Structural, dynamic, electronic, and vibrational properties of flexible, intermediate, and stressed rigid As-Se glasses and liquids from first principles molecular dynamics (original) (raw)
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Physical Review B, 2000
T-modulated differential scanning calorimetry measurements on bulk As x Se 1Ϫx glasses show that the glass transition temperature T g (x) variation at xϽ0.12 is linear with a slope dT g /dxϭ4.1°C/at.% As, and the nonreversing heat flow, ⌬H nr (x) almost vanishes in the 0.291͑1͒ϽxϽ0.37͑1͒ composition range. These thermal results analyzed by agglomeration theory and constraint theory suggest that in addition to As(Se 1/2 ) 3 units, quasitetrahedral SeϭAs͑Se 1/2 ) 3 units also serve to crosslink Se q chains at xϽ 2 5 . The results also suggest that rigidity onsets at r c (1)ϭ2.29(1) and the transition to the stressed rigid phase occurs at r c (2)ϭ2.37(1), below the chemical threshold at r ct ϭ2.40 ͑or xϭ 2 5 ).
Compositional Thresholds and Anomalies in Connection with Stiffness Transitions in Network Glasses
Physical Review Letters, 2013
The structural and dynamical properties of amorphous and liquid As x Se 1Àx (0:2 < x < 0:4) are studied by first principles molecular dynamics. Within the above range of compositions, thresholds and anomalies are found in the behavior of reciprocal space properties that can be correlated to the experimental location of the so-called Boolchand intermediate phase in these glassy networks. These findings are associated with diffusion anomalies for the parent liquid phase, thereby linking structural and dynamical atomicscale fingerprints for the onset of rigidity within the network, while also providing a much more complex picture than the one derived from mean-field approaches of stiffness transitions.
Frontiers in Materials, 2019
Melt dynamics and glass Topological phases of especially dry and homogenized binary As x S 100−x melts/glasses are examined in Modulated-DSC, Raman scattering, and volumetric experiments. In the S-rich glasses (12% < x < 23%), direct evidence for the elusive 537 cm −1 stretch vibrational mode of the Quasi-Tetrahedral (QT), S = As(S 1/2) 3 , local structure is observed in FT-Raman scattering once melts are homogenized and glasses cycled through T g +10 • C for an extended period. The enthalpy of relaxation at T g , H nr (x), fragility index, m(x), Molar volumes, V m (x) each display three distinct regimes of variation. Specifically, m(x) displays a Gaussian like global minimum (fragility window), and H nr (x) displays an abrupt square-well like variation (reversibility window), while V m (x) displays a Gaussian-like local minimum (Volumetric window) in the isostatically rigid phase (22.5% < x < 28.5%). At low x (<20%) in the Flexible phase, glasses are segregated with a S 8-rich nanophase that decouples from the AsS glassy backbone. At medium x (22.5% < x < 28.5%) glassy backbones form an isostatically rigid phase displaying a vanishing H nr (x) term, and compacted structures with corresponding melts being superstrong (m < 20). At high x (28.5% < x < 40%) in the Stressed-Rigid phase, glasses possess an increasing H nr (x) term, and melts become increasingly fragile, with m(x) >20 as x increases. Taken together, these results underscore that superstrong melts yield isostatically rigid glasses, while fragile ones form either Flexible or Stressed-rigid glasses upon cooling. The onset of the rigidity transition near = 2.22, instead of the usual value of = 2.40, is identified with presence of QT local structures in addition to Pyramidal As(S 1/2) 3 local structures in the glassy backbone, and with a small but finite fraction of polymeric S n chains being decoupled from the backbone.
Journal of Physics: Condensed Matter, 2010
A low temperature Monte Carlo dynamics of a Keating like oscillator model is used to study the relationship between the nature of glasses from the viewpoint of rigidity, and the strong-fragile behaviour of glass-forming liquids. The model shows that a Phillips optimal glass formation with minimal enthalpic changes is obtained under a cooling/annealing cycle when the system is optimally constrained by the harmonic interactions, i.e. when it is isostatically rigid. For these peculiar systems, the computed fragility shows also a minimum, which demonstrates that isostatically rigid glasses are strong (Arrhenius-like) glass-forming liquids. Experiments on chalcogenide and oxide glass-forming liquids are discussed under this new perspective and confirm the theoretical prediction for chalcogenide network glasses. PACS numbers: 61.43.Fs-61.20.-x
Onset of rigidity in glasses: From random to self-organized networks
Journal of Non-Crystalline Solids, 2007
We review in this paper the signatures of a new elastic phase that is found in glasses with selected compositions. It is shown that in contrast with random networks, where rigidity percolates at a single threshold, networks that are able to self-organize to avoid stress will remain in an almost stress-free state during a compositional interval, an intermediate phase, that is bounded by a flexible phase and a stressed rigid phase. We report the experimental signatures and describe the theoretical efforts that have been accomplished to characterize the intermediate phase. We illustrate one of the methods used in more detail with the example of Group III chalcogenides and finally suggest further possible experimental signatures of self-organization.
