Remarkably Stable Glassy GeS2 Densified at 8.3 GPa: Hidden Polyamorphism, Contrasting Optical Properties, Raman and DFT Studies, and Advanced Applications.

Glassy GeS2, densified at 8.3 GPa, exhibits a strongly reduced bandgap, predominantly tetrahedral Ge environment, enhanced chemical disorder and partial 3-fold coordination of both germanium and sulfur, assuming two possible reaction paths under high pressure: (i) a simple dissociation 2Ge-S ⇄ Ge-Ge + S-S and (ii) a chemical disproportionation GeS2 ⇄ GeS + S. The observed electronic and structural changes remain intact for at least seven years under ambient conditions but are gradually evolving upon heating. The relaxation kinetics at elevated temperatures, up to the glass transition temperature Tg, suggests that complete recovery of the densified glassy GeS2 over a typical operational T-range of optoelectronic devices will take many thousands of years. The observed logarithmic relaxation and nearly infinite recovery time at room temperature raise questions about the nature of millennia-long phenomena in densified GeS2. Two alternative explanations will be discussed: (1) hidden polyamorphism and (2) continuous structural and chemical changes under high pressure. These investigations offer valuable insights into the behavior of glassy GeS2 under extreme conditions and its potential applications in optoelectronic devices and other advanced technologies.

Pressure-Driven Chemical Disorder in Glassy As2S3 up to 14.7 GPa, Postdensification Effects, and Applications in Materials Design.

A small difference in energy between homopolar and heteropolar bonds and the glass-forming ability of pure chalcogens leads to unexpected trends in densification mechanisms of glassy chalcogenides compared to vitreous oxides. Using high-precision compressibility measurements and in situ high-energy X-ray diffraction up to 14.7 GPa, we show a new densification route in a canonical glass As2S3. After the first reversible elastic step with a maximum pressure of 1.3 GPa, characterized by a strong reduction of voids and cavities, a significant bonding or chemical disorder is developed under higher pressure, reaching a saturation of 30% in the population of As-As bonds above 8-9 GPa. The pressure-driven chemical disorder is accompanied by a remarkable structural relaxation and a strongly diminished optical gap and determines structural, vibrational, and optical properties under and after cold compression. The decompressed recovered glass conserves a dark color and exhibits two relaxation processes: (a) fast (a few days) and (b) slow (months/years at room temperature). The enhanced refractive index of the recovered glass is promising for optical applications with improved functionalities. A nearly permanent red shift in optical absorption after decompression can be used in high-impact-force optical sensors.