Optimization of electrical conductivity in the Na2O-P2O5-AlF3-SO3 glass system

We report on the individual roles of charge carrier density and network modification in sodium ion conducting glasses from the Na₂O-P₂O₅-SO₃-AlF₃ (NAPFS) system. For this, a broad range of glass compositions was considered across the series of 44Na₂O/(56 – x − y)P₂O₅/xAlF₃/ySO₃, 47Na₂O/(53 − x − y)P₂O₅/xAlF₃/ySO₃, and 50Na₂O/(50 − x − y)P₂O₅/xAlF₃/ySO₃, with x = 8, 12, 16, 20 and y = 0, 5, 7, 10, 12. Impedance spectroscopy was conducted on these glasses at frequencies from 10⁻² to 10⁶ Hz and over temperatures from 50 to 250°C, and complemented by structural analyses using Raman spectroscopy and nuclear magnetic resonance data. While the trends in dc conductivity and activation energy follow that of Na₂O content (increasing from 44 to 50 mol%), substantial enhancement of conductivity (by about two orders of magnitude) and correspondingly lower activation energy were also found for constant Na₂O concentration when adjusting SO₃ or AlF₃ within specific limits of glass structure.     

Optical properties,thermal stability,and forming region of high refractive index La2O3–TiO2–Nb2O5 glasses

The high refractive index La₂O₃–TiO₂–Nb₂O₅ glasses were prepared by containerless processing, and the glass‐forming region was determined. The refractive index showed the range from 2.20 to 2.32, and the values were much higher than those of most optical glasses. The completely miscible 30LaO_3/2–(70?x)TiO₂–xNbO_5/2 (0 ≤ x ≤70) system was fabricated to study the compositional dependence of refractive index and optical transmittance. The crucial determinants of the refractive index of oxide glasses, oxygen molar volume, and electronic polarizability of oxygen ions were calculated. The principle of additivity of glass properties was suitable for the calculation of refractive index between glass and compositional oxides. All the glasses were colorless and transparent in the visible to 6.5 μm middle infrared (MIR) region. These results are useful for designing new optical glasses with high refractive index and low wavelength dispersion in wide optical window.     

Elucidating the Effect of Iron Speciation (Fe2+/Fe3+) on Crystallization Kinetics of Sodium Aluminosilicate Glasses

We report on the influence of Fe₂O₃ on the crystallization kinetics of nepheline (Na₂O·Al₂O₃·2SiO₂)‐based sodium aluminosilicate glasses. A series of glasses with varying Al₂O₃/Fe₂O₃ content were synthesized in the system 25Na₂O–(25–x) Al₂O₃–xFe₂O₃–50SiO₂ (x varies between 0 and 5 mol%) through melt‐quench technique. A systematic set of experiments were performed to elucidate the influence of iron speciation (Fe²⁺/Fe³⁺) on the crystallization kinetics of these glasses including: (1) obtaining the details of nonisothermal crystallization kinetics by differential scanning calorimetry, (2) determining the influence of heat treatment on the structure and iron coordination in glasses by X‐ray photoelectron spectroscopy and wet chemistry, and (3) following the crystalline phase evolution in glasses in air and inert environments by X‐ray diffraction and scanning electron microscopy. The crystallization of two polymorphs of NaAlSiO₄—carnegieite (orthorhombic) and nepheline (hexagonal)—was observed in all the glasses, wherein the incorporation of iron promotes the formation of nepheline over carnegieite while shifting the crystallization mechanism from surface to volume. The influence of environment (air versus inert) and iron content on the crystallization kinetics of these glasses is contextualized from the perspective of the devitrification problem usually observed in sodium‐ and alumina‐rich high level nuclear waste glasses.     

Thermodynamic assessment of the pseudoternary Na2O–Al2O3–SiO2 system

Vitrified high‐level radioactive waste that contains high concentrations of Na₂O and Al₂O₃, such as the waste stored at the Hanford site, can cause nepheline to precipitate in the glass upon cooling in the canisters. Nepheline formation removes oxides such as Al₂O₃ and SiO₂ from the host glass, which can reduce its chemical durability. Uncertainty in the extent of precipitated nepheline necessitates operating at an enhanced waste loading margin, which increases operational costs by extending the vitrification mission as well as increasing waste storage requirements. A thermodynamic evaluation of the Na₂O–Al₂O₃–SiO₂ system that forms nepheline was conducted by utilizing the compound energy formalism and ionic liquid model to represent the solid solution and liquid phases, respectively. These were optimized with experimental data and used to extrapolate phase boundaries into regions of temperature and composition where measurements are unavailable. The intent is to import the determined Gibbs energies into a phase field model to more accurately predict nepheline phase formation and morphology evolution in waste glasses to allow for the design of formulations with maximum loading.     

