Glass structure and crystallization via two distinct thermal histories: Melt crystallization and glass crystallization

This study focuses on the effect of structural inhomogeneity of 44.4CaO-33.2SiO₂-8.8Na₂O-6.2Al₂O₃-7.4B₂O₃ glass on its crystallization behavior. The crystallization behavior of the investigated lime-sodium aluminoborosilicate glass system is explored for the two thermal histories of melt crystallization and the heat treatment of quenched glass. The results show that sodium melilite (CaNaAlSi₂O₇) and larnite (β-Ca₂SiO₄) are the dominant crystalline phases during heating (glass crystallization) and cooling (melt crystallization), respectively. In the glass structure from the heat treatment at 610 °C, both Si₂O₇ and AlO₄ units (i.e., four-coordinated Al) are dominant. This may induce sodium melilite (CaNaAlSi₂O₇) crystallization, connecting Si₂O₇ and AlO₄ units with O and linking Ca and Na between interlayers. The melt structure, meanwhile, has more SiO₄ and AlO₆ units in its silicate and aluminate. Therefore, larnite (β-Ca₂SiO₄) crystallizes on cooling by combining SiO₄ units and Ca. The crystallization behavior is further discussed considering the inhomogeneity structure of supercooled liquids before nucleation.     

High Fraction of Penta‐Coordinated Aluminum and Gallium in Lanthanum–Aluminum–Gallium Borates

This study is focused on structural changes induced by increasing treatment temperature of sol‐gel–derived La₂O₃?Al₂O₃?Ga₂O₃?5B₂O₃ system. The structure of samples heated for 30 min up to 900°C was investigated by X‐ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and magic‐angle spinning nuclear magnetic resonance (MAS‐NMR) analysis of ²⁷Al, ¹¹B, and ⁷¹Ga nuclei. The vitreous structure is preserved inclusively after 800°C treatment, and starting with 850°C the only crystalline phase evidenced in XRD patterns is of LaAl_2.03B₄O_10.54 type, of La(Al,Ga)_2.03B₄O_10.54 composition. The FTIR results point out the presence of BO₃, AlO₄, and AlO₆, and starting with 800°C treatment also of BO₄ and AlO₅ structural units, but more detailed information related to boron, aluminum, and gallium environments is obtained from the analysis of MAS‐NMR data. These data evidenced in both amorphous xerogels and in crystallized samples a high fraction of penta‐coordinated aluminum and gallium.     

Ionic Conductivity in Sodium–Alkaline Earth–Aluminosilicate Glasses

The effect of alkaline‐earth ions on Na transport in aluminosilicate glasses was studied by measuring ionic conductivity for a systematic compositional series of Na₂O–RO–Al₂O₃–SiO₂ glasses (R=Mg, Ca, Sr, Ba). The Na transport in aluminosilicate glass could be affected by compositional changes in aluminum coordination and nonbridging oxygen as well as physical properties such as dielectric constant, shear modulus, and ionic packing factor. Through careful experimental designs and measurements, the main determinants among these parameters were identified. ²⁷Al MAS‐NMR indicated that all aluminum species contained in these glasses are four‐coordinated. The activation energy for ion conductivity decreased with increasing aluminum content and decreasing ionic radii of the alkaline‐earth ion in the region where [Al] < [Na]. When the aluminum content exceeded the sodium content ([Al] > [Na]), the composition dependence of the activation energy depended on the specific alkaline earth. These results are explained based on variations in free volume and dielectric constant caused by structural changes around the AlO₄ charge compensation sites. These structure changes occur in response to the smaller size and higher field strength of the alkaline‐earth ions, and are most prevalent in the compositions which require bridging of two AlO₄ sites by the alkaline‐earth ion for charge compensation.     

