Punit Boolchand - Academia.edu (original) (raw)

Papers by Punit Boolchand

Research paper thumbnail of Intermediate Phases, Reversibility Windows, Stress-Free and Non-Aging Networks, and Strong Liquids

Network glasses usually display glass transitions that are hysteretic, i.e., a second scan across... more Network glasses usually display glass transitions that are hysteretic, i.e., a second scan across a glass transition endotherm usually does not replicate the first one. But in recent years, examination of glass transition endotherms of several inorganic systems [1-4] reveal a different pattern: one observes compositional windows across which glass transitions become almost completely non-hysteretic. Specifically, the non-reversing enthalpy (H nr) associated with the glass transition (T g) accessed from modulated Differential Scanning Calorimetry is found to nearly vanish [1-4]. These compositional windows, also called reversibility windows, usually occur [5] in the 2.29 < r < 2.52 range, where r represents the mean coordination number, a measure of network connectivity. Glasses in reversibility windows form ideal stress-free networks. The ideality derives from the optimization of glass forming tendency. The stress-free character of binary Ge x Se 1-x glasses in the reversibility window, 0.20 < x < 0.25, was recently demonstrated [5] in Raman pressure measurements. In these experiments one found that select vibrational modes blue-shift as a function

Research paper thumbnail of Aging, Fragility and Reversibility Window in Bulk Alloy Glasses

Journal of physics. Condensed matter : an Institute of Physics journal, 2003

Non-reversing relaxation enthalpies (∆Hnr) at glass transitions Tg(x) in the PxGexSe1-2x ternary ... more Non-reversing relaxation enthalpies (∆Hnr) at glass transitions Tg(x) in the PxGexSe1-2x ternary display a wide, sharp and deep global minimum (~0) in the 0.09 < x < 0.145 range, within which Tg becomes thermally reversing. In the reversibility window these glasses are found not to age, in contrast to aging observed for fragile glass compositions outside the window. Thermal reversibility and lack of aging are paradigms that molecular glasses in the window share with proteins in transition states, which result from structural self-organization in both systems. In proteins the self-organized structures appear to be at places where life sustaining repeating foldings and unfoldings occur.

Research paper thumbnail of Editorial: Topology of Disordered Networks and Their Applications

Understanding the physicochemical properties of glasses has attracted widespread interest over th... more Understanding the physicochemical properties of glasses has attracted widespread interest over the entire twentieth century. Glasses form disordered network structures and cannot be treated as regular lattices on an atomic scale. It has led not only to contributions in basic science such as the improved description of the glass transition phenomenon but also to contributions in materials science, and these have stimulated worldwide applications as smart window glass, flat panel displays, touch screens, protective sheets for cell phones, realization of high-K Dielectrics in standard 3-Terminal devices (Lucovsky et al., 2003), and phase change memories for information storage. The latter two applications alone represent a $1.6 billion market worldwide in 2020. A profound step forward in understanding network glasses and amorphous materials at an atomic level began nearly 40 years back as the foundations of topolocial constraint theory (TCT) emerged, and interest in manifestations of t...

Research paper thumbnail of Emerging role of local and extended range molecular structures on functionalities of topological phases of (Na2O)x(P2O5)100‐x glasses using Raman scattering and modulated DSC

International Journal of Applied Glass Science, 2020

Raman scattering is a powerful probe of local structure (LS) of glasses. In Sodium Phosphate Glas... more Raman scattering is a powerful probe of local structure (LS) of glasses. In Sodium Phosphate Glasses (SPGs), we show that both LS composed of Qn species and Extended Range Structures (ERS) composed of Long Chains (LCs), Large Rings (LRs), and Small Rings (SRs) can be decoded by Raman scattering. The trimodal distribution of P‐Oterminal stretch modes of Q2 species and P‐Obridging at x < 50% are manifestations of these ERS. These two pairs of triads of modes are uniquely identified with Q2 units present in either LCs, or LRs, or SRs. The existence three phases of c‐NaPO3 composed of 3‐membered rings, 6‐membered rings, and infinitely long chains has facilitated the identification. The Intermediate Phase (IP) in SPGs extends in the 37.5 < x < 46.0% range, the Stressed‐rigid Phase in the 46.0% < x < 50%, and the Flexible Phase in the 18% < x < 37.5% range of soda. We show the IP consists predominantly of LCs (82%), with a minority of LRs (15%) and SRs (3%). The LR‐ a...

