glass network. Such regions act as elementary plastic units whose number increases on increasing the enthalpy of the glass until the material yields. This work establishes X-ray irradiation as a controllable and versatile method, complementary to melt quenching, for tuning structural heterogeneity and thereby tailoring the mechanical and relaxation properties of glasses.

The study of relaxation processes in glasses reveals information on mechanisms governing phenomena such as crystallization. Baglioni et al. show that X-ray irradiation increases the enthalpy of chalcogenide glasses, amplifying the secondary Johari-Goldstein relaxation and reflecting enhanced defect-induced heterogeneity. These results establish X-ray irradiation as a precise tool to control structural heterogeneity and mechanical behavior, providing a versatile approach to designing glasses with tunable relaxation dynamics and tailored mechanical properties.