Frequency and Temperature‐Independent Electrical Transport Properties of 2BaO–0.5Na2O–2.5Nb2O5–4.5B2O3 Glass‐Ceramics

The temperature (300–973 K) and frequency (100 Hz–10 MHz) response of the dielectric and impedance characteristics of 2BaO‐0.5Na₂O–2.5Nb₂O₅–4.5B₂O₃ glasses and glass nanocrystal composites were studied. The dielectric constant of the glass was found to be almost independent of frequency (100 Hz–10 MHz) and temperature (300–600 K). The temperature coefficient of dielectric constant was 8 ± 3 ppm/K in the 300–600 K temperature range. The relaxation and conduction phenomena were rationalized using modulus formalism and universal AC conductivity exponential power law, respectively. The observed relaxation behavior was found to be thermally activated. The complex impedance data were fitted using the least square method. Dispersion of Barium Sodium Niobate (BNN) phase at nanoscale in a glass matrix resulted in the formation of space charge around crystal‐glass interface, leading to a high value of effective dielectric constant especially for the samples heat‐treated at higher temperatures. The fabricated glass nanocrystal composites exhibited P versus E hysteresis loops at room temperature and the remnant polarization (P_r) increased with the increase in crystallite size.     

Y3Al5O12–SiO2 Glasses: Structure and Polyamorphism

Aerodynamic levitation and CO₂ laser melting have been used to synthesize the yttrium aluminosilicate glasses zY₂O₃–yAl₂O₃–xSiO₂ with z/y = 3/5 corresponding to the YAG (Y₃Al₅O₁₂) composition and x between ~5 and ~45 mol%. The low‐ and high‐density (LDA inclusion and HDA matrix) polyamorphic phases in glasses with less than ~14 mol% SiO₂ were identified with backscattering electron imaging. Polarized and depolarized Raman spectra show the formation of various Qⁿ SiO₄ species whose relative populations change smoothly as the SiO₂ content is altered. The AlOs (s = 4–6) and YOz (z = 6–9) polyhedra formed in the YAG glass are preserved upon silica additions while the terminal oxygens of the Q²AlO₄ tetrahedra are gradually bridged to the Qⁿ‐SiO₄ species. The low‐frequency Boson Peak overlaps with the vibrational spectrum and its maximum is redshifted with increasing silica content. Micro‐Raman spectra measured for the LDA and HDA amorphous phases are found to be similar to the spectra of the bulk glass indicating common structural characteristics. The stability of the LDA phase against crystallization appears to be lower than that of the HDA phase. The crystallinity on certain inclusions consisted of YAG microcrystals and a new unidentified microcrystalline phase within Y₄Al_2(1?x)Si_2xO_(9+x) solid solution.     

Dissolution,bioactivity behavior,and cytotoxicity of rare earth-containing bioactive glasses (RE = Gd,Yb)

Rare earth-containing bioactive glasses (RE-BGs) have been poorly explored in the biomaterials field, although RE has optical, nuclear, and magnetic properties that could be used in different biomedical applications. In order to verify whether these glasses can be promising as biomaterials, we studied the dissolution, bioactivity, and cytotoxicity of RE-BGs based on the SiO₂–Na₂O–CaO–P₂O₅–RE₂O₃ (RE = Gd, Yb) system. The glasses were obtained by melting-quenching and their particle size was determined by laser diffraction. Their dissolution behavior was studied in Tris-HCl, while bioactivity was performed in simulated body fluid solution under physiological conditions during several periods. The cytotoxicity test was performed using glass-derived conditioned medium and mesenchymal stem cell derived from deciduous teeth. The dissolution results showed that the glasses dissolved under two different kinetics, which are lower for rare earth-containing glasses, due to the more covalent character of Si–O–RE bonds. The bioactivity results evidenced that all glasses showed bioactivity after 24 hours. However, gadolinium and ytterbium promoted a more calcium phosphate deposition, which contrasts with the slower dissolution kinetics of rare earth-containing glasses. All the glasses were considered biocompatible, showing cell viability higher than 80%. The overall results showed that RE-BGs are promising materials for applications that require bioactivity and/or biocompatibility.     

