Degree of polymerization of the CaAl2Si2O8 aluminosilicate glass

Abstract

The pressure dependence of the refractive index for a glass of the composition 0.50SiO 2 · 0.25Al 2 O 3 · 0.25CaO at pressures up to 5.0 GPa is determined using a polarizing interference microscope and an apparatus with diamond anvils. The densities and compressibilities of the glass are calculated from the measured refractive indices within the framework of the theory of photoelasticity. The results are compared with the data obtained earlier for vitreous silica. The lower compressibility of the glass under investigation as compared to the compressibility of fused silica is in agreement with the degree of polymerization that is calculated with due regard for the formation of triclusters from the relationship NBO / T = (2 O + 3 O t -4 T )/ T , where NBO (nonbridging oxygen) is the number of nonbridging oxygen atoms; O is the number of gram-ions of oxygen, except for of tricluster oxygen; O t is the number of gram-ions of tricluster oxygen; and T is the total number of gram-ions of network formers.

Molecular dynamics studies of CaAl2Si2O8 liquid. Part II: Equation of state and a thermodynamic model

“…Optical measurements by Kuryaeva (2006) and Raman studies by Le Parc et al (2003) do not report evidence for un-mixing, but both studies used material prepared by rapid quenching from the liquid state at room pressure. To have any hope of detecting the two-phase region, samples would have to be annealed below T C , but this temperature is too low at ambient pressure (Fig.…”

Effect of pressure on the refractive index and relative density of the CaO·Al2O3·6SiO2 glass

“…(2) The formula allows taking account of the change in the degree of polymerization at the expense of the formation of highly coordinated aluminum. The calculation of the degree of polymerization was worked out on the example of anorthite glass CaAl 2 Si 2 O 8 [44,45], as the relative% of non-bridging oxygen atoms [18] and highly coordinated aluminum [20] are known for this glass. Suppose that non-bridging oxygen atoms are formed only at the expense of highly coordinated aluminum ($10%) by the equation: Percentage of non-bridging oxygen atoms in this case makes up 1.25% of the total oxygen, which is inconsistent with experimentally obtained value $5% [18].…”

“…However, as glass composition is normally presented in mole fraction or mole percent of oxide components, and to avoid cumbersome equations, a formula was proposed by Kuryaeva [44,45] to calculate NBO/T on the basis of gram-ion composition of oxide components. For the simplicity of calculation it was assumed that tricluster oxygen is formed at the expense of Al 2 O 3 .…”

Influence of pressure on the refractive index and relative density of glasses in the CaO-Al2O3-SiO2 system

“…In our earlier studies [11,14], we used this assumption as the starting point and proposed a model describing the formation of nonbridging oxygen atoms in the structure of aluminosilicate glasses and melts, in which excess charges in aluminum-oxygen tetrahedra are compensated at the expense of doubly charged cations, and a relationship for calculating the degree of polymerization of aluminosilicate glasses and melts with allowance for the formation of triclusters and the presence of 25% aluminum cations with a coordination number of more than four. Taking into account the recent experimental results obtained by Neuville et al [15], according to which the glasses with the ratio of molar concentrations [CaO]/([CaO] + [Al 2 O 3 ]) = 0.5 contain ~10% highly coordinated aluminum, we modified the aforementioned relationship in our previous paper [16]. As a result, this relationship was rewritten in the following form:…”