Elucidating the role of AlO6‐octahedra in aluminum silicophosphate glasses through topological constraint theory

The local structures of sodium aluminum silicophosphate glasses containing unique AlO₆‐octahedra were characterized through nuclear magnetic resonance (NMR) and modeled by topological constraint theory (TCT). Subsequent calculation results of the glass‐transition temperature (T_g) and Vickers hardness (H_v) obtained using TCT were verified by the experimental data, which provided us evidence of the glass former role of the AlO₆‐octahedra and their behavior in the aluminum‐containing glass systems. The glass‐forming behavior of the AlO₆‐octahedra was identified by their displacement of SiO₆‐octahedra based on their corresponding NMR spectrum. The structure featured a constant total amount of AlO₆ and SiO₆‐octahedra (AlO₆‐octahedra increased whereas SiO₆‐octahedra decreased) with an increasing aluminum content, which was caused by the mutual replacement between them. The glass former role of the AlO₆‐octahedra was further supported by the theoretical computation of T_g and H_v through application of TCT. Specifically, the model of the aluminum‐containing glasses reported here is an extension of the conventional TCT that only incorporates the constraints of the glass formers.     

Influence of electropulsing treatment on crystallization and structure of calcium silicate-based melt

Electropulsing treatment (EPT) is a promising technology for controlling the phase transition during the solidification of melts owing to its electric and thermal effects. In this study, the influence of EPT on the crystallization and melt structure of a calcium silicate-based mold flux was investigated. The results showed that the morphology of crystals that precipitated in the mold flux changed from elongated columnar to block shape, and the equivalent grain diameter of the crystals increased with increasing voltage from 0 to 20 V. The mass fraction of Ca₄Si₂O₇F₂ precipitated in the mold flux decreased with increasing impulse voltage, whereas that of Ca₂Mg_0.75Al_0.5Si_1.75O₇ increased. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) analyses suggest that the network structure of both silicate and aluminate was simplified by electropulsing because the simpler structural units of Q₀, Q₁, [AlO₆]⁹⁺, etc., increased with increasing impulse voltage, whereas the complex structural units of Q₂, Q₃, and [AlO₄]⁵⁺ decreased. The extra electric field force is the repulsion force between two oppositely charged ions, which was the root of the network structure simplification and crystallization promotion. The results obtained in this study provide an innovative method for locally controlling the crystallization behavior of mold flux in a mold.     

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.     

Al2O3 triggered in-situ crystallization of Gd3Al2Ga3O12 in glass and enhancement in magnetic and Faraday rotating properties

Nanocrystals doped transparent glasses or glass ceramics have shown promising tunable magnetic and magneto-optical performance. In this study, the Al₂O₃-induced in-situ crystallization of Gd₃Al₂Ga₃O₁₂ in tellurite glass was reported. 10–20 nm-cubic Gd₃Al₂Ga₃O₁₂ (Ia3d space group) nanocrystals formed in tellurite glass with Al₂O₃ content ≤ 0.75 mol% by 400 °C-heat treatment. When Al₂O₃ content in glass was higher than 0.75 mol%, excessive Al₂O₃ triggered the crystallization of orthorhombic GdAlO₃ (Pnma) in which the Gd and Al ions existed as octahedral GdO₈ and AlO₆ units at a temperature higher than 398 °C. The in-situ crystallization influenced the glass network structure, broke the linkage of tetrahedral TeO₄, and BO₄, and formed trigonal pyramids TeO₃ and BO₃ instead. At the same time, nuclear magnetic resonance spectra revealed the conversion of AlO₄→ AlO₆, GaO₄→ GaO₆, and the changes from bridging oxygen to non-bridging oxygen as well. From energy-dispersive X-Ray analysis, Gd³⁺ clusters were observed, leading to the ferromagnetism of glass. Electron paramagnetic resonance spectra witnessed an enhancement of the Zeeman effect which is the reason for the improvement of Faraday rotation. Tellurite glass with 0.75 mol% Al₂O₃ after 400 °C-annealing (A75) showed a giant Verdet constant of 93 rad/T.m at 633 nm which is superior to most of the values from the literature.     

