X-ray spectroscopy reveals high-pressure transformations in glasses

Understanding the atomic-scale structure and densification of network-forming glasses at high pressures is of vital importance for various fields, from materials science to Earth and planetary sciences. For example, studying pressure-induced local structural changes in simple glasses, such as amorphous SiO₂, helps us to understand the behaviour of silicate melts in the Earth’s deep interior.

In such tetrahedral glasses, the transformation from low-density to high-density amorphous states involves a coordination change from four-fold (tetrahedral) to six-fold (octahedral) order, typically occurring at pressures between 15 to 50 GPa [1].

Recently, researchers have investigated these glasses under ultra-high pressures beyond 1 Mbar, relevant to core–mantle boundary conditions. Several studies on archetypal oxide glasses suggest a second densification stage, leading to an amorphous state with coordination numbers greater than six (CN>6) [1], while others find a persistent six-fold coordination (CN=6) [2,3].

Overcoming challenges

A major challenge in studying glassy materials at extreme pressures is the weak scattering signals from tiny amorphous samples (typically loaded in a diamond anvil cell), complicating standard diffraction-based techniques. X-ray absorption spectroscopy (XAS) is a powerful technique for studying disordered systems, as it directly probes local structural correlations without interference from the pressure cell environment.

XAS was used to investigate pressure-induced transformations in GeSe₂ chalcogenide glass up to 160 GPa at the beamline BM23. Glassy GeSe₂ serves as an ideal model system for studying pressure effects in amorphous systems with XAS. The proximity of the Ge and Se K-edge allows for scanning both K-edges in the same compression run, providing insights into local structural changes at both cation (Ge) and anion (Se) sites.

A more accurate structural analysis is achieved by simultaneously fitting extended X-ray absorption fine structure (EXAFS) signals from both edges (Figure 1).  

Fig. 1: a) Experimental and best-fit calculated EXAFS signals at the Ge K-edge. Only the Ge-Se distribution (model-1) is considered in the calculated signal. b) Experimental and best-fit theoretical EXAFS signals at the Ge K-edge. Distributions of Ge-Se and Ge-Ge distances (model-2) were considered to produce the total calculated signal. c) Fourier transforms of experimental and calculated EXAFS signals reported in panels (a) and (b). d) Experimental and best-fit theoretical EXAFS signals at the Se K-edge. Only the Se-Ge distribution (model-1) is considered in the calculated signal. e) Experimental and best-fit theoretical EXAFS signals at the Se K-edge. Distributions of Se-Ge and Se-Se distances (model-2) were considered to produce the total calculated signal. f) Fourier transforms of experimental and simulated EXAFS signals reported in panels (d) and (e). Intensity of the Fourier transforms in (f) was multiplied by two.

This double-edge EXAFS refinement in extended K-space provided reliable structural information on changes in the nearest neighbour Ge-Se distribution and the evolution of chemical disorder (Ge-Ge and Se-Se “wrong” bonds).  Additionally, changes in the electronic configuration (p-like density of states of Ge and Se atoms) were monitored using near-edge XAS. These configurations, directly involved in chemical bonding, offered valuable insights into the pressure-induced semiconductor–metal transition (Figure 2), structural rearrangements, and the evolution of chemical disorder. 
 

Fig. 2: Energy shift of Ge and Se K-edges under pressure.  

The main conclusion of this study is that the transformation from low- to high-density amorphous–amorphous states in this glass occurs within the 10-30 GPa pressure range, with the conversion from tetrahedral to octahedral coordination complete above 80 GPa. No evidence was found of another high-density state with coordination numbers greater than six (CN>6) within the investigated pressure range (up to 160 GPa).

EXAFS analysis indicates that a significant increase in chemical disorder, such as the number of Ge-Ge or Se-Se “wrong” bonds, plays a crucial role in the pressure behaviour of GeSe₂ glass. Enhanced chemical disorder under pressure, particularly in the Se-Se short-range correlations, was observed through near-edge XAS features, such as a blue shift of the Se K-edge (Figure 2), and molecular dynamics simulations.

While not all observations in this particular system can be generalized, this study has implications for understanding the pressure response of the local atomic and electronic structures in simple tetrahedral network-forming glasses, especially chalcogenide systems with chemical disorder, under ultrahigh pressures beyond 1 Mbar. 

Principal publication and authors
Structural and electronic transformations of GeSe₂ glass under high pressures studied by X-ray absorption spectroscopy, E. Mijit (a,b), M. Durandurdu (c), J.E.F.S. Rodrigues (b), A. Trapananti (a), S.J. Rezvani (a), A. D. Rosa (b), O. Mathon (b), T. Irifune (d), A. Di Cicco (a), Proc. Natl. Acad. Sci. 121(14) 2024; https://doi.org/10.1073/pnas.2318978121
(a) University of Camerino, Camerino (Italy)
(b) ESRF
(c) Abdullah Gül University, Kayseri (Turkey)
(d) Ehime University, Matsuyama (Japan)

References
[1] Y. Kono et al., Proc. Natl. Acad. Sci. 113, 3436-3441 (2016).
[2] G. Spiekermann et al., Phys. Rev. X 9, 011025 (2019).
[3] S. Petitgirard et al., Geochem. Perspect. Lett. 9, 32-37 (2019).