Research paper thumbnail of Signature of Coordination Defects in the Vibrational Spectrum of Amorphous Chalcogenides

Frontiers in Materials, 2019

Using model structures obtained from molecular dynamics simulations, we calculate the Infrared (I... more Using model structures obtained from molecular dynamics simulations, we calculate the Infrared (IR) spectrum of amorphous and liquid As 2 Se 3. The calculated spectrum is in rather good agreement with the experimental counterpart and contains the signatures of coordination defects represented by quasi-tetrahedral Se=AsSe 3/2 units which dominate the vibrational spectrum beyond 300 cm −1 with a typical vibration for the As=Se bond near 370 cm −1. Conversely, the most prominent peaks around 100 and 200 cm −1 are mainly due to vibrations of As atoms in pyramidal geometry. The study of the thermal behavior indicates that such defects increase with temperature so that their presence in the glass is reminiscent of the liquid state. They are, furthermore, metastable in character, as are other defects such as 1-and 3-fold Se, and relax to a few percent on the picosecond timescale once an instantaneous quench has been achieved from high temperature to room temperature.

Research paper thumbnail of Linking Melt Dynamics With Topological Phases and Molecular Structure of Sodium Phosphate Glasses From Calorimetry, Raman Scattering, and Infrared Reflectance

Frontiers in Materials, 2019

Research paper thumbnail of Stiffness transitions inSixSe1−xglasses from Raman scattering and temperature-modulated differential scanning calorimetry

Physical Review B, 2000

Temperature-modulated differential scanning calorimetry ͑MDSC͒ measurements on Si x Se 1Ϫx glasse... more Temperature-modulated differential scanning calorimetry ͑MDSC͒ measurements on Si x Se 1Ϫx glasses show glass transitions to be thermally reversing in character in the composition window 0.20ϽxϽ0.27. Raman scattering shows a trimodal distribution of vibrational modes, identified with corner-sharing ͑CS͒, edge-sharing ͑ES͒, and Se-chain modes ͑CM͒. In the floppy region, 0ϽxϽ0.19, these modes are symmetric and are characteristic of a random network. In the transition region, 0.20ϽxϽ0.27, the ES and CM split into doublets suggesting appearance of extended range structural correlations. In the percolatively rigid region, xϾ0.27, the CS mode in addition to the ES and CM also splits into a doublet indicating growth of substantial medium range structure. The large compositional width (0.20ϽxϽ0.27) associated with Raman elastic thresholds coincides with the thermally reversing window from MDSC.

Research paper thumbnail of Glassy materials with enhanced thermal stability

MRS Bulletin, 2017

Ever since the inception of academic glass research, practitioners have often wondered-what is so... more Ever since the inception of academic glass research, practitioners have often wondered-what is so special about the select few melts that can bypass crystallization and be supercooled to form bulk glasses at the glass-transition temperature (T g)? With the important strides made in glass science in the past 35 years, both in theory and experiments, we now have a wealth of new information on the crucial role of network topology in decoding the origin of the glass-forming tendency. The Phillips-Thorpe Rigidity Theory 1-3 has been pivotal in elucidating the physics of network glasses. The key parameter in the theory is the number of constraints per atom, n c, , due to chemical bonds, in particular, bond-stretching and bondbending interactions. When n c = 3, glassy networks are optimally constrained, fulfi lling the Maxwell criteria for rigidity (i.e., n c = 3 for three-dimensional networks). Such optimally constrained networks within the Phillips-Thorpe approach possess a mean coordination number of 2.40. At n c < 3, networks possess low-frequency fl oppy modes and are fl exible, those with n c > 3, possess stress-creating redundant bonds and are stressed-rigid, while those with n c = 3 form networks that belong to an intermediate phase (IP). The comparative functionalities of the underlined three topological phases have evoked much interest in glass science. The fi rst tests of these topology-driven ideas were applied to the chalcogenide glasses. These glasses represent alloys of Group VI (S, Se) elements with Group IV (Si, Ge) or Group V (P, As) elements. An attractive feature of these elements is that their chemical bonding conforms to the 8-N bonding rule, where N represents the number of valence electrons. Thus, the coordination number r acquired by Ge, As, and Se usually is 4, 3, and 2, respectively. One can thus enumerate the mechanical constraints per atom, n c , from a knowledge of the chemical stoichiometry alone of an alloyed glass composition such as Ge x As y Se 100-x-y .