Viscosity and structure evolution of bearing-BaO slag melt with the low CaO/SiO2 mass ratio of 0.7

The effect of BaO on the viscosity of experimental slag with the CaO/SiO₂ ratio of 0.7 was studied based on the rotating cylinder method, and the structure evolution analysis was performed using Fourier transform infrared (FT-IR), X-ray photoelectron spectroscopy (XPS) and ²⁷Al MAS-NMR spectra. The results indicated that the viscosity of molten slag decreased gradually with BaO content rising from 0 wt.% and 5 wt.% due to the dominant effect of free O^2- rather than Ba²⁺. The viscous flow activation energy (Eη) of molten slag was calculated, presenting a similar change trend with that of viscosity. The structure analysis demonstrated that, with the increasing BaO content, the [SiO₄]-tetrahedral structures and Si-O-Al bonds were destroyed due to an increase in the relative fraction of free oxygen (O^2-). For the Al-related structural units, the ²⁷Al magic angles spinning nuclear magnetic resonance (²⁷Al MAS NMR) spectra analysis illustrated that the concentration of [AlO₄] units reduced, whereas that of [AlO₅] and [AlO₆] units increased because of the increase of free oxygen and nonbridged oxygen (O⁰). The results of structure analysis agreed well with the viscosity variations of experimental slag.     

Control of Oxidation State of Eu Ions in Na2O–Al2O3–SiO2 Glasses

Glasses doped with well‐controlled Eu³⁺ and Eu²⁺ ions have attracted considerable interest due to the possibility of tuning the wavelength range of the emitted light from violet to red by using their ⁵D₀→⁷F_j and 5d–4f electron transitions. Glasses were prepared to dope Eu³⁺ ions in a Na₂O–Al₂O₃–SiO₂ system, and the changes in the valence state of Eu³⁺ ions and the glass structure surrounding the Eu atoms during heating under H₂ atmosphere were investigated using fluorescence spectroscopy, X‐ray absorption fine‐structure spectroscopy, and ²⁷Al magic‐angle spinning solid‐state nuclear magnetic resonance spectroscopy. The reduction behavior of Eu³⁺ ions was dependent on the Al/Na molar ratio of the glass. For Al/Na < 1, the Al³⁺ ions formed the AlO₄ network structure accompanied by the Na⁺ ions as charge compensators; the Eu³⁺ ions occupied the interstitial positions in the SiO₄ network structure and were not reduced even under heating in H₂ gas. On the other hand, in the glasses containing Al₂O₃ with the Al/Na ratio exceeding unity, the Eu³⁺ ions commenced to be coordinated by the AlO₄ units in addition to the SiO₄ network structure. When heated in H₂ gas, H₂ gas molecules reacted with the AlO₄ units surrounding Eu³⁺ ions to form AlO₆ units terminated with OH bonds, and reduced Eu³⁺ ions to Eu²⁺ via the extracted electrons.     

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.     

Structural modification associated with Al2O3 addition in oxyfluoride glasses: Thermal and mechanical properties

In this work, the effect of gradual addition of Al₂O₃ substituting SiO₂ on the structural, thermal, and mechanical properties of SiO₂–BaF₂–K₂O–GdF₃–Sb₂O₃‐based oxyfluoride glasses have been studied. The X‐ray diffraction (XRD) patterns and differential scanning calorimetric (DSC) curves indicate that there is a distinct primary crystallization corresponding to BaGdF₅ phase formation in the samples without (0AlG) and with 5 mol% substitution of Al₂O₃ (5AlG) while the sample with 10 mol% of Al₂O₃ (10AlG) does not show such crystallization event. Further, the activation energy (E_a) for fluoride crystal formation is higher for the 5AlG in comparison to the 0AlG glass as determined by Kissinger, Augis‐Bennett and Ozawa models. Fourier transform infrared (FTIR) and Raman spectroscopy analysis confirmed the structural modification with the gradual addition of Al₂O₃ in the glass matrix revealing dominant presence of AlO₄ tetrahedral units in 10AlG sample unlike in 5AlG sample which exhibited the manifestation of AlO₆ units. Such structural variation has further been substantiated from the estimated elastic properties like Young's modulus (E), shear modulus (G), bulk modulus (K), longitudinal modulus (L), and mean ultrasonic velocity (U_m) by showing a decrease for 5AlG sample in comparison with 0AlG sample followed by subsequent increase for 10AlG sample.     