Effect of Ta2O5 Substituting on Thermal and Optical Properties of High Refractive Index La2O3–Nb2O5 Glass System Prepared by Aerodynamic Levitation Method

High refractive index glasses with nominal composition of 0.35La₂O₃–(0.65?x)Nb₂O₅–xTa₂O₅ (x ≤ 0.35) were prepared by aerodynamic levitation method. The effect of Ta₂O₅ substituting on their thermal and optical properties was investigated. All the glasses obtained were colorless and transparent. Differential thermal analyzer results show that as the content of Ta₂O₅ increased, the thermal stability of the glasses increased but the glass‐forming ability decreased. The transmittance spectra of all the obtained glasses exhibited a wide transmittance window ranging from 380 to 5500 nm. As the content of Ta₂O₅ increased, the refractive index of the glasses was enhanced from 2.15 to 2.21 and the dispersion was reduced with the Abbe number increasing from 20 to 27.     

Thermal and optical properties of La2O3–Ga2O3– (Nb2O5 or Ta2O5) ternary glasses

La₂O₃–Ga₂O₃–M₂O₅ (M = Nb or Ta) ternary glasses were fabricated using an aerodynamic levitation technique, and their glass‐forming regions and thermal and optical properties were investigated. Incorporation of adequate amounts of Nb₂O₅ and Ta₂O₅ drastically improved the thermal stabilities of the glasses against crystallization. Optical transmittance measurements revealed that all the glasses were transparent over a wide wavelength range from the ultraviolet to the mid‐infrared. The refractive indices of the glasses increased and the Abbe number decreased upon substituting Ga₂O₃ with Nb₂O₅, and the decrease in the Abbe number was significantly suppressed when Ta₂O₅ was incorporated into the glass. As a result, excellent compatibility between high refractive index and lower wavelength dispersion was realized in La₂O₃–Ga₂O₃–Ta₂O₅ glasses. Analysis based on the single‐oscillator Drude–Voigt model provided more systematical information and revealed that this compatibility was due to an increase in the electron density of the glass.     

Thermal Stability,Optical Transmittance,and Refractive Index Dispersion of La2O3–Nb2O5–Al2O3 Glasses

La₂O₃–Nb₂O₅–Al₂O₃ high‐refractive‐index glasses were fabricated by containerless processing, and the glass‐forming region was determined. The thermal stability, density, optical transmittance, and the refractive index dispersion of these glasses were investigated. All the glasses were colorless and transparent in the visible to near infrared (NIR) region and had high refractive index with low wavelength dispersion. Some of these glasses were found to have significantly high glass‐forming ability. These results indicate that the ternary glasses are suitable for optical applications in the visible to NIR region. The effects of the substitution of Al₂O₃ for Nb₂O₅ on optical properties were discussed on the basis of the Drude–Voigt equation. It was suggested that the substitution of Al₂O₃ for Nb₂O₅ increased the molecular density and suppressed a decrease in the refractive index, even when both the average oscillator strength and inherent absorption wavelength decreased in La₂O₃–Nb₂O₅–Al₂O₃ glasses. These results are helpful for designing new optical glasses controlled to have a higher refractive index and lower wavelength dispersion.     

Novel insights into electrical transport mechanism in ionic‐polaronic glasses

Transformation of electrical transport from ionic to polaronic in glasses, which are a potential class of new cathode materials, has been investigated in four series containing WO₃/MoO₃ and Li⁺/Na⁺ ions, namely: xWO₃–(30?0.5x)Li₂O–(30?0.5x)ZnO–40P₂O₅, xWO₃–(30?0.5x)Na₂O–(30.5x)ZnO–40P₂O₅, xMoO₃–(30?0.5x)Li₂O–(30?0.5x)ZnO–40P₂O₅, and xMoO₃–(30?0.5x)Na₂O–(30?0.5x)ZnO–40P₂O₅, 0 ≤ x ≤ 60, (mol%). This study reports a detailed analysis of the role of structural modifications and its implications on the origin of electrical transport in these mixed ionic‐polaron glasses. Raman spectra show the clustering of WO₆ units by the formation of W–O–W bonds in glasses with high WO₃ content while the coexistence of MoO₄ and MoO₆ units is evidenced in glasses containing MoO₃ with no clustering of MoO₆ octahedra. Consequently, DC conductivity of tungstate glasses with either Li⁺ or Na⁺ exhibits a transition from ionic to polaronic showing a minimum at about 20‐30 mol% of WO₃ as a result of ion‐polaron interactions followed by a sharp increase for six orders of magnitude as WO₃ content increases. The formation of WO₆ clusters involved in W‐O‐W linkages for tungsten glasses plays a key role in significant increase in DC conductivity. On the other hand, DC conductivity is almost constant for glasses containing MoO₃ suggesting an independent ionic and polaronic transport pathways for glasses containing 10‐50 mol% of MoO₃.     

