Basically, this method is a modification of the solution-doping process such as employed for the production of alumina- or rare-earth-doped silica for optical fiber preforms. This is to enable a design strategy for glasses with high dopant capacity, but also to provide new insight at the structural origin of the excess in the vibrational density of states and the Boson peak of aluminosilicate materials.īinary (100-x) SiO 2 – x Al 2O 3 glasses with 2.05 < × < 7 mol % were prepared by reactive powder sintering of nanoscale silica 15, 16, 17.
We now explore the low-frequency modes of binary aluminosilicate glasses through analysis of low temperature heat capacity and low frequency Raman scattering so as to find quantitative scaling parameters and, subsequently, relations between chemical composition, intermediate-range order and the length-scale of elastic heterogeneity. On the other hand, high dopant capacity allows for shorter fiber lengths and/or significantly higher power levels 14. However, typical strategies for compositional design remain empirical due to the lack of understanding regarding the structural role of alumina on intermediate length scale. This is primarily for improving the solubility of rare-earth ions and to prevent their clustering 13. In the latter, alumina is one of the most significant dopants used in silica-based active fiber laser applications.
Here, we consider the binary system of SiO 2-Al 2O 3, both as a fundamental model for a mixed tetrahedral network 11 and for the wide relevance of aluminosilicates, ranging from geosciences to optical fiber and high-power laser gain media 12. g., optical attenuation 8, mechanical behaviour 9 or ion mobility 10. While the exact consequences of such intermediate-range structural features remain unclear, various links have been identified with macroscopic properties, e. Heterogeneity then manifests not only in the excess of heat capacity, but also in the terahertz frequency range (0.1–3 THz) which is detectable by low-frequency Raman spectroscopy 5, or in the inelastic scattering of X-rays (IXS) and neutrons (INS) 6, 7. The underlying excess of vibrational states is often taken as a universal signature of structural heterogeneity at intermediate length scale 2, resulting from dynamic heterogeneity in the liquid state which leads to spatial gradients in fictive temperature 3, 4. In accordance with Loewensteinian dynamics, this proves that mild alumina doping increases structural homogeneity on molecular scale.Īt low temperature (<50 K), the heat capacity C p of glasses and other non-crystalline materials exhibits a characteristic deviation from the Debye model 1. At the same time, the average inter-particle distance increases slightly due to the presence of oxygen tricluster species.
We find that this value decreases from about 3.9 nm to 3.3 nm when mildly increasing the alumina content from zero (vitreous silica) to 7 mol%. Both experiments allow for the extraction of the average dynamic correlation length as a function of alumina content. From correlation with low-frequency Raman spectroscopy, we obtain the Raman coupling coefficient. We determine the vibrational density of states VDoS g(ω) using low-temperature heat capacity data. Here, we consider the low-frequency vibrational modes of such materials for assessing structural heterogeneity on molecular scale. In binary aluminosilicate liquids and glasses, heterogeneity on intermediate length scale is a crucial factor for optical fiber performance, determining the lower limit of optical attenuation and Rayleigh scattering, but also clustering and precipitation of optically active dopants, for example, in the fabrication of high-power laser gain media.
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