Sealing Glass‐Ceramics with Near Linear Thermal Strain,Part I: Process Development and Phase Identification

A widely adopted approach to form matched seals in metals having high coefficient of thermal expansion (CTE), e.g. stainless steel, is the use of high CTE glass‐ceramics. With the nucleation and growth of Cristobalite as the main high‐expansion crystalline phase, the CTE of recrystallizable lithium silicate Li₂O–SiO₂–Al₂O₃–K₂O–B₂O₃–P₂O₅–ZnO glass‐ceramic can approach 18 ppm/°C, matching closely to the 18 ppm/°C–20 ppm/°C CTE of 304L stainless steel. However, a large volume change induced by the α‐β inversion between the low‐ and high‐ Cristobalite, a 1^st order displacive phase transition, results in a nonlinear step‐like change in the thermal strain of glass‐ceramics. The sudden change in the thermal strain causes a substantial transient mismatch between the glass‐ceramic and stainless steel. In this study, we developed new thermal profiles based on the SiO₂ phase diagram to crystallize both Quartz and Cristobalite as high expansion crystalline phases in the glass‐ceramics. A key step in the thermal profile is the rapid cooling of glass‐ceramic from the peak sealing temperature to suppress crystallization of Cristobalite. The rapid cooling of the glass‐ceramic to an initial lower hold temperature is conducive to Quartz crystallization. After Quartz formation, a subsequent crystallization of Cristobalite is performed at a higher hold temperature. Quantitative X‐ray diffraction analysis of a series of quenched glass‐ceramic samples clearly revealed the sequence of crystallization in the new thermal profile. The coexistence of two significantly reduced volume changes, one at ~220°C from Cristobalite inversion and the other at ~470°C from Quartz inversion, greatly improves the linearity of the thermal strains of the glass‐ceramics, and is expected to improve the thermal strain match between glass‐ceramics and stainless steel over the sealing cycle.     

Effect of the Al2O3/SiO2 mass ratio on the crystallization behavior of CaO-SiO2-MgO-Al2O3 slags using confocal laser scanning microscopy

The crystallization behavior of a CaO-SiO₂-MgO-Al₂O₃ slag system with varying Al₂O₃/SiO₂ mass ratios from 0.03 to 1.10 has been investigated using a confocal laser scanning microscopy (CLSM). The resulting continuous cooling transformation (CCT) and time-temperature-transformation (TTT) curves showed that the initial crystallization temperature increased and the incubation time for crystallization slightly decreased with increasing Al₂O₃/SiO₂ ratio. The crystal growth rate first increased and then decreased with decreasing isothermal temperature. X-ray diffraction (XRD) analysis suggested that Ca₂MgSi₂O₇ or Ca₃MgSi₂O₈ precipitated as the primary phase at lower Al₂O₃/SiO₂ ratios, while the Ca₂Al₂SiO₇ phase was preferred at higher Al₂O₃/SiO₂ ratios. The observed crystalline phases correlated well with the expected thermodynamic predictions from FactSage. In addition, structural analysis using ²⁷Al magic angle spinning nuclear magnetic resonance (²⁷Al MAS-NMR) microscopy of the as-quenched slags indicated the presence of a higher ratio of tetrahedral [AlO₄]^5-structural units with increasing Al₂O₃/SiO₂ ratio, which enhanced the polymerization of tetrahedral [AlO₄]^5- and [SiO₄]^4- structural units to form Ca₂Al₂SiO₇.     