Effect of the Glass Structure on Emission of Rare‐Earth‐Doped Borate Glasses

A series of glasses composed of xB₂O₃–8Al₂O₃‐(90?x)Na₂O–R₂O₃ (x = 65, 70, 75, 80, 85; R = Dy³⁺, Tb³⁺, Sm³⁺) were prepared through melt‐quenching. Structural evolution was induced by varying the glass composition. Increasing the glass network former B₂O₃ enhanced the luminescence of rare‐earth ions, as observed in the emission spectra. The mechanism of the glass structural evolution was investigated by the NMR spectra analysis. The dispersant effect of the glass structure was believed to promote the better distribution of the rare‐earth ions in the matrix and reduced the concentration quenching between them. The relationship between the glass structure and its optical properties was established.     

Structure-property relations in crack-resistant alkaline-earth aluminoborosilicate glasses studied by solid state NMR

The effect of the average ionic potential ξ = Ze/r of the network modifier cations on crack initiation resistance (CR) and Young's modulus E has been measured for a series of alkaline-earth aluminoborosilicate glasses with the compositions 60SiO₂–10Al₂O₃–10B₂O₃–(20−x)M₍₂₎O–xM’O (0 ≤ x ≤ 20; M, M’ = Mg, Ca, Sr, Ba, Na). Systematic trends indicating an increase of CR with increasing ionic potential, ξ, have been correlated with structural properties deduced from the NMR interaction parameters in ²⁹Si, ²⁷Al, ²³Na, and ¹¹B solid state NMR. ²⁷Al NMR spectra indicate that the aluminum atoms in these glasses are essentially all four-coordinated, however, the average quadrupolar coupling constant <C_Q> extracted from lineshape analysis increases linearly with increasing average ion potential computed from the cation composition. A similar linear correlation is observed for the average ²⁹Si chemical shift, whereas the fraction of four-coordinate boron decreases linearly with increasing ξ. Altogether the results indicate that in pure alkaline-earth boroaluminosilicate glasses the crack resistance/E-modulus trade-off can be tailored by the alkaline-earth oxide inventory. In contrast, the situation looks more complicated in glasses containing both Na₂O and the alkaline-earth oxides MgO, CaO, SrO, and BaO. For 60SiO₂–10Al₂O₃–10B₂O₃–10MgO–10Na₂O glass, the NMR parameters, interpreted in the context of their correlations with ionic potentials, are consistent with a partial network former role of the MgO component, enhancing crack resistance. Altogether the presence of MgO in aluminoborosilicate glasses helps overcome the trade-off issue between high crack resistance and high elasticity modulus present in borosilicate glasses, thereby offering additional opportunities for the design of glasses that are both very rigid and very crack resistant.     

Effect of BaO substitution for CaO on the structural and thermal properties of SiO2–B2O3–Al2O3–CaO–Na2O–P2O5 bioactive glass system used for implant coating applications

The present study replaced 3.30 and 9.00 mol.% BaO for CaO in a SiO₂–B₂O₃–Al₂O₃–CaO–Na₂O–P₂O₅ bioactive glass system used for implant coating applications. Variations of the glass structure, thermal properties, cytotoxicity, and radiopacity of glasses were studied. As demonstrated by the results, upon adding barium oxide to the glass structure, the weight density increased significantly, while a slight decrease in oxygen density was determined. Introducing barium oxide into glass composition did not cause any considerable change in the spectra of FTIR and Raman. It was demonstrated that the amount of bridging oxygen in the glass structure remained quite unaffected. The hot stage microscopy evaluations revealed further shrinkage of barium-containing frits due to lower viscosity and hence, higher viscous flow of these glasses. By substituting barium oxide for calcium oxide and increasing its concentration, the glass transition temperature (T_g) and the dilatometric softening temperature (T_d) decreased, while the thermal expansion coefficient increased. Moreover, upon substituting 9 mol.% barium oxide for calcium oxide, a 30 °C reduction in maximum sintering temperature (T_ms) of the glass was obtained, whereas the shrinkage rate was increased 1.7 times. It was indicated that the sintering process of barium-incorporated glasses would easily proceed without any phase crystallization. The barium-incorporated glasses exhibited more radiopacity. Additionally, no cytotoxic effect was caused by the substitution, and the Ba-containing glasses could be used for biomedical applications and implant coating as well.     