Research paper thumbnail of Universal amorphous-amorphous transition in GexSe100−x glasses under pressure

Scientific Reports, 2016

Pressure induced structural modifications in vitreous GexSe100−x (where 10 ≤ x ≤ 25) are investig... more Pressure induced structural modifications in vitreous GexSe100−x (where 10 ≤ x ≤ 25) are investigated using X-ray absorption spectroscopy (XAS) along with supplementary X-ray diffraction (XRD) experiments and ab initio molecular dynamics (AIMD) simulations. Universal changes in distances and angle distributions are observed when scaled to reduced densities. All compositions are observed to remain amorphous under pressure values up to 42 GPa. The Ge-Se interatomic distances extracted from XAS data show a two-step response to the applied pressure; a gradual decrease followed by an increase at around 15–20 GPa, depending on the composition. This increase is attributed to the metallization event that can be traced with the red shift in Ge K edge energy which is also identified by the principal peak position of the structure factor. The densification mechanisms are studied in details by means of AIMD simulations and compared to the experimental results. The evolution of bond angle distri...

Research paper thumbnail of Reversibility Window, Aging, and Nanoscale Phase Separation in GexAsxS1-2x Bulk Alloy Glasses

The non-reversing enthalpy (∆H nr) near T g , in bulk Ge x As x S 1-2x glasses is found to displa... more The non-reversing enthalpy (∆H nr) near T g , in bulk Ge x As x S 1-2x glasses is found to display a global minimum (~0) in the 0.11 < x < 0.15 range, the reversibility window. Furthermore, the ∆H nr term is found to age for glass compositions below (x < 0.11) and above (x > 0.15) the window but not in the window. In analogy to corresponding selenides, glass compositions in the window represent the intermediate phase, those at x < 0.11 are floppy, and those at x > 0.15 stressed-rigid. Raman scattering shows floppy and stressed rigid networks to be partially nanoscale phase separated, an aspect of structure that contributes to a narrowing of the reversibility window width and to suppression of the ∆H nr term in S-rich glasses qualitatively in relation to corresponding Se-rich glasses. Ideas based on Lagrangian bonding constraints have proved to be central to understand the elastic behavior of network glasses 1,2. In the 1980s, these considerations led to the

Research paper thumbnail of Elastic phases in GexSbxSe100-2x ternary glasses

Research paper thumbnail of Molecular structure, glass transition temperature variation, agglomeration theory, and network connectivity of binary P-Se glasses

Physical Review B, 2001

Raman scattering and 31 P NMR results show that the backbone of binary P x Se 1Ϫx glasses is comp... more Raman scattering and 31 P NMR results show that the backbone of binary P x Se 1Ϫx glasses is composed of Se n-chain fragments, pyramidal P͑Se 1/2 ͒ 3 units, quasitetrahedral SeϭP͑Se 1/2 ͒ 3 units, and ethylenelike P 2 ͑Se 1/2 ͒ 4 units at low P content (xϽ0.47). Concentrations of the various building blocks independently established from each spectroscopic probe are found to be correlated. Theoretical predictions for the glass transition variation T g (x) from agglomeration theory are compared to the observed T g (x) trends established from temperaturemodulated differential scanning calorimetry. The comparison shows that a stochastic network description is an appropriate one of glasses at low x (xϽ0.12). At medium x (0.12ϽxϽ0.47), substantial medium-range structure evolves in the form of polymeric ethylenelike units that comprise elements of the barely rigid backbone. At higher x (xϾ0.47), a rapid phase separation of monomeric P 4 Se 3 units from the backbone takes place, leading to a molecular glass with a rather low T g at xϾ0.50.

Research paper thumbnail of Rigidity transitions and molecular structure ofAsxSe1−xglasses

Physical Review B, 2000

T-modulated differential scanning calorimetry measurements on bulk As x Se 1Ϫx glasses show that ... more 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).