Effect of ZnO content change on the structure and properties of zinc borophosphate glasses

The glass system xZnO-10B₂O₃-(100 ? x)P₂O₅ was prepared by cooling using the melt-quenching technique in the composition series x from 30 to 60 mol %. The density, thermal expansion coefficient, glass transition temperature, and softening temperature of the glass system were determined. Structural characterization was performed by using a combination of IR and Raman spectroscopy and ¹¹B/³¹P solid state NMR spectroscopy data. In particular, variations in the phosphate network structure upon the addition of ZnO were investigated. Data analysis indicated that increasing the ZnO content and decreasing the P₂O₅ content increased the extent of cross-linking between the phosphate and borate units in the glass network; this was because incorporation of ZnO polyhedra into the structural network of ZnO increased the network dimensionality. These structural changes were confirmed by Raman and IR spectroscopy.     

Effect of P2O5 on the Nonisothermal Sinter‐Crystallization Process of a Lithium Aluminum Silicate Glass

Dense (~98.5%), lithium aluminum silicate glass‐ceramics were obtained via the sinter‐crystallization of glass particle compacts at relatively low temperatures, that is, 790–875°C. The effect of P₂O₅ on the glass‐ceramics' sinter‐crystallization behavior was evaluated. We found that P₂O₅ does not modify the surface crystallization mechanism but instead delays the crystallization kinetics, which facilitates viscous flow sintering. Our glass‐ceramics had virgilite (Li_xAl_xSi_3‐xO₆; 0.5 < x < 1), a crystal size <1 μm, and a linear thermal expansion coefficient of 2.1 × 10^?6°C^?1 in the temperature range 40–500°C. The overall heat treatment to obtain these GCs was quite short, at ~25 min.     

Formation of Bi2ZnB2O7 Nanocrystals in ZnO–Bi2O3–B2O3 Glass Induced by Femtosecond Laser

We report on the formation of Bi₂ZnB₂O₇ crystal structures with designated patterns in ZnO–Bi₂O₃–B₂O₃ glass by femtosecond laser direct writing. The crystallization mechanism in glass is investigated by crystallization kinetics analysis and simulation of the three‐dimensional temperature field distribution. The crystallized regions show larger third‐order optical nonlinearity than the unirradiated region in glass by Z‐scan technique. This finding is of great potential in application of nonlinear optical integrated devices and development of new nonlinear materials.     

Effect of Li2O on the crystallization behavior of CaO-Al2O3-SiO2-Li2O-Ce2O3 mold slags

The effect of Li₂O on the crystallization properties of CaO-Al₂O₃-SiO₂-Li₂O-Ce₂O₃ slags was investigated. With increasing the Li₂O content, LiAlO₂ and CaCeAlO₄ were the main crystalline phases. LiAlO₂ formed for the charge compensating of Li⁺ ions to [AlO₄^5?]-tetrahedrons, and CaCeAlO₄ formed as a result of the charge balance of Ce³⁺ ions, Ca²⁺ ions, and [AlO₆^9?]-octahedrons. Increasing the content of Li₂O to 10%, the crystallization temperature was the highest, and the incubation time was the shortest. The crystallization ability was strong due to the three factors of strengthening the interaction between ions and ion groups, decreasing the polymerization degree, and increasing the melting temperature. Further increasing the content of Li₂O, the crystallization performance was obviously suppressed, because the melting temperature and the force between the cations and the anion groups decreased.     