Preparation of Mn2+-Cu+ co-doped P2O5–ZnO–Al2O3 glasses and their red fluorescence properties

To make a Mn²⁺-doped red glass phosphor that can be excited with ultraviolet (UV) light of light-emitting diodes (LEDs), 60P₂O₅-35ZnO-5Al₂O₃-8MnO-xCu₂O glasses (x = 0-1.00) were prepared by a melt-quenching method at 1200-1400°C for 30-180 minutes in atmospheric air, and the redox of Mn and Cu as well as fluorescence properties were investigated. The Mn²⁺ ion was not reduced and oxidized in the melting, quenching, and annealing processes. The valence of Cu in the glasses changed in the order of 0, 1+, and 2+ with the increase in the amount of Cu₂O and in the melting temperature and time. In this study, a 60P₂O₅-35ZnO-5Al₂O₃-8MnO-0.10Cu₂O glass melted at 1250°C for 90 minutes, having the highest Cu⁺ concentration, showed the strongest Mn²⁺ red fluorescence under the UV light at 275 nm. This strong Mn²⁺ red fluorescence has been caused by the energy transfer from excited Cu⁺ ions to Mn²⁺ ions.     

Thermal Conductivity of Molten B2O3, B2O3–SiO2, Na2O–B2O3, and Na2O–SiO2 Systems

With the aid of the transient hot‐wire method, the thermal conductivity of molten B₂O₃, B₂O₃–SiO₂, Na₂O–SiO₂, and Na₂O–B₂O₃ systems was measured along with their temperature and composition. It was observed that the thermal conductivity of pure B₂O₃ increased with temperature, until about 1400 K, and then decreased subsequently. Using the MAS‐NMR, 3Q‐MAS, and Raman spectroscopy, the structure of B₂O₃ and SiO₂ in the B₂O₃–SiO₂ system was confirmed. Findings show that an addition of B₂O₃ into the pure SiO₂ system causes a significant decrease in thermal conductivity, due to the formation of boroxol rings. The thermal conductivity of the Na₂O–SiO₂ system was measured and its phonon mean free path was calculated. In addition, a positive linear relation between viscosity and thermal conductivity was observed. In the Na₂O–B₂O₃ system, it was found that a change in the relative fraction of 4‐coordinated boron has an influence on the thermal conductivity when the concentration of Na₂O is between 10 and 30 mol%, in which case the tetraborate unit is dominant.     

Coupled Experimental and Thermodynamic Optimization of the Na2O–FeO–Fe2O3–Al2O3 System: Part 1. Phase Diagram Experiments

As part of the complete thermodynamic modeling of the Na₂O–FeO–Fe₂O₃–Al₂O₃ system, the Na₂O–FeO–Fe₂O₃–Al₂O₃ phase diagrams in air (1583 and 1698 K) and at Fe saturation (1573 and 1673 K) were investigated using the quenching method followed by Electron Probe Micro‐Analyzer (EPMA) and X‐ray Diffraction (XRD) phase analysis. General features of the phase diagrams in this system were well revealed for the first time. A complete meta‐oxide solid solution between NaAlO₂ and NaFeO₂ was observed. An extensive solid solution of Na₂(Al,Fe)₁₂O₁₉ Na‐β?‐alumina was found and the existence of a miscibility gap in this solution was confirmed. Several compatibility triangles of three‐phase assemblages were also identified in air and at Fe saturation.     