Research paper thumbnail of Fast-Ion Conduction and Flexibility of Glassy Networks

Physical Review Letters, 2007

We observe two thresholds in the variations of electrical conductivity of dry solid electrolyte (... more We observe two thresholds in the variations of electrical conductivity of dry solid electrolyte (AgI) x (AgPO 3) 1-x glasses, when the AgI additive concentration x increases to 9.5% and to 37.8%. Raman scattering complemented by calorimetric measurements confirm that these thresholds are signatures of the rigidity phase transitions; at x = 9.5% from a stressed rigid to an isostatically (stress free) rigid phase, and at x = 37.8% from isostatically rigid to a flexible phase. In the flexible phase, the electrical conductivity seems to increase as a power of x, this is in good agreement with the theoretical prediction based on 3d percolation. The solid electrolytes, AgI, Ag 2 S, Ag 2 Se, exist in a non-crystalline or glassy phase, usually not as stoichiometric solids but as additives in base network glasses [1]. These additives can either segregate [1, 2] as separate phases, or uniformly mix [1] with the base glass to form homogeneous solid electrolyte glasses. Gaining a more complete

Research paper thumbnail of Light-Induced Giant Softening of Network Glasses Observed near the Mean-Field Rigidity Transition

Physical Review Letters, 2004

The longitudinal acoustic (LA) mode of bulk Ge x Se 1ÿx glasses is examined in Brillouin scatteri... more The longitudinal acoustic (LA) mode of bulk Ge x Se 1ÿx glasses is examined in Brillouin scattering (BS) over the 0:15 < x < 1=3 range using near band-gap radiation (647:1 nm). The LA-mode frequency (LA) softens with increasing laser power in an athermal and reversible manner by nearly 30% (LA = LA) near x x c 0:191 or mean coordination number, r c 2 2x c 2:382, close to the mean-field rigidity percolation transition (r t 2:40). BS is a bulk probe of elasticity, and photosoftening is maximized here when network stress is minimized near the elastic phase transition.

Research paper thumbnail of Comment on “Microscopic Theory of Network Glasses”

Physical Review Letters, 2003

Research paper thumbnail of Direct Evidence for Stiffness Threshold in Chalcogenide Glasses

Physical Review Letters, 1997

Research paper thumbnail of Search for a structural response to the intermediate phase inGexSe1−xglasses

Research paper thumbnail of Origin of giant photocontraction in obliquely deposited amorphousGexSe1−xthin films and the intermediate phase

Physical Review B, 2008

Obliquely deposited amorphous Ge x Se 100-x thin-films at several compositions in the 15% < x < 3... more Obliquely deposited amorphous Ge x Se 100-x thin-films at several compositions in the 15% < x < 33.3% range , and at several obliqueness angles in the 0 < α < 80° range at each x were evaporated on Si and glass substrates. Here α designates the angle between film normal and direction of vapor transport. Raman scattering, ir reflectance and optical absorption measurements were undertaken to characterize the vibrational density of states and optical band gaps. Edge views of films in SEM confirm the columnar structure of obliquely (α = 80°) deposited films. Films, mounted in a cold stage flushed with N 2 gas, were irradiated to UV radiation from a Hg-Xe arc lamp, and photo-contraction (PC) of

Research paper thumbnail of Midgap states, Raman scattering, glass homogeneity, percolative rigidity and stress transitions in chalcogenides

physica status solidi (b), 2012

Raman scattering from binary GexSe100 − x glasses excited using 1064 nm radiation display vibrati... more Raman scattering from binary GexSe100 − x glasses excited using 1064 nm radiation display vibrational modes whose linewidths significantly exceed those observed using 647 nm radiation. In these glasses, 1064 nm radiation excites midgap states while 647 nm radiation excites conduction band tail states. Presence of midgap states in glasses, ascribed to coordination defects, is responsible for vibrational mode broadening that smears glass compositional variation of mode frequency using 1064 nm radiation but not with the 647 nm radiation. In the latter, mode‐frequency variation of Corner‐ (CS) and Edge‐sharing (ES) tetrahedral units, in specially prepared homogeneous glasses, display thresholds near the rigidity (x = 19.5%) and stress (x = 26.0%) transitions, opening an intermediate phase (IP) that correlates well with the reversibility window (blue region) observed in calorimetric measurements (figure on the right). magnified imageCompositional trends in the non‐reversing enthalpy of r...