Rigidity transitions in glasses driven by changes in network dimensionality and structural groupings
EPL (Europhysics Letters), 2014
The method of in situ high-pressure neutron diffraction is used to investigate the structure of B 2 O 3 glass on compression in the range from ambient to 17.5(5) GPa. The experimental results are supplemented by molecular dynamics simulations made using a newly developed aspherical ion model. The results tie together those obtained from other experimental techniques to reveal three densification regimes. In the first, BO 3 triangles are the predominant structural motifs as the pressure is increased from ambient to 6.3(5) GPa, but there is an alteration to the intermediate range order which is associated with the dissolution of boroxol rings. In the second, BO 4 motifs replace BO 3 triangles at pressures beyond 6.3 GPa and the dissolution of boroxol rings continues until it is completed at 11-14 GPa. In the third, the B-O coordination number continues to increase with pressure to give a predominantly tetrahedral glass, a process that is completed at a pressure in excess of 22.5 GPa. On recovery of the glass to ambient from a pressure of 8.2 GPa, triangular BO 3 motifs are recovered but, relative to the uncompressed material, there is a change to the intermediate range order. The comparison between experiment and simulation shows that the aspherical ion model is able to provide results of unprecedented accuracy at pressures up to at least 10 GPa.
Physical Review B, 2009
We propose that the molar volume minimum observed in barium silicate glasses ͑1−x͒SiO 2 − xBaO is related to the onset of an adaptative rigid glassy network. We obtain in the compositional window 29% Ͻ x Ͻ 33% a dramatic decrease in stressed rigid local units from the Raman analysis and at x =31% the onset of barium ionic conduction. A random bond model ͓J. Barré et al., Phys. Rev. Lett. 94, 208701 ͑2005͔͒ and constraint counting algorithms permit defining of the free energy of the system, and analyzing of the elastic nature of three compositional ranges of interest: a stressed rigid phase at low x, a flexible phase at high x, and a stress-free intermediate phase where space filling is optimized.
Physical Review B, 2008
The structure of binary As_xS_{1-x} glasses is elucidated using modulated-DSC, Raman scattering, IR reflectance and molar volume experiments over a wide range (8%<x<41%) of compositions. We observe a reversibility window in the calorimetric experiments, which permits fixing the three elastic phases; flexible at x<22.5%, intermediate phase (IP) in the 22.5%<x<29.5% range, and stressed-rigid at x>29.5%. Raman scattering supported by first principles cluster calculations reveal existence of both pyramidal (PYR, As(S1/2)3) and quasi-tetrahedral(QT, S=As(S1/2)3) local structures. The QT unit concentrations show a global maximum in the IP, while the concentration of PYR units becomes comparable to those of QT units in the phase, suggesting that both these local structures contribute to the width of the IP. The IP centroid in the sulfides is significantly shifted to lower As content x than in corresponding selenides, a feature identified with excess chalcogen partially segregating from the backbone in the sulfides, but forming part of the backbone in selenides. These ideas are corroborated by the proportionately larger free volumes of sulfides than selenides, and the absence of chemical bond strength scaling of Tgs between As-sulfides and As-selenides. Low-frequency Raman modes increase in scattering strength linearly as As content x of glasses decreases from x = 20% to 8%, with a slope that is close to the floppy mode fraction in flexible glasses predicted by rigidity theory. These results show that floppy modes contribute to the excess vibrations observed at low frequency. In the intermediate and stressed rigid elastic phases low-frequency Raman modes persist and are identified as boson modes. Some consequences of the present findings on the optoelectronic properties of these glasses is commented upon.
Theory of the structural glass transition: a pedagogical review
Advances in Physics, 2015
The random first-order transition (RFOT) theory of the structural glass transition is reviewed in a pedagogical fashion. The rigidity that emerges in crystals and glassy liquids is of the same fundamental origin. In both cases, it corresponds with a breaking of the translational symmetry; analogies with freezing transitions in spin systems can also be made. The common aspect of these seemingly distinct phenomena is a spontaneous emergence of the molecular field, a venerable and well-understood concept. In crucial distinction from periodic crystallisation, the free energy landscape of a glassy liquid is vastly degenerate, which gives rise to new length and time scales while rendering the emergence of rigidity gradual. We obviate the standard notion that to be mechanically stable a structure must be essentially unique; instead, we show that bulk degeneracy is perfectly allowed but should not exceed a certain value. The present microscopic description thus explains both crystallisation and the emergence of the landscape regime followed by vitrification in a unified, thermodynamics-rooted fashion. The article contains a self-contained exposition of the basics of the classical density functional theory and liquid theory, which are subsequently used to quantitatively estimate, without using adjustable parameters, the key attributes of glassy liquids, viz., the relaxation barriers, glass transition temperature, and cooperativity size. These results are then used to quantitatively discuss many diverse glassy phenomena, including: the intrinsic connection between the excess liquid entropy and relaxation rates, the non-Arrhenius temperature dependence of α-relaxation, the dynamic heterogeneity, violations of the fluctuation-dissipation theorem, glass ageing and rejuvenation, rheological and mechanical anomalies, super-stable glasses, enhanced crystallisation near the glass transition, the excess heat capacity and phonon scattering at cryogenic temperatures, the Boson peak and plateau in thermal conductivity, and the puzzling midgap electronic states in amorphous chalcogenides.
Emergence of Crystal-like Atomic Dynamics in Glasses at the Nanometer Scale
Physical Review Letters, 2013
The vibrational dynamics of a permanently densified silica glass is compared to the one of an -quartz polycrystal, the silica polymorph of the same density and local structure. The combined use of inelastic x-ray scattering experiments and ab initio numerical calculations provides compelling evidence of a transition, in the glass, from the isotropic elastic response at long wavelengths to a microscopic regime as the wavelength decreases below a characteristic length of a few nanometers, corresponding to about 20 interatomic distances. In the microscopic regime the glass vibrations closely resemble those of the polycrystal, with excitations related to the acoustic and optic modes of the crystal. A coherent description of the experimental results is obtained assuming that the elastic modulus of the glass presents spatial heterogeneities of an average size a $ =2.