Understanding the structure,thermal, and optical properties in Al2O3-incorporated La2O3-TiO2-Nb2O5 glasses

Glasses in the 30La₂O₃-40TiO₂-30Nb₂O₅ system are known to have excellent optical properties such as refractive indices over 2.25 and wide transmittance within the visible to mid-infrared (MIR) region. However, titanoniobate glasses also tend to crystallize easily, significantly limiting their applications in optical glasses due to processing challenges. Therefore, the 30La₂O₃-40TiO₂-(30−x) Nb₂O₅-xAl₂O₃ (LTNA) glass system was successfully synthesized using a aerodynamic containerless technique, which improves glass thermal stability and expands the glass-forming region. The effects of Al₂O₃ on the structure, thermal, and optical properties of base composition glasses were investigated by XRD, DSC, NMR, Raman spectroscopy, and optical measurements. DSC results indicated that as the content of Al₂O₃ increased, the thermal stability of the glasses and glass-forming ability increased, as the 30La₂O₃-40TiO₂-25Nb₂O₅-5Al₂O₃ (Nb-Al-5) glass obtained the highest ΔT value (103.5°C). Structural analysis indicates that the proportion of [AlO₄] units increases gradually and participates in the glass network structure to increase connectivity, promoting more oxygen to become bridging oxygen and form [AlO₄] tetrahedral linkages to [TiO₅] and [NbO₆] groups. The refractive index values of amorphous glasses remained above 2.1 upon Al₂O₃ substitution, and a transmittance exceeding 65% in the visible and mid-infrared range. The crystallization activation energies of 30La₂O₃-40TiO₂-30Nb₂O₅ (Nb-Al-0) and Nb-Al-5 glasses were calculated to be 611.7 and 561.4 kJ/mol, and the Avrami parameters are 5.28 and 4.96, respectively. These results are useful to design new optical glass with good thermal stability, high refractive index and low wavelength dispersion for optical applications such as lenses, endoscopes, mini size lasers, and optical couplers.     

The role of PbO/Bi2O3 insertion on the shielding characteristics of novel borate glasses

Impacts of lead and bismuth oxides insertion on a novel glass system of P₂O₅, B₂O₃, Li₂O, Al₂O₃ according the formula 25B₂O₃–25P₂O₅–10Li₂O–5Al₂O₃–5ZnO-xPbO+ (30-x)Bi₂O₃, x = 5,10, 15, 20, 25 mol%. The mass attenuation coefficient (μ_m = μ/ρ) simulated between 0.015 and 15 MeV using MCNP and calculated theoretically using Phy-X/PSD program. Based on the simulated μ_m, other significant parameters such as linear attenuation coefficient (LAC), half and tenth value layer (HVL, TVL), mean free path (MFP), and effective atomic number (Z_eff) were calculated for fabricated glasses. The G-P fitting methods were used to calculate the exposure and energy absorption buildup factors (EBF and EABF) for fabricated glasses. Furthermore, fast neutron removal cross sections (Σ_R) were calculated theoretically for fabricated glasses. The prepared glasses were effective shielding material which can reduce fast neutrons as well as gamma rays.     

Understanding the structural origin of crystalline phase transformations in nepheline (NaAlSiO4)‐based glass‐ceramics

Nepheline (Na₆K₂Al₈Si₈O₃₂) is a rock‐forming tectosilicate mineral which is by far the most abundant of the feldspathoids. The crystallization in nepheline‐based glass‐ceramics proceeds through several polymorphic transformations — mainly orthorhombic, hexagonal, cubic — depending on their thermochemistry. However, the fundamental science governing these transformations is poorly understood. In this article, an attempt has been made to elucidate the structural drivers controlling these polymorphic transformations in nepheline‐based glass‐ceramics. Accordingly, two different sets of glasses (meta‐aluminous and per‐alkaline) have been designed in the system Na₂O–CaO–Al₂O₃–SiO₂ in the crystallization field of nepheline and synthesized by the melt‐quench technique. The detailed structural analysis of glasses has been performed by ²⁹Si, ²⁷Al, and ²³Na magic‐angle spinning — nuclear magnetic resonance (MAS NMR), and multiple‐quantum MAS NMR spectroscopy, while the crystalline phase transformations in these glasses have been studied under isothermal and non‐isothermal conditions using differential scanning calorimetry (DSC), X‐ray diffraction (XRD), and MQMAS NMR. Results indicate that the sequence of polymorphic phase transformations in these glass‐ceramics is dictated by the compositional chemistry of the parent glasses and the local environments of different species in the glass structure; for example, the sodium environment in glasses became highly ordered with decreasing Na₂O/CaO ratio, thus favoring the formation of hexagonal nepheline, while the cubic polymorph was the stable phase in SiO₂–poor glass‐ceramics with (Na₂O+CaO)/Al₂O₃ > 1. The structural origins of these crystalline phase transformations have been discussed in the paper.