Research paper thumbnail of Intermediate Phases, Reversibility Windows, Stress-Free and Non-Aging Networks, and Strong Liquids

Network glasses usually display glass transitions that are hysteretic, i.e., a second scan across... more Network glasses usually display glass transitions that are hysteretic, i.e., a second scan across a glass transition endotherm usually does not replicate the first one. But in recent years, examination of glass transition endotherms of several inorganic systems [1-4] reveal a different pattern: one observes compositional windows across which glass transitions become almost completely non-hysteretic. Specifically, the non-reversing enthalpy (H nr) associated with the glass transition (T g) accessed from modulated Differential Scanning Calorimetry is found to nearly vanish [1-4]. These compositional windows, also called reversibility windows, usually occur [5] in the 2.29 < r < 2.52 range, where r represents the mean coordination number, a measure of network connectivity. Glasses in reversibility windows form ideal stress-free networks. The ideality derives from the optimization of glass forming tendency. The stress-free character of binary Ge x Se 1-x glasses in the reversibility window, 0.20 < x < 0.25, was recently demonstrated [5] in Raman pressure measurements. In these experiments one found that select vibrational modes blue-shift as a function

Research paper thumbnail of Aging, Fragility and Reversibility Window in Bulk Alloy Glasses

Journal of physics. Condensed matter : an Institute of Physics journal, 2003

Non-reversing relaxation enthalpies (∆Hnr) at glass transitions Tg(x) in the PxGexSe1-2x ternary ... more Non-reversing relaxation enthalpies (∆Hnr) at glass transitions Tg(x) in the PxGexSe1-2x ternary display a wide, sharp and deep global minimum (~0) in the 0.09 < x < 0.145 range, within which Tg becomes thermally reversing. In the reversibility window these glasses are found not to age, in contrast to aging observed for fragile glass compositions outside the window. Thermal reversibility and lack of aging are paradigms that molecular glasses in the window share with proteins in transition states, which result from structural self-organization in both systems. In proteins the self-organized structures appear to be at places where life sustaining repeating foldings and unfoldings occur.

Research paper thumbnail of Editorial: Topology of Disordered Networks and Their Applications

Understanding the physicochemical properties of glasses has attracted widespread interest over th... more Understanding the physicochemical properties of glasses has attracted widespread interest over the entire twentieth century. Glasses form disordered network structures and cannot be treated as regular lattices on an atomic scale. It has led not only to contributions in basic science such as the improved description of the glass transition phenomenon but also to contributions in materials science, and these have stimulated worldwide applications as smart window glass, flat panel displays, touch screens, protective sheets for cell phones, realization of high-K Dielectrics in standard 3-Terminal devices (Lucovsky et al., 2003), and phase change memories for information storage. The latter two applications alone represent a $1.6 billion market worldwide in 2020. A profound step forward in understanding network glasses and amorphous materials at an atomic level began nearly 40 years back as the foundations of topolocial constraint theory (TCT) emerged, and interest in manifestations of t...

Research paper thumbnail of Emerging role of local and extended range molecular structures on functionalities of topological phases of (Na2O)x(P2O5)100‐x glasses using Raman scattering and modulated DSC

International Journal of Applied Glass Science, 2020

Raman scattering is a powerful probe of local structure (LS) of glasses. In Sodium Phosphate Glas... more Raman scattering is a powerful probe of local structure (LS) of glasses. In Sodium Phosphate Glasses (SPGs), we show that both LS composed of Qn species and Extended Range Structures (ERS) composed of Long Chains (LCs), Large Rings (LRs), and Small Rings (SRs) can be decoded by Raman scattering. The trimodal distribution of P‐Oterminal stretch modes of Q2 species and P‐Obridging at x < 50% are manifestations of these ERS. These two pairs of triads of modes are uniquely identified with Q2 units present in either LCs, or LRs, or SRs. The existence three phases of c‐NaPO3 composed of 3‐membered rings, 6‐membered rings, and infinitely long chains has facilitated the identification. The Intermediate Phase (IP) in SPGs extends in the 37.5 < x < 46.0% range, the Stressed‐rigid Phase in the 46.0% < x < 50%, and the Flexible Phase in the 18% < x < 37.5% range of soda. We show the IP consists predominantly of LCs (82%), with a minority of LRs (15%) and SRs (3%). The LR‐ a...