Formation and thermal stability of dual glass phases in the h-BN/SiO2/Yb-Si-Al-O composites

The glass phases formation and microstructural evolution in the h-BN/SiO₂/Yb-Si-Al-O composite were investigated. Owing to the introduction of Al₂O₃, SiO₂ could transform into amorphous silica at a lower temperature (∼1600 °C). On the other hand, with increasing temperature, Al³⁺ gradually dissolved into ytterbium silicate (Yb₂Si₂O₇) and then accelerated the formation of amorphous Yb-Si-Al-O glass phase. Up to 1880 °C, fine spherical Yb-Si-Al-O glass particles and irregular amorphous silica distributed uniformly, which contributes to the excellent mechanical properties of the composite. The thermal stability study disclosed that more [AlO₄] units could effectively inhibit crystallization of the Yb-Si-Al-O glass phase, but mechanical properties of composite still decreased slowly with increasing the heat treatment temperature. For example, the flexural strength of the composite with 1.5 wt.% Al₂O₃ decreased from 297 ± 30 to 284 ± 22 MPa as the treatment temperature rose from 800 to 1200 °C.     

Effect of drawing temperature on the structure and free volume of semicrystalline polyester yarns

Supercritical carbon dioxide (scCO₂), an environmentally friendly solvent, can change the fine structure of fibers depending on treatment temperature and pressure. This article presents the results of an investigation into the effects of drawing temperature, scCO₂ exposure and tension on structural changes, and mesomorphic transitions in partially oriented yarn (POY) and oriented poly(ethylene terephthalate) (PET) yarns. Samples of POY and PET yarns were uniaxially drawn at different temperatures to obtain filaments with different structures for subsequent exposure to scCO₂ in the presence and absence of tension at a temperature of 80^oC and under a pressure of 220 bar. Structural investigation of the samples was performed by combining the results obtained from differential scanning calorimetry, Fourier transform infrared, mechanical deformation, and density and birefringence measurements, all of which exhibited a good correlation. The fractional free volume in the PET samples was considered for interpreting structure–property relations. Results showed that the development of a transient structure strongly depends on process temperature and that for an identical draw ratio, PET yarns drawn at 23^oC are significantly more oriented than those drawn at 68^oC. Two crystallization mechanisms, that is strain‐induced and thermal‐induced crystallization, are involved that are frequently used to explain the structural changes during yarn deformation at different temperatures. The degree of crystallinity and orientation factors for PET yarns does not proportionally increase with increasing temperature of the drawing process; indeed, the highest value is obtained at a temperature of 100^oC, whereas the lowest is observed at a temperature of 68^oC, which is near the glass transition temperature. Tension under scCO₂ exposure also produces significant effects in term of causing structural changes. POLYM. ENG. SCI., 55:2030–2041, 2015. © 2014 Society of Plastics Engineers     

Identification of the Zn Substitution Sites in La–Zn Substituted SrAl12O19 from 27Al Solid‐State NMR Studies

The La–Zn substituted hexagonal strontium aluminate, Sr_1?xLa_xAl_12?xZn_xO₁₉, with the magnetoplumbite structure and having five different coordination environments for Al with different symmetries, is investigated using ²⁷Al solid‐state NMR to get detailed information on the sites of substitution of Zn and the associated changes in the local coordination environments of Al. The objective of the study was to get information on the local structural variations in the isostructural La–Co substituted strontium ferrite, Sr_1?xLa_xFe_12?xCo_xO₁₉, showing enhanced magnetic performance on substitution. The NMR studies on the aluminate give direct evidence for the sites of substitution and the changes in the local coordination environments. It is found that Zn is substituted at the 2a and 4f₂ AlO₆ octahedral sites. However, an interesting observation from the NMR studies is the stabilization of the Al site occupancy at the penta‐coordinated 2b site over the distorted tetrahedral 4eAl site, without any substitution at these sites. Large changes in the quadrupolar coupling constant of the 2a and 4e sites are observed between x = 0.2 and 0.3, corresponding to the compositional region showing higher performance in the case of Sr_1?xLa_xFe_12?xCo_xO₁₉, indicating the role of distortion of local coordination environments on suitable substitution in controlling the performance parameters.