Alkali-free aluminosilicate glasses, characterized by high hardness, high strain point, high electrical insulation, excellent chemical durability, and tunable thermal expansion coefficients, are preferred substrate materials for high-end OLED displays. In this study, glass samples with a fixed total alkaline-earth oxide content of 26.17% (molar fraction) and varying Mg/Ca/Sr ratios were prepared by the melt-quenching method. The coordination environments of ²⁹Si and oxygen atoms were characterized by magic-angle spinning nuclear magnetic resonance (MAS-NMR) and X-ray photoelectron spectroscopy (XPS), and first-principles calculations were performed to resolve atomic-scale network-structure evolution and elucidate the structural origin of the ternary MgO-CaO-SrO mixed alkaline-earth effect (MAEE). The results show that the short-range structures of network formers (Si-O, Al-O, B-O) exhibit high stability during alkaline-earth substitution. Electron localization function (ELF) analysis quantitatively shows that high-field-strength Mg²⁺ induces stronger localization of oxygen electron clouds than Sr²⁺. Partial density of states (PDOS) analysis reveals that Mg 3p does not hybridize with O 2p, whereas Sr 4p exhibits p-p hybridization with O 2p, providing an electronic-level explanation for the contrasting network constraints exerted by the two cations. Coordination number analysis further distinguishes their structural roles: Mg²⁺ acts as a network intermediate (CN = 4-5), while Ca²⁺ and Sr²⁺ behave as typical network modifiers. Under equimolar Mg/Ca mixing conditions, the nonlinear effect is most pronounced, with the NBO fraction reaching 28.6%, the free volume fraction expanding from ~1.04% to ~2.75%, and the number of low-polymerization Qⁿ units increasing. These findings suggest that the competition among alkaline-earth cations in local coordination, orbital hybridization, and charge compensation is the fundamental origin of the nonlinear structural response, providing a theoretical basis for the compositional design of high-performance electronic substrate glasses.

A series of glass samples with a fixed total alkaline-earth oxide content (26.17%) and varying Mg/Ca/Sr ratios were prepared by conventional melt-quenching at 1650 ℃ and subsequently annealed at 600 ℃. The local coordination environments of ²⁹Si and oxygen atoms were characterized using magic-angle spinning nuclear magnetic resonance (MAS-NMR) and X-ray photoelectron spectroscopy (XPS). To investigate the atomic-scale evolution of the network, ab initio molecular dynamics (AIMD) simulations were conducted using the CP2K package. The 321-atom models used the PBE exchange-correlation functional and Goedecker-Teter-Hutter (GTH) pseudopotentials. Structural descriptors-including radial distribution functions (RDFs), bond-angle distributions, coordination-number curves n(r) obtained from RDF integration, Qⁿ distributions, oxygen coordination ratios, and free volume fractions-were extracted from the optimized configurations to quantify short- and medium-range network structures. Electronic-structure analyses, including electron localization function (ELF) and partial density of states (PDOS), were performed to characterize bond ionicity and orbital hybridization.

The results indicated that while the short-range structures of Si-O, Al-O, and B-O units remained highly stable across compositions, medium-range topological connectivity was strongly influenced by the MAEE. ELF analysis revealed that in the Mg-rich N3 composition, the bond-region ELF is 0.503, lower than 0.524 in the Sr-rich N5 composition (ΔELF = -0.021). This directly confirmed that high-field-strength Mg²⁺ exerted stronger polarization and localization on oxygen electron clouds, resulting in more ionic Mg-O bonds. PDOS analysis further showed that Mg 3p was buried at -37.6 eV with an O 2p energy separation exceeding 33 eV (no hybridization), whereas Sr 4p lay at -14.3 eV with an O 2p separation of only ~11 eV (p-p hybridization). The weaker electrostatic constraint from Sr²⁺ allowed O 2p orbitals to shift toward the Fermi level, consistent with the ELF findings. In the CaO-SrO substitution sequence, larger Sr²⁺ ions acted primarily as long-range charge compensators for [AlO₄]^- units, increasing the BO fraction to 74.8% in sample N2. In Mg-rich systems, high-field-strength Mg²⁺ induced more NBOs; however, the short and strong Mg-O ionic bonds locally constrained the network. Coordination number analysis from RDF integration clearly distinguished the structural roles: Mg²⁺ exhibited a CN distribution of 4-5 (network intermediate behavior), while Ca²⁺ and Sr²⁺displayed higher and broader CN distributions (typical network modifier behavior), resolving the ambiguity in bond-angle distribution differentiation. Notably, when MgO and CaO coexist in equimolar proportions within a high-SrO matrix (sample N6), pronounced nonlinear variations emerged: the NBO content reached 28.6%, and the free volume fraction, quantitatively evaluated by probe-sphere scanning (probe radius 1.2 Å, grid spacing 0.1 Å), increased from ~1.04% (N3, Mg-rich) to ~2.75% (N5, Sr-rich). This expansion was primarily attributed to the dilation of modifier coordination environments rather than to rupture of the network-former framework.

This study systematically elucidates the regulating role of alkaline-earth cation ratios on the network structure of alkali-free aluminosilicate glasses in the MgO-CaO-SrO ternary system by combining NMR and XPS characterization with AIMD simulations. The short-range structural units (Si-O, Al-O, B-O) exhibit high stability against compositional variation (Si-O CN = 4.00 ± 0.01; B-O predominantly [BO₃] with average CN = 3.08 ± 0.12), whereas the medium-range topological structure exhibits a pronounced nonlinear MAEE. High-field-strength Mg²⁺ promotes NBO formation with strong electron localization (ELF = 0.503) and no Mg 3p/O 2p hybridization, resulting in highly ionic Mg-O bonds that locally constrain the network. In contrast, larger-radius Sr²⁺ preferentially compensates [AlO₄]^- charges with weaker electron localization (ELF = 0.524) and exhibits Sr 4p/O 2p p-p hybridization (energy separation ~11 eV), facilitating network condensation. Coordination number analysis further differentiates the structural roles: Mg²⁺ functions as a network intermediate (CN = 4-5) and Ca²⁺/Sr²⁺ function as network modifiers (higher and broader CN distributions). Under equimolar Mg/Ca mixing (sample N6), the nonlinear response is most pronounced, with the NBO fraction reaching 28.6% and the free volume fraction expanding from ~1.04% (N3) to ~2.75% (N5), accompanied by an increase in low-polymerization Qⁿ units. These findings demonstrate that competitive coordination, orbital hybridization, and charge-compensation behaviors among alkaline-earth cations are the fundamental origin of the nonlinear structural response. The close agreement between experimental observations and simulation trends confirms the reliability of AIMD for resolving the structural evolution of complex glass networks and provides a theoretical basis for the compositional design of high-performance electronic substrate glasses.