Research paper thumbnail of Signature of Coordination Defects in the Vibrational Spectrum of Amorphous Chalcogenides

Frontiers in Materials, 2019

Using model structures obtained from molecular dynamics simulations, we calculate the Infrared (I... more Using model structures obtained from molecular dynamics simulations, we calculate the Infrared (IR) spectrum of amorphous and liquid As 2 Se 3. The calculated spectrum is in rather good agreement with the experimental counterpart and contains the signatures of coordination defects represented by quasi-tetrahedral Se=AsSe 3/2 units which dominate the vibrational spectrum beyond 300 cm −1 with a typical vibration for the As=Se bond near 370 cm −1. Conversely, the most prominent peaks around 100 and 200 cm −1 are mainly due to vibrations of As atoms in pyramidal geometry. The study of the thermal behavior indicates that such defects increase with temperature so that their presence in the glass is reminiscent of the liquid state. They are, furthermore, metastable in character, as are other defects such as 1-and 3-fold Se, and relax to a few percent on the picosecond timescale once an instantaneous quench has been achieved from high temperature to room temperature.

Research paper thumbnail of Linking Melt Dynamics With Topological Phases and Molecular Structure of Sodium Phosphate Glasses From Calorimetry, Raman Scattering, and Infrared Reflectance

Frontiers in Materials, 2019

Research paper thumbnail of Stiffness transitions inSixSe1−xglasses from Raman scattering and temperature-modulated differential scanning calorimetry

Physical Review B, 2000

Temperature-modulated differential scanning calorimetry ͑MDSC͒ measurements on Si x Se 1Ϫx glasse... more Temperature-modulated differential scanning calorimetry ͑MDSC͒ measurements on Si x Se 1Ϫx glasses show glass transitions to be thermally reversing in character in the composition window 0.20ϽxϽ0.27. Raman scattering shows a trimodal distribution of vibrational modes, identified with corner-sharing ͑CS͒, edge-sharing ͑ES͒, and Se-chain modes ͑CM͒. In the floppy region, 0ϽxϽ0.19, these modes are symmetric and are characteristic of a random network. In the transition region, 0.20ϽxϽ0.27, the ES and CM split into doublets suggesting appearance of extended range structural correlations. In the percolatively rigid region, xϾ0.27, the CS mode in addition to the ES and CM also splits into a doublet indicating growth of substantial medium range structure. The large compositional width (0.20ϽxϽ0.27) associated with Raman elastic thresholds coincides with the thermally reversing window from MDSC.

Research paper thumbnail of Glassy materials with enhanced thermal stability

MRS Bulletin, 2017

Ever since the inception of academic glass research, practitioners have often wondered-what is so... more Ever since the inception of academic glass research, practitioners have often wondered-what is so special about the select few melts that can bypass crystallization and be supercooled to form bulk glasses at the glass-transition temperature (T g)? With the important strides made in glass science in the past 35 years, both in theory and experiments, we now have a wealth of new information on the crucial role of network topology in decoding the origin of the glass-forming tendency. The Phillips-Thorpe Rigidity Theory 1-3 has been pivotal in elucidating the physics of network glasses. The key parameter in the theory is the number of constraints per atom, n c, , due to chemical bonds, in particular, bond-stretching and bondbending interactions. When n c = 3, glassy networks are optimally constrained, fulfi lling the Maxwell criteria for rigidity (i.e., n c = 3 for three-dimensional networks). Such optimally constrained networks within the Phillips-Thorpe approach possess a mean coordination number of 2.40. At n c < 3, networks possess low-frequency fl oppy modes and are fl exible, those with n c > 3, possess stress-creating redundant bonds and are stressed-rigid, while those with n c = 3 form networks that belong to an intermediate phase (IP). The comparative functionalities of the underlined three topological phases have evoked much interest in glass science. The fi rst tests of these topology-driven ideas were applied to the chalcogenide glasses. These glasses represent alloys of Group VI (S, Se) elements with Group IV (Si, Ge) or Group V (P, As) elements. An attractive feature of these elements is that their chemical bonding conforms to the 8-N bonding rule, where N represents the number of valence electrons. Thus, the coordination number r acquired by Ge, As, and Se usually is 4, 3, and 2, respectively. One can thus enumerate the mechanical constraints per atom, n c , from a knowledge of the chemical stoichiometry alone of an alloyed glass composition such as Ge x As y Se 100-x-y .

