Aluminosilicate glasses are considered to follow the Al-avoidance principle, which states that Al-O-Al linkages are energetically less favorable, such that, if there is a possibility for Si-O-Al linkages to occur in a glass composition, Al-O-Al linkages are not formed. The current paper shows that breaching of the Al-avoidance principle is essential for understanding the distribution of network-forming AlO and SiO structural units in alkaline-earth aluminosilicate glasses. The present study proposes a new modified random network (NMRN) model, which accepts Al-O-Al linkages for aluminosilicate glasses. The NMRN model consists of two regions, a network structure region (NS-Region) composed of well-separated homonuclear and heteronuclear framework species and a channel region (C-Region) of nonbridging oxygens (NBOs) and nonframework cations. The NMRN model accounts for the structural changes and devitrification behavior of aluminosilicate glasses. A parent Ca- and Al-rich melilite-based CaO-MgO-AlO-SiO (CMAS) glass composition was modified by substituting MgO for CaO and SiO for AlO to understand variations in the distribution of network-forming structural units in the NS-region and devitrification behavior upon heat treating. The structural features of the glass and glass-ceramics (GCs) were meticulously assessed by advanced characterization techniques including neutron diffraction (ND), powder X-ray diffraction (XRD), Si andAl magic angle spinning (MAS)-nuclear magnetic resonance (NMR), and in situ Raman spectroscopy. ND revealed the formation of SiO and AlO tetrahedral units in all the glass compositions. Simulations of chemical glass compositions based on deconvolution of Si MAS NMR spectral analysis indicate the preferred formation of Si-O-Al over Si-O-Si and Al-O-Al linkages and the presence of a high concentration of nonbridging oxygens leading to the formation of a separate NS-region containing both SiO and AlO tetrahedra (Si/Al) (heteronuclear) in addition to the presence of Al-O-Al bonds; this region coexists with a predominantly SiO-containing (homonuclear) NS-region. In GCs, obtained after heat treatment at 850 °C for 250 h, the formation of crystalline phases, as revealed from Rietveld refinement of XRD data, may be understood on the basis of the distribution of SiO and AlO structural units in the NS-region. The in situ Raman spectra of the GCs confirmed the formation of a Si/Al structural region, as well as indicating interaction between the Al/Si region and SiO-rich region at higher temperatures, leading to the formation of additional crystalline phases.
The
presence of lanthanide-tellurite “anti-glass”
nanocrystalline phases not only affects the transparency in glass–ceramics
(GCs) but also influences the emission of a dopant ion. Therefore,
a methodical understanding of the crystal growth mechanism and local
site symmetry of doped luminescent ions when embedded into the precipitated
“anti-glass” phase is crucial, which unfolds the practical
applications of GCs. Here, we examined the Ln2Te6O15 “anti-glass” nanocrystalline phase growth
mechanism and local site symmetry of Eu3+ ions in transparent
GCs produced from 80TeO2–10TiO2–(5
– x)La2O3–5Gd2O3–xEu2O3 glasses, where x = 0, 1, 2. A crystallization
kinetics study identifies a unique crystal growth mechanism via a
constrained nucleation rate. The extent of “anti-glass”
phase precipitation and its growth in GCs with respect to heat-treatment
duration is demonstrated using X-ray diffraction (XRD) and field emission
scanning electron microscopy (FE-SEM) analysis. Qualitative analysis
of XRD confirms the precipitation of both La2Te6O15 and Gd2Te6O15 nanocrystalline
phases. Rietveld refinement of powder X-ray diffraction patterns reveals
that Eu3+ ions occupy “Gd” sites in Gd2Te6O15 over “La” sites
in La2Te6O15. Raman spectroscopy
reveals the conversion of TeO3 units to TeO4 units with Eu2O3 addition. This confirms the
polymerizing role of Eu2O3 and consequently
high crystallization tenacity with increasing Eu2O3 concentration. The measured Eu3+ ion photoluminescence
spectra revealed its local site symmetry. Moreover, the present GCs
showed adequate thermal cycling stability (∼50% at 423 K) with
the highest activation energy of around 0.3 eV and further suggested
that the present transparent GCs would be a potential candidate for
the fabrication of red-light-emitting diodes (LEDs) or red component
phosphor in W-LEDs.
Exploring the reasons for the initiation of Al–O–Al bond formation in alkali-earth alumino silicate glasses is a key topic in the glass-science community.
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