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ABSTRACT Local structure, bonding, and the distribution of vacancies in stoichiometric and highly carbon‐deficient transition metal carbides (TMCs) ZrC, HfC, NbC, and TaC is studied using high‐resolution¹³C magic angle spinning (MAS) NMR spectroscopy. Isotopic enrichment of these TMCs with¹³C allows for remarkable signal enhancement and fast MAS of dielectric‐diluted samples, yielding high‐resolution NMR spectra of these TMCs for the first time. The¹³C MAS NMR spectra display positive correlation between the¹³C isotropic shift of the stoichiometric C site and the metal‐carbon nearest‐neighbor distance in these TMCs. More importantly, these spectra reveal the presence of multiple local carbon environments that can be attributed to either vacancies or oxygen and nitrogen impurities on the carbon sub‐lattice. Comparison between the NMR spectra of stoichiometric and highly carbon deficient TMCs shows that¹³C NMR can be used to differentiate between random distribution and clustering of vacancies and impurities in these lattices. 

ABSTRACT Homogeneous glass formation in binary rare‐earth silicate systems has thus far been precluded due to the presence of extensive liquid–liquid immiscibility and a strong tendency of these liquids toward crystallization. In this study, we demonstrate homogeneous glass formation in the Sc₂O₃–SiO₂binary system within a narrow compositional window (37–39 mol% Sc₂O₃) near a deep eutectic between the compounds Sc₂Si₂O₇and Sc₂SiO₅, using containerless laser melting under aerodynamic levitation. The atomic structure of these unusual glasses is investigated using multinuclear (²⁹Si,⁴⁵Sc,¹⁷O) solid‐state nuclear magnetic resonance (NMR) and Raman spectroscopy. The spectroscopic results, when taken together, provide a comprehensive picture of the structure of these glasses characterized by pyrosilicate [Si₂O₇]⁶⁻anionic units interconnected by Sc cations in ScO₆coordination polyhedra, via Si–O–Sc linkages. A significant fraction (∼6%) of the oxygen atoms in the structure is present as free oxide (FO) ions in Sc–O–Sc linkages, providing connectivity between the ScO₆polyhedra. The formation of the FO species via oxygen disproportionation reaction is promoted by the uniquely high field strength of the Sc³⁺ions, and the resulting structural frustration is hypothesized to suppress crystallization of the stable pyrosilicate phase in these liquids, enabling glass formation in an otherwise non‐glass‐forming binary system. These findings highlight the critical role of rare‐earth cation field strength in controlling oxygen speciation, structure, and glass‐forming ability in this binary silicate system. 

ABSTRACT We investigate the kinetics of physical aging in single‐ and mixed‐modifier phosphate glasses following temperature down‐jumps to aging temperatures above their calorimetric glass transition temperature but below their fictive temperatureT_f, using fast scanning calorimetry (FSC). This technique enables ultrafast heating and cooling (up to ∼30 000°C/s), enabling structural relaxation experiments over very short timescales on a single sample by repeatedly resetting its thermal history. Our results indicate that the non‐exponentiality (Kohlrausch–Williams–Watts) parameter remains nearly temperature‐independent for both aging and shear relaxation. Modeling of the aging kinetics with the Tool–Narayanaswamy–Moynihan formalism revealed negligible nonlinearity, indicating that the role of the fictive temperature in governing aging kinetics diminishes at these elevated aging temperatures above . This reduction in nonlinearity can be understood as a consequence of greater spatial homogeneity in structural rearrangements during aging, as more of the energy landscape becomes accessible atT > . Additionally, the average timescales for enthalpy and recovery are closely coupled to that of shear relaxation in the corresponding supercooled liquid for the single‐modifier system, whereas a large temporal decoupling is observed for the mixed‐modifier case. These findings may have important implications for understanding and bridging the behavior between the glassy and supercooled liquid states, and for elucidating mixed‐modifier effects on different relaxation timescales. 