Research paper thumbnail of Universal amorphous-amorphous transition in GexSe100−x glasses under pressure

Scientific Reports, 2016

Pressure induced structural modifications in vitreous GexSe100−x (where 10 ≤ x ≤ 25) are investig... more Pressure induced structural modifications in vitreous GexSe100−x (where 10 ≤ x ≤ 25) are investigated using X-ray absorption spectroscopy (XAS) along with supplementary X-ray diffraction (XRD) experiments and ab initio molecular dynamics (AIMD) simulations. Universal changes in distances and angle distributions are observed when scaled to reduced densities. All compositions are observed to remain amorphous under pressure values up to 42 GPa. The Ge-Se interatomic distances extracted from XAS data show a two-step response to the applied pressure; a gradual decrease followed by an increase at around 15–20 GPa, depending on the composition. This increase is attributed to the metallization event that can be traced with the red shift in Ge K edge energy which is also identified by the principal peak position of the structure factor. The densification mechanisms are studied in details by means of AIMD simulations and compared to the experimental results. The evolution of bond angle distri...

Research paper thumbnail of Reversibility Window, Aging, and Nanoscale Phase Separation in GexAsxS1-2x Bulk Alloy Glasses

The non-reversing enthalpy (∆H nr) near T g , in bulk Ge x As x S 1-2x glasses is found to displa... more The non-reversing enthalpy (∆H nr) near T g , in bulk Ge x As x S 1-2x glasses is found to display a global minimum (~0) in the 0.11 < x < 0.15 range, the reversibility window. Furthermore, the ∆H nr term is found to age for glass compositions below (x < 0.11) and above (x > 0.15) the window but not in the window. In analogy to corresponding selenides, glass compositions in the window represent the intermediate phase, those at x < 0.11 are floppy, and those at x > 0.15 stressed-rigid. Raman scattering shows floppy and stressed rigid networks to be partially nanoscale phase separated, an aspect of structure that contributes to a narrowing of the reversibility window width and to suppression of the ∆H nr term in S-rich glasses qualitatively in relation to corresponding Se-rich glasses. Ideas based on Lagrangian bonding constraints have proved to be central to understand the elastic behavior of network glasses 1,2. In the 1980s, these considerations led to the

Research paper thumbnail of Elastic phases in GexSbxSe100-2x ternary glasses

Research paper thumbnail of Molecular structure, glass transition temperature variation, agglomeration theory, and network connectivity of binary P-Se glasses

Physical Review B, 2001

Raman scattering and 31 P NMR results show that the backbone of binary P x Se 1Ϫx glasses is comp... more Raman scattering and 31 P NMR results show that the backbone of binary P x Se 1Ϫx glasses is composed of Se n-chain fragments, pyramidal P͑Se 1/2 ͒ 3 units, quasitetrahedral SeϭP͑Se 1/2 ͒ 3 units, and ethylenelike P 2 ͑Se 1/2 ͒ 4 units at low P content (xϽ0.47). Concentrations of the various building blocks independently established from each spectroscopic probe are found to be correlated. Theoretical predictions for the glass transition variation T g (x) from agglomeration theory are compared to the observed T g (x) trends established from temperaturemodulated differential scanning calorimetry. The comparison shows that a stochastic network description is an appropriate one of glasses at low x (xϽ0.12). At medium x (0.12ϽxϽ0.47), substantial medium-range structure evolves in the form of polymeric ethylenelike units that comprise elements of the barely rigid backbone. At higher x (xϾ0.47), a rapid phase separation of monomeric P 4 Se 3 units from the backbone takes place, leading to a molecular glass with a rather low T g at xϾ0.50.

Research paper thumbnail of Rigidity transitions and molecular structure ofAsxSe1−xglasses

Physical Review B, 2000

T-modulated differential scanning calorimetry measurements on bulk As x Se 1Ϫx glasses show that ... more 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).

