•Multiple glass network formers significantly controls the separation of glass transition and crystallization temperatures•The glass transition (Tg) increases linearly from 845 to 903 °C between 25-100 mol% mullite component•Crystallization temperature exhibited a distinct maximum nearly 200 °C greater than Tg at 50 mol% mullite•Crystallization of LaPO4 and mullite occurs separately for 25-35% mullite, but concurrently for higher mullite contents•The structural complexity of the glass tetrahedral network revealed preferences for SiOSi, AlOSi, and AlOP linkages•AlOAl linkages are limited and SiOP linkages are absent•La2O3 preferentially depolymerizes PO4 tetrahedra and stabilizes AlO4 tetrahedra•DSC suggests that compositional unmixing in the glass may well precede crystallization from the supercooled liquid stateRare earth oxide–aluminate–phosphate–silicate (REAPS) glasses are useful precursors for ceramic-matrix-composites (CMCs) with important thermal and mechanical properties. It is important to determine the glass structure, relaxation and crystallization properties for designing and controlling CMC formation. Transparent 3Al2O3·2SiO2-LaPO4 glasses containing 25–80 mol% mullite (3Al2O3·2SiO2) component were prepared by quenching from high temperature melts using a containerless technique. Glass transformation and crystallization behavior were examined by differential scanning calorimetry and X-ray diffraction. The glass transition onset increased from 845 to 906 °C with mullite content. The temperature interval between Tg and crystallization was maximized at 200 °C for 50 mol% mullite glass. Below 40 mol% mullite, successive appearance of LaPO4 (monazite) and mullite gave rise to two crystallization peaks, while at higher mullite content, only one combined exotherm was observed. A glass structure model constructed from 27Al, 31P and 29Si magic angle spinning (MAS) NMR and Raman spectroscopy results indicated that Si4 + and P5 + remained tetrahedrally bonded while Al3 + ions were predominantly in four-fold coordination with some five-coordinated sites. The presence of La2O3 component resulted in an increased proportion of AlO4 tetrahedra. The PO4 polymerization state varied from Q3 to Q2 with increasing LaPO4 content. The SiO4, AlO4, and PO4 units form a continuous network with PO4 tetrahedra attached to aluminosilicate framework through two or three P–O–Al linkages. SiO4 tetrahedra crosslink with AlO4 tetrahedra to form Q4(4Al) and Q4(3Al) units. The glass structure model helps explain the crystallization sequence as a function of mullite content and the formation of different CMC textures.

A homogeneous mullite gel (3:2 Al2O3:SiO2 molar ratio) was made from tetraethoxysilane (TEOS) and aluminum isopropoxide (Al(OCH(CH3)2)3) at room temperature within a relatively quick three days. Mullite was the only crystalline phase to form during calcination; metastable phases like aluminosilicate spinel did not appear. When heated at 20 °C/min, crystalline mullite (65 mol% Al2O3) started forming at 575 °C and reached 27 wt.% by 900 °C. Major crystallization occurred at ~ 1000 °C with a concurrent increase of Al2O3 concentration (68–69 mol%) in the mullite phase. The alumina content decreased towards stoichiometric (3Al2O3·2SiO2) mullite at even higher temperatures. When the gel remained in the amorphous state, low temperature preheating significantly improved crystallization at higher temperatures. After preheating at 425 °C for 24 h, 78 wt.% of the final product was crystallized when it was subsequently annealed at 750 °C for 5 min. Only 20 wt.% crystallized without preheating.