Glasses denser than the supercooled liquid

When aged below the glass transition temperature, Tg, the density of a glass cannot exceed that of the metastable supercooled liquid (SCL) state, unless crystals are nucleated. The only exception is when another polyamorphic SCL state exists, with a density higher than that of the ordinary SCL. Experimentally, such polyamorphic states and their corresponding liquid–liquid phase transitions have only been observed in network-forming systems or those with polymorphic crystalline states. In otherwise simple liquids, such phase transitions have not been observed, either in aged or vapor-deposited stable glasses, even near the Kauzmann temperature. Here, we report that the density of thin vapor-deposited films of N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD) can exceed their corresponding SCL density by as much as 3.5% and can even exceed the crystal density under certain deposition conditions. We identify a previously unidentified high-density supercooled liquid (HD-SCL) phase with a liquid–liquid phase transition temperature (TLL) ∼35 K below the nominal glass transition temperature of the ordinary SCL. The HD-SCL state is observed in glasses deposited in the thickness range of 25 to 55 nm, where thin films of the ordinary SCL have exceptionally enhanced surface mobility with large mobility gradients. The enhanced mobility enables vapor-deposited thin films to overcome kinetic barriers for relaxation and access the HD-SCL state. The HD-SCL state is only thermodynamically favored in thin films and transforms rapidly to the ordinary SCL when the vapor deposition is continued to form films with thicknesses more than 60 nm.

Polyamorphism of vapor-deposited amorphous selenium in response to light

Enhanced surface mobility is critical in producing stable glasses during physical vapor deposition. In amorphous selenium (a-Se) both the structure and dynamics of the surface can be altered when exposed to above-bandgap light. Here we investigate the effect of light on the properties of vapor-deposited a-Se glasses at a range of substrate temperatures and deposition rates. We demonstrate that deposition both under white light illumination and in the dark results in thermally and kinetically stable glasses. Compared to glasses deposited in the dark, stable a-Se glasses formed under white light have reduced thermal stability, as measured by lower density change, but show significantly improved kinetic stability, measured as higher onset temperature for transformation. While light induces enhanced mobility that penetrates deep into the surface, resulting in lower density during vapor deposition, it also acts to form more networked structures at the surface, which results in a state that is kinetically more stable with larger optical birefringence. We demonstrate that the structure formed during deposition with light is a state that is not accessible through liquid quenching, aging, or vapor deposition in the dark, indicating the formation of a unique amorphous solid state.

Sound attenuation in finite-temperature stable glasses

We find that sound attenuation changes dramatically with the stability and the temperature of a model glass former.

Effects of microstructure formation on the stability of vapor-deposited glasses

Glasses formed by physical vapor deposition (PVD) are an interesting new class of materials, exhibiting properties thought to be equivalent to those of glasses aged for thousands of years. Exerting control over the structure and properties of PVD glasses formed with different types of glass-forming molecules is now an emerging challenge. In this work, we study coarse-grained models of organic glass formers containing fluorocarbon tails of increasing length, corresponding to an increased tendency to form microstructures. We use simulated PVD to examine how the presence of the microphase-separated domains in the supercooled liquid influences the ability to form stable glasses. This model suggests that increasing molecule tail length results in decreased thermodynamic stability of the molecules in PVD films. The reduced stability is further linked to the reduced ability of these molecules to equilibrate at the free surface during PVD. We find that, as the tail length is increased, the relaxation times near the surface of the supercooled equilibrium liquid films of these molecules are slowed and become essentially bulk-like, due to the segregation of the fluorocarbon tails to the free surface. Surface diffusion is also markedly reduced due to clustering of the molecules at the surface. Based on these results, we propose a trapping mechanism where tails are unable to move between local phase-separated domains on the relevant deposition time scales.

Sound attenuation in stable glasses

We examine the wavevector dependence of sound attenuation in simulated glasses with a wide range of stabilities.

Controlled physical and optical traits of magnesium-zinc sulphophosphate glass: Role of europium ions

Trivalent rare earth ions doped sulfophosphate glasses became demanding owing to their several notable attributes that are advantageous for diverse photonic devices. To fulfil such goal, preparation of sulfophosphate glasses with optimized composition by selecting appropriate modifier and subsequent characterizations are essential. Driven by this idea, we synthesized a new series of europium (Eu3+) ions doped magnesium-zinc-sulfophosphate glasses of composition (65–x) P2O5–20MgO-15ZnSO4–xEu2O3 (x = 0.0, 0.5, 1.0, 1.5 and 2.0 mol%) using simple melt-quenching method. As-prepared glasses were characterized thoroughly at room temperature via various analytical techniques to determine the Eu2O3 concentration-dependent physical and optical properties. Transparent (pinkish) and thermally stable glasses were achieved. XRD pattern confirmed the amorphous nature of the studied glasses. Glass density was increased from 2.603 to 2.789 g/cm-3 with the increase of Eu2O3 contents from 0 to 2.0 mol%. FTIR spectra revealed the characteristics bonding vibrations (symmetric and asymmetric stretching and bending of nS (P-O), naS (P-O-P), nS (P-O-P), nS P3O, nS (P-O-P)of phosphate networks linkages. The UV-Vis-NIR spectra of the glasses disclosed six significant absorption peaks centred at 360, 380, 394, 414, 465, and 531 nm accompanied by two NIR peaks around 2091 and 2205 nm allocated to various transitions from the ground state to the excited states of Eu3+ ion. Furthermore, the optical absorption data were further used to calculate the energies of direct (2.0 to 3.85 eV) and indirect (3.74 to 5.0 eV) band gap as well as Urbach energies (0.1909 to 0.2440 eV). The photoluminescence (PL) emission spectra of glasses displayed four peaks entered at 593, 613, 654 and 701 nm assigned to the 5Do→7Fo, 5Do→7F2, 5Do→7F3 and 5Do→7F4 transitions of Eu3+ ion. The PL peak at 613 nm showed the highest emission intensity. The PL intensity was enhanced with the increase of Eu3+ content up to 1.5 mol% and quenched thereafter. It was concluded that controlled physical and optical properties can be obtained by appropriately optimizing the glass composition useful for photonic purposes.