Research paper thumbnail of Fast-Ion Conduction and Flexibility of Glassy Networks

Physical Review Letters, 2007

We observe two thresholds in the variations of electrical conductivity of dry solid electrolyte (... more We observe two thresholds in the variations of electrical conductivity of dry solid electrolyte (AgI) x (AgPO 3) 1-x glasses, when the AgI additive concentration x increases to 9.5% and to 37.8%. Raman scattering complemented by calorimetric measurements confirm that these thresholds are signatures of the rigidity phase transitions; at x = 9.5% from a stressed rigid to an isostatically (stress free) rigid phase, and at x = 37.8% from isostatically rigid to a flexible phase. In the flexible phase, the electrical conductivity seems to increase as a power of x, this is in good agreement with the theoretical prediction based on 3d percolation. The solid electrolytes, AgI, Ag 2 S, Ag 2 Se, exist in a non-crystalline or glassy phase, usually not as stoichiometric solids but as additives in base network glasses [1]. These additives can either segregate [1, 2] as separate phases, or uniformly mix [1] with the base glass to form homogeneous solid electrolyte glasses. Gaining a more complete

Research paper thumbnail of Light-Induced Giant Softening of Network Glasses Observed near the Mean-Field Rigidity Transition

Physical Review Letters, 2004

The longitudinal acoustic (LA) mode of bulk Ge x Se 1ÿx glasses is examined in Brillouin scatteri... more The longitudinal acoustic (LA) mode of bulk Ge x Se 1ÿx glasses is examined in Brillouin scattering (BS) over the 0:15 < x < 1=3 range using near band-gap radiation (647:1 nm). The LA-mode frequency (LA) softens with increasing laser power in an athermal and reversible manner by nearly 30% (LA = LA) near x x c 0:191 or mean coordination number, r c 2 2x c 2:382, close to the mean-field rigidity percolation transition (r t 2:40). BS is a bulk probe of elasticity, and photosoftening is maximized here when network stress is minimized near the elastic phase transition.

Research paper thumbnail of Comment on “Microscopic Theory of Network Glasses”

Physical Review Letters, 2003

Research paper thumbnail of Direct Evidence for Stiffness Threshold in Chalcogenide Glasses

Physical Review Letters, 1997

Research paper thumbnail of Search for a structural response to the intermediate phase inGexSe1−xglasses

Research paper thumbnail of Origin of giant photocontraction in obliquely deposited amorphousGexSe1−xthin films and the intermediate phase

Physical Review B, 2008

Obliquely deposited amorphous Ge x Se 100-x thin-films at several compositions in the 15% < x < 3... more Obliquely deposited amorphous Ge x Se 100-x thin-films at several compositions in the 15% < x < 33.3% range , and at several obliqueness angles in the 0 < α < 80° range at each x were evaporated on Si and glass substrates. Here α designates the angle between film normal and direction of vapor transport. Raman scattering, ir reflectance and optical absorption measurements were undertaken to characterize the vibrational density of states and optical band gaps. Edge views of films in SEM confirm the columnar structure of obliquely (α = 80°) deposited films. Films, mounted in a cold stage flushed with N 2 gas, were irradiated to UV radiation from a Hg-Xe arc lamp, and photo-contraction (PC) of

Research paper thumbnail of Midgap states, Raman scattering, glass homogeneity, percolative rigidity and stress transitions in chalcogenides

physica status solidi (b), 2012

Raman scattering from binary GexSe100 − x glasses excited using 1064 nm radiation display vibrati... more Raman scattering from binary GexSe100 − x glasses excited using 1064 nm radiation display vibrational modes whose linewidths significantly exceed those observed using 647 nm radiation. In these glasses, 1064 nm radiation excites midgap states while 647 nm radiation excites conduction band tail states. Presence of midgap states in glasses, ascribed to coordination defects, is responsible for vibrational mode broadening that smears glass compositional variation of mode frequency using 1064 nm radiation but not with the 647 nm radiation. In the latter, mode‐frequency variation of Corner‐ (CS) and Edge‐sharing (ES) tetrahedral units, in specially prepared homogeneous glasses, display thresholds near the rigidity (x = 19.5%) and stress (x = 26.0%) transitions, opening an intermediate phase (IP) that correlates well with the reversibility window (blue region) observed in calorimetric measurements (figure on the right). magnified imageCompositional trends in the non‐reversing enthalpy of r...