Abstract The vibrational and configurational entropic contributions of modifier Li cations to the glass‐to‐liquid transition and the fragility of supercooled (Li,Na)PO₃liquids are investigated using calorimetric measurements of their ⁷Li and ⁶Li isotopologues. Intriguingly, a measurable difference in excess entropy ΔS^excis observed between the isotopologues, which implies that the modifier Li cations must contribute significantly to the vibrational and configurational entropy of these supercooled liquids. These modifier contributions are shown to influence the thermodynamic and kinetic fragility of these liquids, with more pronounced effects observed in mixed‐alkali systems. The larger deviation of heat capacity from the Dulong–Petit limit at the glass transition temperatureT_gin mixed‐alkali glasses compared to their single‐alkali counterparts is attributed to the coupled motion of dissimilar alkali ions in the former. Such motions act as a catalytic step allowing the onset of network rearrangement to occur in mixed‐alkali systems without the need for full vibrational excitation of the network atT_g. The lighter ⁶Li isotope facilitates this coupled motion more efficiently than the heavier ⁷Li isotope, resulting in a greater departure from the Dulong–Petit limit of ⁶Li‐containing mixed‐alkali glasses, compared to their ⁷Li isotopologues. 

Abstract The viscosity, fragility index, and shear relaxation dynamics of a range of sodium borosilicate (NBS) liquids have been studied using a combination of differential scanning calorimetry, beam‐bending viscometry, and shear‐mechanical spectroscopy (SMS). Our results show a non‐monotonic compositional variation of the isokom temperatureT₁₂corresponding to a viscosity of 10¹² Pa s and the fragility indexm, consistent with the known structural evolution involving boron coordination change and non‐bridging oxygen formation. A negative correlation is observed betweenT₁₂andm. On the other hand, the quantity (T_g − T₁₂), whereT_gis the glass transition temperature, becomes increasingly positive asmdecreases below ∼50, which suggests an increase in the temporal decoupling between enthalpy and shear relaxation with decreasing fragility in these NBS liquids. SMS measurements, which involve frequency‐ and temperature‐dependent scans of storage and loss moduli, are shown to yield shear relaxation timescales that are consistent with those derived from viscosity via the Maxwell model. Furthermore, the dynamical heterogeneity associated with the shear relaxation of these liquids as estimated from the Kohlrausch–Williams–Watts stretching exponentβ, remains temperature‐independent nearT_gin fragile NBS liquids but increases with temperature for the strongest liquid. This observation supports recent findings of distinctβ(T) behavior in strong versus fragile liquids. Similarly, consistent with the recent results for organic liquids, no correlation is observed betweenmandβnearT_gfor NBS liquids. 

Abstract The Q‐speciation and the role of modifier dynamics on network relaxation in the supercooled mixed‐alkali–alkaline‐earth (MAAE) Na–Ba metaphosphate liquids are investigated using a combination of³¹P nuclear magnetic resonance (NMR) spectroscopy, calorimetric, electrical conductivity, and rheological measurements. Progressive replacement of Na with Ba in these glasses is shown to result in an increasing disproportionation of Q²species via the reaction: 2Q² = Q¹+ Q³. Unlike mixed‐alkali liquids, the Na–Ba metaphosphate liquids display a monotonic variation in isothermal electrical conductivity, glass transition temperature, calorimetric and kinetic fragility, and isothermal viscosity. It is hypothesized that this monotonic variation arises from the lack of elastic facilitation of network relaxation via coupled hopping of Na–Ba pairs as these modifier cations are prohibited from mixing randomly due to the differences between their size, mass, charge, and mobility. Isobaric heat capacity measurements provide supporting evidence in favor of a such a nonrandom mixing between the modifier cations in Na–Ba metaphosphate glasses and liquids. 

The ionic transport in glasses and supercooled liquids exhibits complex dynamical behavior characterized by non-exponential correlation functions, often described by stretched exponential decay. This study investigates the connection between the Haven ratio HR, which measures the deviation of diffusivity of modifier alkali cations from random walk due to their backward-correlated hopping, and the stretching exponent β of the orientational correlation function associated with the nuclear magnetic resonance spin–lattice relaxation (NMR SLR) of these alkali nuclides. By analyzing the temperature-dependent NMR SLR rate data of alkali ions in a wide range of supercooled oxide network liquids, this study reveals a hitherto unknown approximate equality between HR and β. This relationship is shown to be consistent with a model of backward-correlated hopping of mobile modifier ions in a temporally frozen oxide network. Estimation of NMR SLR β for individual alkali ions in mixed-alkali systems offers a pathway to estimate species-specific HR values that are otherwise experimentally inaccessible. These findings, when taken together, suggest that β can serve as a proxy for HR and offer new insight into the microscopic nature of glassy ion transport.