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

Structural and electronic transformations of GeSe2 glass under high pressures studied by X-ray absorption spectroscopy

Pressure-induced transformations in an archetypal chalcogenide glass (GeSe2) have been investigated up to 157 GPa by X-ray absorption spectroscopy (XAS) and molecular dynamics (MD) simulations. Ge and Se K-edge XAS data allowed simultaneous tracking of the correlated local structural and electronic changes at both Ge and Se sites. Thanks to the simultaneous analysis of extended X-ray absorption fine structure (EXAFS) signals of both edges, reliable quantitative information about the evolution of the first neighbor Ge-Se distribution could be obtained. It also allowed to account for contributions of the Ge-Ge and Se-Se bond distributions (chemical disorder). The low-density to high-density amorphous-amorphous transformation was found to occur within 10 to 30 GPa pressure range, but the conversion from tetrahedral to octahedral coordination of the Ge sites is completed above [Formula: see text] 80 GPa. No convincing evidence of another high-density amorphous state with coordination number larger than six was found within the investigated pressure range. The number of short Ge-Ge and Se-Se "wrong" bonds was found to increase upon pressurization. Experimental XAS results are confirmed by MD simulations, indicating the increase of chemical disorder under high pressure.

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.

Thermobaric history as a tool to govern properties of glasses: case of dipropylene glycol

The elastic properties of high- and low-pressure glasses of dipropylene glycol were determined for the first time under conditions of isothermal compression up to 1 GPa at 77 K and isobaric heating of 77-300 K at 0.05 GPa and 1 GPa. A strong dependence of the elastic properties of glasses on their thermobaric history has been revealed: glasses obtained at high pressure have not only higher densities (3.9%), but also noticeably higher elastic moduli. This effect is especially pronounced in the shear modulus: high-pressure glass has a 30% higher shear modulus than low-pressure glass. The behavior of elastic moduli during the glass-to-liquid transition also depends on the thermobaric history. Glass produced at low pressure but heated at high pressure has anomalous temperature dependences of the elastic moduli. Heating dipropylene glycol glasses at different pressures allowed us to refine the Tg(P) dependence.

Transient Mesoscopic Immiscibility, Viscosity Anomaly, and High Internal Pressure at the Semiconductor-Metal Transition in Liquid Ga2Te3

Gallium tellurides appear to be promising phase-change materials (PCMs) of the next generation for brain-inspired computing and reconfigurable optical metasurfaces. They are different from the benchmark PCMs because of sp³ gallium hybridization in both cubic Ga₂Te₃ and amorphous pulsed laser deposition (PLD) films. Liquid Ga₂Te₃ also shows a viscosity η(T) anomaly just above melting when η(T) first increases and only then starts decreasing. We used high-energy X-ray diffraction to observe a transient mesoscopic immiscibility that suggested dense metallic liquid droplets in a semiconducting melt. The η(T) shape was consistent with this finding. A vanishing first sharp diffraction peak that also shifts to a higher Q indicates a high internal pressure in the metallic melt, which produces a remarkable asymmetry of the Ga-Te nearest neighbor distances and is reminiscent of high-pressure rhombohedral Ga₂Te₃. The observed phenomena provide a realistic scenario for a fast, multilevel SET-RESET response, which also unravels similar trends in the purported density-driven liquid polyamorphism of water, phosphorus, sulfur, and other materials.

Deciphering Fast Ion Transport in Glasses: A Case Study of Sodium and Silver Vitreous Sulfides

High-capacity solid-state batteries are promising future products for large-scale energy storage and conversion. Sodium fast ion conductors including glasses and glass ceramics are unparalleled materials for these applications. Rational design and tuning of advanced sodium sulfide electrolytes need a deep insight into the atomic structure and dynamics in relation with ion-transport properties. Using pulsed neutron diffraction and Raman spectroscopy supported by first-principles simulations, we show that preferential diffusion pathways in vitreous sodium and silver sulfides are related to isolated sulfur S_iso, that is, the sulfur species surrounded exclusively by mobile cations with a typical stoichiometry of M/S_iso ≈ 2. The S_iso/S_tot fraction appears to be a reliable descriptor of fast ion transport in glassy sulfide systems over a wide range of ionic conductivities and cation diffusivities. The S_iso fraction increases with mobile cation content x, tetrahedral coordination of the network former and, in case of thiogermanate systems, with germanium disulfide metastability and partial disproportionation, GeS₂ → GeS + S, leading to the formation of additional sulfur, transforming into S_iso. A research strategy enabling to achieve extended and interconnected pathways based on isolated sulfur would lead to glassy electrolytes with superior ionic diffusion.

Chemical and Structural Variety in Sodium Thioarsenate Glasses Studied by Neutron Diffraction and Supported by First-Principles Simulations

Sodium-conducting sulfide glasses are promising materials for the next generation of solid-state batteries. Deep insight into the glass structure is required to ensure a functional design and tailoring of vitreous alloys for energy applications. Using pulsed neutron diffraction supported by first-principles molecular dynamics, we show a structural diversity of Na₂S-As₂S₃ sodium thioarsenate glasses, consisting of long corner-sharing (CS) pyramidal chains CS-(AsSS_2/2)_k, small As_pS_q rings (p + q ≤ 11), mixed corner- and edge-sharing oligomers, edge-sharing (ES) dimers ES-As₂S₄, and isolated (ISO) pyramids ISO-AsS₃, entirely or partially connected by sodium species. Polysulfide S-S bridges and structural units with homopolar As-As bonds complete the glass structure, which is basically different from structural motifs predicted by the equilibrium phase diagram. In contrast to superionic silver and sodium sulfide glasses, characterized by a significant population of isolated sulfur species S_iso (0.20 < S_iso/S_tot < 0.28), that is, sulfur connected to only mobile cations M⁺ with a usual M/S_iso stoichiometry of 2, poorly conducting Na₂S-As₂S₃ alloys exhibit a modest S_iso fraction of 6.2%.

Mercury Thiogermanate Glasses HgS-GeS2: Vibrational, Macroscopic, and Electric Properties

Glasses in the pseudo-binary system (HgS)_x (GeS₂)_1-x were synthesized over the concentration range of 0.0 ≤ x ≤ 0.5. The fundamental glass properties (macroscopic, electric, and vibrational) were studied using differential scanning calorimetry (DSC), direct current (dc) electrical measurements, Raman spectroscopy supported by DFT modeling, and X-ray diffraction. Mercury species in thiogermanate glasses essentially form chain-like (HgS_2/2) fragments substituting bridging sulfur between corner- and edge-sharing GeS_4/2 tetrahedra. This structural evolution results in a significant monotonic decrease of the glass transition temperatures from 480 to 270 °C. The room-temperature dc conductivity changes non-monotonically with increasing HgS content x over a limited range of 4 × 10^-15 to 7 × 10^-13 S cm^-1. The electronic transport in insulating HgS-GeS₂ glasses occurs via extended electronic states. Tetrahedral HgS_4/4 fragments also appear in the glass network with increasing x. Their exact population needs further advanced structural studies using diffraction techniques.