Topological hardening through oxygen triclusters in calcium aluminosilicate glasses

Abstract

Calcium aluminosilicate (CAS) glass systems are industrially significant due to their favorable optical, mechanical, and thermal properties, as well as serving as a basis for many glasses in nuclear waste confinement and alkali-free display substrates. [1][2][3][4] The properties which make them industrially desirable are closely linked to their complex structure. In general, the glass system is comprised of network forming silica and alumina units, both ideally in tetrahedral arrangement with four bonded oxygens. Bonded oxygens which bridge to neighboring network formers are appropriately called bridging oxygens (BOs). An interesting feature of the CAS glass system occurs when varying the ratio, R, of [Al 2 O 3 ]/[CaO]. When R < 1 (i.e., percalcic regime), calcium cations act as charge compensators for the negatively charged (AlO 4/2 ) − tetrahedra. Excess calcium cations decrease network connectivity, resulting in non-bridging oxygens (NBOs). At R = 1, previous studies consider the network to be fully connected with no NBOs. 5 However, multiple studies have revealed that a finite concentration of NBOs exist at this ratio. 6,7 When R > 1 (i.e., peraluminous systems), the [AlO 4/2 ] − units are in excess, with insufficient calcium cations available for charge balancing. Two mechanisms have been proposed to compensate for the insufficient population of modifier cations: the formation of highly coordinated alumina (i.e., [5] Al and/or [6] Al), [8][9][10][11] or the formation of three-bonded oxygens (TBO), also known as triclusters. 12 While triclusters are present in crystalline polymorphs, 12,13 verification of triclusters in glassy systems has been limited spectroscopically, and hence molecular dynamics (MD) has been the primary

Octahedral oxide glass network in ambient pressure neodymium titanate

“…Rare-earth titanate glasses generally exhibit 1.9 ≤ ≤ 2.1 4 , so the EPSR-predicted would suggest more O-Ti 3 than previously reported. A significant fraction of these triclusters, a term first used for O linking three Al-O 4 tetrahedra in aluminosilicate networks 32 , 33 , leads to topological hardening and lower glass-forming ability, similar to networks containing large fractions of Al 2 O 3 34 or TiO 2 35 . In the cases of pure Al 2 O 3 or TiO 2 , the large fraction of triclusters prevents glass formation via melt-quenching, though amorphous forms can be prepared by other means 36 .…”

High‐coordinated alumina and oxygen triclusters in modified aluminosilicate glasses

“…48 It has also been suggested by Ren et al 14 that the excess NBOs observed at R = 1 linearly correlate with the percentage of triclusters in the system. Recent reports by Welch et al 15 suggested that this correlation extends to peraluminous compositions in which excess charge (instead of excess NBOs) follows a linear correlation with the percentage of triclusters for the CAS system.…”

“…In contrast, MD studies in the literature show increasing support that the percentage of [5] Al is minimal (less than ∼3%) in both the peraluminous and metaluminous region ranging from R = 0.067 to R = 2. 2, 15,48,55,69,70 These recent works have a stronger emphasis on using computational approaches which have inherent disadvantages such as the high quench rate and the use of empirical interatomic potentials. Numerous studies have focused on examining different potential forms by comparing simulated structural features with results from neutron diffraction with excellent agreement being found.…”

“…(Bottom) The percent increase in the Young's modulus. Reproduced with permission 15 the dominating mechanisms in peraluminous regimes and have been incorporated into several predictive models. 8 Definitive evidence for triclusters with techniques such as NMR could help resolve this debate, but it has been hypothesized that obtaining such a peak assignment may be impossible.…”

“…11 Structural explanations in the current literature remain divided into these glass systems, and the debate between high-coordinated Al, triclusters, or the extent to which both occur in the same glass, continues. 3,[13][14][15] Due to the technological significance of aluminosilicate melts, structural models have consistently been developed for these systems to aid in the design of new applications. [16][17][18] This requires a thorough understanding of the glass structure and its impact on macroscopic properties.…”