Role of Substitution Pathway in Governing Density and Molar Volume Trends of Y2O3-Modified Sodium Borate Glasses

Abstract

The structural role and volumetric dynamics of yttrium oxide substitution were investigated in the glass series 42.5Na2O.(57.5-x) B2O3.xY2O3(x = 0, 0.5, 1, 2.5, 3, 4, 5 mol%) synthesized via the conventional melt-quenching technique [1,2]. X-ray diffraction analysis confirmed the fully amorphous nature of all prepared specimens, characterized by broad diffuse halo patterns centered near 2??30-35o without discrete crystalline reflections [1,3]. Archimedes density measurements revealed a monotonic decrease in glass density from 2.6214g/cm3 to2.1137 g/cm3 (19.37% reduction) with increasing Y2O3 concentration from 0 to 5mol% [1,4]. Concurrently, the molar volume exhibited a substantial expansion, increasing from25.32 cm3/mol to 35.10 cm3/mol (38.63% growth) [1,4]. This unexpected structural relaxation—despite the significantly higher molecular weight of Y2O3 (225.81g/mol) relative to B2O3 (69.62g/mol)—is attributed to network expansion caused by the replacement of covalent B–O linkages with longer, predominantly ionic Y–O bonds bridging isolated borate units[4,5]. Comparative analysis with established literature demonstrates that the directional behaviour of physical density in rare-earth-doped borates is fundamentally dictated by the specific oxide component being replaced rather than being an intrinsic property of the dopant cation itself [5,6]. These findings provide critical clarification on compositional pathways in modified alkali borate glasses [1,7].

Introduction

Alkali borate glasses are important materials in glass technology and condensed matter physics because of their optical transparency, low melting temperatures, and unique structural behaviour known as the boron anomaly. The addition of alkali oxides such as Na?O modifies the borate network by converting trigonal BO? units into tetrahedral BO? units. Rare-earth oxides, particularly yttrium oxide (Y?O?), can further enhance optical, nonlinear optical, and radiation-shielding properties.

The present study investigates the structural and physical properties of the ternary glass system 42.5Na?O · (57.5−x)B?O? · xY?O? (x = 0–5 mol%) to understand how replacing the glass-forming oxide B?O? with Y?O? influences density and molar volume.

Experimental Methodology

Glass samples were synthesized using the conventional melt-quenching technique. Sodium carbonate, boric acid, and yttrium oxide were mixed, calcined at 450°C, melted at 900–950°C, and rapidly quenched into preheated molds. The samples were subsequently annealed at 320°C to remove residual thermal stresses.

Structural and physical characterization was performed using:

• X-ray diffraction (XRD): To confirm the amorphous nature of the synthesized glasses.

• Archimedes’ principle: To measure bulk density.

• Molar volume calculations: To investigate changes in the glass structure and packing efficiency.

Results and Discussion

1. XRD Analysis

All synthesized samples exhibited broad diffuse diffraction halos without sharp Bragg peaks, confirming that the glass samples remained amorphous throughout the investigated Y?O? concentration range.

2. Density and Molar Volume

Increasing Y?O? concentration produced a continuous decrease in density and an increase in molar volume.

The density decreased by 19.37%, while the molar volume increased by 38.63%. Although Y?O? increases the average molecular weight, the substantial expansion in molar volume dominates, resulting in an overall reduction in density.

3. Structural Origin of Network Expansion

The decrease in density is attributed to structural depolymerization and reduced packing efficiency. Substitution of B?O? with Y?O? disrupts the rigid borate network containing covalently bonded BO? and BO? units.

Y³? ions form longer and predominantly ionic Y–O bonds (approximately 2.25 Å) compared with shorter covalent B–O bonds (approximately 1.37–1.47 Å). This difference increases structural spacing, weakens network connectivity, and promotes volumetric expansion.

4. Comparison with Other Glass Systems

The physical effects of oxide substitution depend strongly on the role of the substituted oxide:

• B?O? → Y?O?: Network depolymerization, increased molar volume, and decreased density.

• P?O? → ZnO: Zn–O–P cross-linking, structural compaction, and increased density.

• Na?O → Y?O?: Y³? ions occupy interstitial spaces, resulting in network compaction and increased density.

These comparisons demonstrate that density changes in rare-earth-doped glasses are governed not only by molecular weight but also by the structural role of the substituted oxide.

Conclusion

A series of 42.5Na2O?(57.5?x)B2O3 ?xY2O3(x = 0–5 mol%) glasses was successfully prepared via melt-quenching and characterized. The core findings are: 1) XRD analysis confirmed complete amorphous phase purity across all compositions. 2) Density decreased monotonically from 2.6214 g/cm3to 2.1137 g/c m3(19.37% reduction), while molar volume expanded from 25.32 c m3/mol to 35.10 c m3/mol (+38.63%). 3) The structural expansion is caused by replacing short covalent B–O linkages with longer ionic Y–O bonds, resulting in network depolymerization. 4) Comparison with 42.5Na2O ?(57.5 ? x)P2O5 ?xZnO glasses [1] proves that density trends depend strictly on whether the dopant displaces a network former or a modifier oxide.

References

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Copyright

Copyright © 2026 Sagar Vijay Kale. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.